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How to remove two chars from the beginning of a line
Question: I'm a complete Python noob. How can I remove two characters from the beginning
of each line in a file? I was trying something like this:
#!/Python26/
import re
f = open('M:/file.txt')
lines=f.readlines()
i=0;
for line in lines:
line = line.strip()
#do something here
Answer: You were off to a good start. Try this in your loop:
for line in lines:
line = line[2:]
# do something here
The [2:] is called
"[slice](http://docs.python.org/tutorial/introduction.html#strings)" syntax,
it essentially says "give me the part of this sequence which begins at index 2
and continues to the end (since no end point was specified after the colon).
|
exec() bytecode with arbitrary locals?
Question: Suppose I want to execute code, for example
value += 5
inside a namespace of my own (so the result is essentially `mydict['value'] +=
5`). There's a function `exec()`, but I have to pass a string there:
exec('value += 5', mydict)
and passing statements as strings seems strange (e.g. it's not colorized that
way). Can it be done like:
def block():
value += 5
???(block, mydict)
? The obvious candidate for last line was `exec(block.__code__, mydict)`, but
no luck: it raises `UnboundLocalError` about `value`. I believe it basically
executes `block()`, not _the code inside block_ , so assignments aren't easy –
is that correct?
Of course, another possible solution would be to disassembly
`block.__code__`...
FYI, I got the question because of [this
thread](http://mail.python.org/pipermail/python-
ideas/2009-August/005552.html). Also, this is why some (me undecided) call for
new syntax
using mydict:
value += 5
* * *
Note how this doesn't throw error but doesn't change `mydict` either:
def block(value = 0):
value += 5
block(**mydict)
Answer: You can pass bytecode instead of a string to `exec`, you just need to make the
right bytecode for the purpose:
>>> bytecode = compile('value += 5', '<string>', 'exec')
>>> mydict = {'value': 23}
>>> exec(bytecode, mydict)
>>> mydict['value']
28
Specifically, ...:
>>> import dis
>>> dis.dis(bytecode)
1 0 LOAD_NAME 0 (value)
3 LOAD_CONST 0 (5)
6 INPLACE_ADD
7 STORE_NAME 0 (value)
10 LOAD_CONST 1 (None)
13 RETURN_VALUE
the load and store instructions must be of the _NAME persuasion, and this
`compile` makes them so, while...:
>>> def f(): value += 5
...
>>> dis.dis(f.func_code)
1 0 LOAD_FAST 0 (value)
3 LOAD_CONST 1 (5)
6 INPLACE_ADD
7 STORE_FAST 0 (value)
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
...code in a function is optimized to use the _FAST versions, and those don't
work on a dict passed to `exec`. If you started somehow with a bytecode using
the _FAST instructions, you could patch it to use the _NAME kind instead, e.g.
with [bytecodehacks](http://mail.python.org/pipermail/python-announce-
list/2004-July/003280.html) or some similar approach.
|
wxPython: Items in BoxSizer don't expand horizontally, only vertically
Question: I have several buttons in various sizers and they expand in the way that I
want them to. However, when I add the parent to a new wx.BoxSizer that is used
to add a border around all the elements in the frame, the sizer that has been
added functions correctly vertically, but not horizontally.
The following code demonstrates the problem:
#! /usr/bin/env python
import wx
import webbrowser
class App(wx.App):
def OnInit(self):
frame = MainFrame()
frame.Show()
self.SetTopWindow(frame)
return True
class MainFrame(wx.Frame):
title = 'Title'
def __init__(self):
wx.Frame.__init__(self, None, -1, self.title)
panel = wx.Panel(self)
#icon = wx.Icon('icon.png', wx.BITMAP_TYPE_PNG)
#self.SetIcon(icon)
sizer = wx.FlexGridSizer(rows=2, cols=1, vgap=10, hgap=10)
button1 = wx.Button(panel, -1, 'BUTTON')
sizer.Add(button1, 0, wx.EXPAND)
buttonSizer = wx.FlexGridSizer(rows=1, cols=4, vgap=10, hgap=5)
buttonDelete = wx.Button(panel, -1, 'Delete')
buttonSizer.Add(buttonDelete, 0, 0)
buttonEdit = wx.Button(panel, -1, 'Edit')
buttonSizer.Add(buttonEdit, 0, 0)
buttonNew = wx.Button(panel, -1, 'New')
buttonSizer.Add(buttonNew, 0, 0)
buttonSizer.AddGrowableCol(0, 0)
sizer.Add(buttonSizer, 0, wx.EXPAND|wx.HORIZONTAL)
sizer.AddGrowableCol(0, 0)
sizer.AddGrowableRow(0, 0)
mainSizer = wx.BoxSizer(wx.EXPAND)
mainSizer.Add(sizer, 0, wx.EXPAND|wx.ALL, 10)
#panel.SetSizerAndFit(sizer)
#sizer.SetSizeHints(self)
panel.SetSizerAndFit(mainSizer)
mainSizer.SetSizeHints(self)
if __name__ == '__main__':
app = App(False)
app.MainLoop()
Commenting out lines **57** and **58** and uncommenting lines **55** and
**56** removes the extra BoxSizer and shows how I expect everything to
function (without the whitespace of course).
I am completely stuck with this problem and still have no clue as to how to
fix it.
Answer: First of all, you're passing some flags incorrectly. BoxSizer takes
wx.HORIZONTAL or wx.VERTICAL, not wx.EXPAND. sizer.Add does not take
wx.HORIZONTAL.
If you have a VERTICAL BoxSizer, wx.EXPAND will make the control fill
horizontally, while a proportion of 1 or more (second argument to Add) will
make the control fill vertically. It's the opposite for HORIZONTAL BoxSizers.
sizer = wx.BoxSizer(wx.VERTICAL)
sizer.Add(widget1, 0, wx.EXPAND)
sizer.Add(widget2, 1)
widget1 will expand horizontally. widget2 will expand vertically.
If you put a sizer in another sizer, you need to be sure to have its
proportion and EXPAND flags set so that its insides will grow how you want
them to.
I'll leave the rest to you.
|
models.py with ManyToMany and progmatically adding data via a shell script
Question: First post to stackoverflow I did do a search and came up dry. I also own the
django book (Forcier,Bissex,Chun) and they don't explain how to do this. In
short I can't figure out how to progmatically add a data via a python shell
script to the ManyToMay model..
from django.db import models
from django.contrib import admin
class Client(models.Model):
client = models.CharField(max_length=256, primary_key=True)
access = models.DateField()
description = models.TextField()
host = models.CharField(max_length=256)
lineEnd = models.CharField(max_length=256)
options = models.TextField()
owner = models.CharField(max_length=100)
root = models.CharField(max_length=256)
submitOptions = models.CharField(max_length=256)
update = models.DateField()
def __unicode__(self):
return str(self.client)
admin.site.register(Client)
class Change(models.Model):
"""This simply expands out 'p4 describe' """
change = models.IntegerField(primary_key=True)
client = models.ManyToManyField(Client)
desc = models.TextField()
status = models.CharField(max_length=128)
def __unicode__(self):
return str(self.change)
admin.site.register(Change)
Here is what I have which works but I don't know how to add the ManyToMany. I
can't seem to figure out how to progmatically call it. I know the row in SQL
exists.
\--- massImport.py ---
# Assume the client "clientspec" exists. I know how to create that if
neeeded.
changes = [ { 'change': 123, 'desc': "foobar", status': "foobar",
client': "clientspec", }]
for item in changes:
entry = Change(
change = item['change'],
desc = item['desc'],
status = item['status'],
# client = Client.objects.filter(client=item['client'])
)
entry.save()
Can anyone show me where the error of my ways is. I would really appreciate
it. Thanks!!
Answer: Turns out Tiago was very close..
# Assume the client "clientspec" exists. I know how to create that if
neeeded.
changes = [ { 'change': 123, 'desc': "foobar", status': "foobar",
client': "clientspec", }]
for item in changes:
entry = Change()
entry.change = item['change']
entry.desc = item['desc']
entry.status = item['status']
entry.time = datetime.datetime.fromtimestamp(float(item['time']))
entry.client.add(Client.objects.get(client=item['client']))
entry.save()
So.. I will give props to Tiago
|
Can't get Beaker sessions to work (KeyError)
Question: I'm a newb to the Python world and am having the dangest time with getting
sessions to work in my web frameworks. I've tried getting Beaker sessions to
work with the webpy framework and the Juno framework. And in both frameworks I
always get a KeyError when I try to start the session.
Here is the error message in webpy (its pretty much the exact same thing when
I try to use beaker sessions in Juno too)...
ERROR
<type 'exceptions.KeyError'> at /
'beaker.session'
Python /Users/tyler/Dropbox/Code/sites/webpy1/code.py in GET, line 15
Web GET http://localhost:1234/
15. session = web.ctx.environ['beaker.session']
CODE
import web
import beaker.session
from beaker.middleware import SessionMiddleware
urls = (
'/', 'index'
)
class index:
def GET(self):
session = web.ctx.environ['beaker.session']
return "hello"
app = web.application(urls, globals())
if __name__ == "__main__": app.run()
Answer: You haven't created the session object yet, so you can't find it in the
environment (the `KeyError` simply means "`beaker.session` is not in this
dictionary").
Note that I don't know either webpy nor beaker very well, so I can't give you
deeper advice, but from what I understand from the docs and source this should
get you started:
if __name__ == "__main__": app.run(SessionMiddleware)
|
How useful would be a Smalltalk source code browser for other programming languages?
Question: I'm working on an IDE for python, ruby and php.
Never having used Smallltalk myself (even it was very popular when I was at
university) I wonder if the classic Smalltalk Browser which displays only one
method is really an improvment or to classical file editing or not.
I myself like to have the overview of as much as possible in a class. Now i
use a 24" 1280x1920 display in a two column mode which can display **a lot**
of lines at ones.
I personally have to wonder what is the benefit if you for example also have
good code folding editor where a user can fold for example all def's
(functions code bodies) with one keystroke.
But I see the request to make xxx more smalltalkish from time to time in
newsgroups. I know some might want an image based version but the browser was
the second most different Smalltalk invention.
Answer: The Smalltalk browser has two parts: the top one shows the packages, classes,
protocols and methods/messages, the bottom one shows the content of one
method. This is very usefull if you design/code your program thinking more on
signatures and names, rather than as a web of lines of codes.
If you concentrate on signature, this could lead to a more "object oriented"
style, where the system is designed as a collaboration of objects sending
messages to each other. In this paradigm, the methods names are somehow more
important than how they are implemented.
If you have a very large screen (I got one myself :-)) you would open several
smalltalk browsers allowing you to browse (and code) in several different
packages and classes. Moreover, you would probably also have a worspace and a
xUnit to test and play with your objects.
I suggest you look at the Whisker editor created for Squeak. It proposes a
quite nice balance between names and signatures navigation, and lines of code
exploration. You would need to try it, as the screenshot does not show the
dynamic of it, and how you navigate thru the "boxes" of codes.
<http://www.mindspring.com/~dway/smalltalk/whisker.html>
|
Python re question - sub challenge
Question: I want to add a href links to all words prefixed with # or ! or @ If this is
the text
Check the #bamboo and contact @Fred re #bamboo #garden
should be converted to:
Check the <a href="/what/bamboo">#bamboo</a> and contact <a href="/who/fred">@Fred</a> re <a href="/what/bamboo">#bamboo</a> <a href="/what/garden">#garden</a>
Note that # and @ go to different places.
This is as far as I have got, just doing the hashes...
matched = re.sub("[#](?P<keyword>\w+)", \
'<a href="/what/(?P=keyword)">(?P=keyword)</a>', \
text)
Any re gurus able to point me in the right direction. Do I need to do separate
matches for each symbol?
Answer: I'd do it with a single match and a function picking the "place". I.e.:
import re
places = {'#': 'what',
'@': 'who',
'!': 'why',
}
def replace(m):
all = m.group(0)
first, rest = all[0], all[1:]
return '<a href="/%s/%s">%s</a>' % (
places[first], rest, all)
markedup = re.sub(r'[#!@]\w+', replace, text)
|
relevent query to how to fetch public key from public key server
Question:
import urllib
response = urllib.urlopen('http://pool.sks-keyservers.net/')
print 'RESPONSE:', response
print 'URL :', response.geturl()
headers = response.info()
print 'DATE :', headers['date']
print 'HEADERS :'
print '---------'
print headers
data = response.read()
print 'LENGTH :', len(data)
print 'DATA :'
print '---------'
print data
This code enable me to see some webpage information and contents. what
actually i had query that how to fetch the public key from any public key
server using python function.
Answer: [This spec](http://tools.ietf.org/html/draft-shaw-openpgp-hkp-00) should help
you along, and stop you asking the same question repeatedly :). Basically you
need to perform an HTTP GET request to the key server. Most key servers work
on TCP port 11371 so you need to ensure that your firewall will let you
through.
For example, this URL will give you God's public key:
[http://pool.sks-
keyservers.net:11371/pks/lookup?op=get&search=0x1278A1862492D908&options=mr](http://pool.sks-
keyservers.net:11371/pks/lookup?op=get&search=0x1278A1862492D908&options=mr)
The response is the key.
In python:
import urllib
f = urllib.urlopen('http://pool.sks-keyservers.net:11371/pks/lookup?op=get&search=0x1278A1862492D908&options=mr')
data = f.read()
print data
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: SKS 1.1.0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=NhFO
-----END PGP PUBLIC KEY BLOCK-----
|
Python socket error occured
Question: I wrote this code.
import socket
host = 'localhost'
port = 3794
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.setsockopt(socket.SOL_SOCKET, socket.SO_BROADCAST, 1)
s.bind((host, port))
while 1:
print 'Type message you want to send...'
msg = raw_input()
if msg == '':
s.close()
break
s.sendall(msg)
and next execute this code.
Traceback (most recent call last):
File "socket.py", line 11, in ?
s.bind((host, port))
File "<string>", line 1, in bind
socket.error: (99, 'Cannot assign requested address')
What's wrong?
Do you know solutions?
Answer: This means that you already have a socket bound to 3794 port.
It may be another application or it means that port didn't got released yet
after the previous run of your own script (it happens, if script terminated
improperly).
Simply try to use another port number - I believe everything will work fine.
|
Calculating a SHA hash with a string + secret key in python
Question: Amazon Product API now requires a signature with every request which I'm
trying to generate ushing Python.
The step I get hung up on is this one:
"Calculate an RFC 2104-compliant HMAC with the SHA256 hash algorithm using the
string above with our "dummy" Secret Access Key: 1234567890. For more
information about this step, see documentation and code samples for your
programming language."
Given a string and a secret key (in this case 1234567890) how do I calculate
this hash using Python?
\----------- UPDATE -------------
The first solution using HMAC.new looks correct however I'm getting a
different result than they are.
<http://docs.amazonwebservices.com/AWSECommerceService/latest/DG/index.html?rest-
signature.html>
According to Amazon's example when you hash the secret key 1234567890 and the
following string
GET
webservices.amazon.com
/onca/xml
AWSAccessKeyId=00000000000000000000&ItemId=0679722769&Operation=I
temLookup&ResponseGroup=ItemAttributes%2COffers%2CImages%2CReview
s&Service=AWSECommerceService&Timestamp=2009-01-01T12%3A00%3A00Z&
Version=2009-01-06
You should get the following signature:
`'Nace+U3Az4OhN7tISqgs1vdLBHBEijWcBeCqL5xN9xg='`
I am getting this:
`'411a59403c9f58b4a434c9c6a14ef6e363acc1d1bb2c6faf9adc30e20898c83b'`
Answer:
import hmac
import hashlib
import base64
dig = hmac.new(b'1234567890', msg=your_bytes_string, digestmod=hashlib.sha256).digest()
base64.b64encode(dig).decode() # py3k-mode
'Nace+U3Az4OhN7tISqgs1vdLBHBEijWcBeCqL5xN9xg='
|
Is this a bug in Django formset validation?
Question: Manual example:
<http://docs.djangoproject.com/en/1.0/topics/forms/formsets/#formset-
validation> (I'm using Django 1.0.3 to run on Google App Engine)
Code:
from django import forms
from django.forms.formsets import formset_factory
class ArticleForm1(forms.Form):
title = forms.CharField()
pub_date = forms.DateField()
class ArticleForm2(forms.Form):
title = forms.CharField()
class ArticleForm3(forms.Form):
title = forms.CharField()
pub_date = forms.CharField()
ArticleFormSet1 = formset_factory(ArticleForm1)
ArticleFormSet2 = formset_factory(ArticleForm2)
ArticleFormSet3 = formset_factory(ArticleForm3)
data = {
'form-TOTAL_FORMS': u'2',
'form-INITIAL_FORMS': u'0',
'form-0-title': u'',
'form-0-pub_date': u'16 June 1904',
'form-1-title': u'', # <-- this title is missing but required
'form-1-pub_date': u'16 June 1904', # <-- this date is missing but required
}
formset = ArticleFormSet1(data)
print "Should be False: %s" % formset.is_valid()
formset = ArticleFormSet2(data)
print "Should be False: %s" % formset.is_valid()
formset = ArticleFormSet3(data)
print "Should be False: %s" % formset.is_valid()
Output:
$ .../ActiveStatePython2.5/python.exe formset_bug.py
Should be False: False
Should be False: True
Should be False: False
ActiveState Python 2.5.4.4, Django 1.0.3 final.
It looks as if it's not validating in the case of only one CharField (but 2
CharFields or a CharField and a DateField works).
I Googled for such a bug and couldn't find anything. I haven't yet tried
Django 1.1, but it's much easier to use 1.0 on GAE for now.
Answer: Okay, I understand more about Django now. This is not a bug.
From <http://docs.djangoproject.com/en/1.0/topics/forms/formsets/#formset-
validation>:
"The formset is smart enough to ignore extra forms that were not changed."
From <http://stackoverflow.com/questions/927619/django-formset-isvalid-
failing-for-extra-forms>:
"Formsets pass empty_permitted=True to all "extra" forms, and a form with
empty_permitted that hasn't been modified should always pass validation."
When I set form-TOTAL_FORMS to u'2', it no longer has any extra, and the
validation does what I expect.
When there is part of a second field (either as DateField or CharField), the
field has some stuff and is no longer extra.
|
Does a UDP service have to respond from the connected IP address?
Question: [Pyzor](http://pyzor.org) uses UDP/IP as the communication protocol. We
recently switched the public server to a new machine, and started getting
reports of many timeouts. I discovered that I could fix the problem if I
changed the IP that was queried from `eth0:1` to `eth0`.
I can reproduce this problem with a simple example:
This is the server code:
#! /usr/bin/env python
import SocketServer
class RequestHandler(SocketServer.DatagramRequestHandler):
def handle(self):
print self.packet
self.wfile.write("Pong")
s = SocketServer.UDPServer(("0.0.0.0", 24440), RequestHandler)
s.serve_forever()
This is the client code (`188.40.77.206` is `eth0`. `188.40.77.236` is the
same server, but is `eth0:1`):
>>> import socket
>>> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
>>> s.sendto('ping', 0, ("188.40.77.206", 24440))
4
>>> s.recvfrom(1024)
('Pong', ('188.40.77.206', 24440))
>>> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
>>> s.sendto('ping', 0, ("188.40.77.236", 24440))
4
>>> s.recvfrom(1024)
[never gets anything]
The server gets the "ping" packet in both cases (and therefore sends the
"pong" packet in both cases).
Oddly, this **does** work from some places (i.e. I'll get a response from both
IPs). For example, it works from `188.40.37.137` (same network/datacenter,
different server), but also from `89.18.189.160` (different datacenter). In
those cases, the `recvfrom` response does have the `eth0` IP, rather than the
one that was connected to.
Is this just a rule of UDP? Is this a problem/limitation with the
[Python](http://python.org) `UDPServer` class? Is it something I'm doing
incorrectly? Is there any way that I can have this work apart from simply
connecting to the `eth0` IP (or listening on the specific IP rather than
`0.0.0.0`)?
Answer: I came across this with a TFTP server. My server had two IP addresses facing
the same network. Because UDP is connectionless, there can be issues with IP
addresses not being set as expected in that situation. The sequence I had was:
1. Client sends the initial packet to the server at a particular IP address
2. Server reads the client's source address from the incoming packet, and sends a response.
1. However, in the response, the server's "source address" is set according to the routing tables, and it gets set to the **other** IP address.
2. It wasn't possible to control the server's "source" IP address because the OS didn't tell us which IP address the request came in through.
3. The client gets a response from the "other" IP address, and rejects it.
The solution in my case was to specifically bind the TFTP server to the IP
address that I wanted to listen to, rather than binding to all interfaces.
I found some text that may be relevant in a [Linux man page for
**`tftpd`**](http://man.cx/tftpd%288%29) (TFTP server). Here it is:
Unfortunately, on multi-homed systems, it is impossible for tftpd to
determine the address on which a packet was received. As a result, tftpd
uses two different mechanisms to guess the best source address to use for
replies. If the socket that inetd(8) passed to tftpd is bound to a par‐
ticular address, tftpd uses that address for replies. Otherwise, tftpd
uses ‘‘UDP connect’’ to let the kernel choose the reply address based on
the destination of the replies and the routing tables. This means that
most setups will work transparently, while in cases where the reply
address must be fixed, the virtual hosting feature of inetd(8) can be
used to ensure that replies go out from the correct address. These con‐
siderations are important, because most tftp clients will reject reply
packets that appear to come from an unexpected address.
See [this answer](http://stackoverflow.com/a/3929208/60075) which shows that
on Linux it _is_ possible to read the local address for incoming UDP packets,
and set it for outgoing packets. It's possible in C; I'm not sure about Python
though.
|
What is the oldest time that can be represented in python?
Question: I have written a function comp(time1, time2) which will return true when time1
is lesser than time2. I have a scenario where time1 should always be lesser
than time2. I need time1 to have the least possible value(date). How to find
this time and how to form the corresponding object.
Answer: If using the [datetime](https://docs.python.org/2/library/datetime.html)
module, [date](https://docs.python.org/2/library/datetime.html#date-objects),
[time](https://docs.python.org/2/library/datetime.html#time-objects), and
[datetime](https://docs.python.org/2/library/datetime.html#datetime-objects)
objects all have a `min` and `max` attribute.
>>> from datetime import date, time, datetime
>>> date.min
datetime.date(1, 1, 1)
>>> date.max
datetime.date(9999, 12, 31)
>>> time.min
datetime.time(0, 0)
>>> time.max
datetime.time(23, 59, 59, 999999)
>>> datetime.min
datetime.datetime(1, 1, 1, 0, 0)
>>> datetime.max
datetime.datetime(9999, 12, 31, 23, 59, 59, 999999)
|
Python: what kind of literal delimiter is "better" to use?
Question: What is the best literal delimiter in Python and why? Single ' or double "?
And most important, why?
I'm a beginner in Python and I'm trying to stick with just one. I know that in
PHP, for example " is preferred, because PHP does not try to search for the
'string' variable. Is the same case in Python?
Answer: ' because it's one keystroke less than ". Save your wrists!
They're otherwise identical (except you have to escape whichever you choose to
use, if they appear inside the string).
|
gotchas where Numpy differs from straight python?
Question: Folks,
is there a collection of gotchas where Numpy differs from python, points that
have puzzled and cost time ?
> "The horror of that moment I shall never never forget !"
> "You will, though," the Queen said, "if you don't make a memorandum of it."
For example, NaNs are always trouble, anywhere. If you can explain this
without running it, give yourself a point --
from numpy import array, NaN, isnan
pynan = float("nan")
print pynan is pynan, pynan is NaN, NaN is NaN
a = (0, pynan)
print a, a[1] is pynan, any([aa is pynan for aa in a])
a = array(( 0, NaN ))
print a, a[1] is NaN, isnan( a[1] )
(I'm not knocking numpy, lots of good work there, just think a FAQ or Wiki of
gotchas would be useful.)
Edit: I was hoping to collect half a dozen gotchas (surprises for people
learning Numpy).
Then, if there are common gotchas or, better, common explanations, we could
talk about adding them to a community Wiki (where ?) It doesn't look like we
have enough so far.
Answer: Because `__eq__` does not return a bool, using numpy arrays in any kind of
containers prevents equality testing without a container-specific work around.
Example:
>>> import numpy
>>> a = numpy.array(range(3))
>>> b = numpy.array(range(3))
>>> a == b
array([ True, True, True], dtype=bool)
>>> x = (a, 'banana')
>>> y = (b, 'banana')
>>> x == y
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
This is a horrible problem. For example, you cannot write unittests for
containers which use `TestCase.assertEqual()` and must instead write custom
comparison functions. Suppose we write a work-around function
`special_eq_for_numpy_and_tuples`. Now we can do this in a unittest:
x = (array1, 'deserialized')
y = (array2, 'deserialized')
self.failUnless( special_eq_for_numpy_and_tuples(x, y) )
Now we must do this for every container type we might use to store numpy
arrays. Furthermore, `__eq__` might return a bool rather than an array of
bools:
>>> a = numpy.array(range(3))
>>> b = numpy.array(range(5))
>>> a == b
False
Now each of our container-specific equality comparison functions must also
handle that special case.
Maybe we can patch over this wart with a subclass?
>>> class SaneEqualityArray (numpy.ndarray):
... def __eq__(self, other):
... return isinstance(other, SaneEqualityArray) and self.shape == other.shape and (numpy.ndarray.__eq__(self, other)).all()
...
>>> a = SaneEqualityArray( (2, 3) )
>>> a.fill(7)
>>> b = SaneEqualityArray( (2, 3) )
>>> b.fill(7)
>>> a == b
True
>>> x = (a, 'banana')
>>> y = (b, 'banana')
>>> x == y
True
>>> c = SaneEqualityArray( (7, 7) )
>>> c.fill(7)
>>> a == c
False
That seems to do the right thing. The class should also explicitly export
elementwise comparison, since that is often useful.
|
Python/Suds: Type not found: 'xs:complexType'
Question: I have the following simple python test script that uses
[Suds](https://fedorahosted.org/suds/) to call a SOAP web service (the service
is written in ASP.net):
from suds.client import Client
url = 'http://someURL.asmx?WSDL'
client = Client( url )
result = client.service.GetPackageDetails( "MyPackage" )
print result
When I run this test script I am getting the following error (used code markup
as it doesn't wrap):
No handlers could be found for logger "suds.bindings.unmarshaller"
Traceback (most recent call last):
File "sudsTest.py", line 9, in <module>
result = client.service.GetPackageDetails( "t3db" )
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 240, in __call__
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 379, in call
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 240, in __call__
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 422, in call
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 480, in invoke
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 505, in send
File "build/bdist.cygwin-1.5.25-i686/egg/suds/client.py", line 537, in succeeded
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/binding.py", line 149, in get_reply
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 303, in process
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 88, in process
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 104, in append
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 181, in append_children
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 104, in append
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 181, in append_children
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 104, in append
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 181, in append_children
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 102, in append
File "build/bdist.cygwin-1.5.25-i686/egg/suds/bindings/unmarshaller.py", line 324, in start
suds.TypeNotFound: Type not found: 'xs:complexType'
Looking at the source for the WSDL file's header (reformatted to fit):
<?xml version="1.0" encoding="utf-8" ?>
<wsdl:definitions xmlns:http="http://schemas.xmlsoap.org/wsdl/http/"
xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
xmlns:s="http://www.w3.org/2001/XMLSchema"
xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding/"
xmlns:tns="http://http://someInternalURL/webservices.asmx"
xmlns:tm="http://microsoft.com/wsdl/mime/textMatching/"
xmlns:mime="http://schemas.xmlsoap.org/wsdl/mime/"
targetNamespace="http://someURL.asmx"
xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/">
I am guessing based on the last line of output:
suds.TypeNotFound: Type not found: 'xs:complexType'
That I need to use Sud's [doctor
class](https://fedorahosted.org/suds/wiki/Documentation#FIXINGBROKENSCHEMAs)
to fix the schema but being a SOAP newbie I don't know what exactly needs
fixed in my case. Does anyone here have any experience using Suds to
fix/correct schema?
Answer: **Ewall** 's resource is a good one. If you try to search in suds trac
tickets, you could see that other people have problems [similar to
yours](https://fedorahosted.org/suds/ticket/220), but with different object
types. It can be a good way to learn from it's examples and how they import
their namespaces.
> The problem is that your wsdl contains a schema definition that references
> the (...) but fails to import the
> "http://schemas.xmlsoap.org/soap/encoding/" namespace (and associated
> schema) properly. The schema can be patched at runtime using the schema
> ImportDoctor as discussed here:
> <https://fedorahosted.org/suds/wiki/Documentation#FIXINGBROKENSCHEMAs>.
>
> **This is a fairly common problem.**
>
> A commonly referenced schema **(that is not imported)** is the SOAP section
> 5 encoding schema. This can now be fixed as follows:
(all emphasis were mine).
You could try the lines that these documentations provide adding the
namespaces presented in your WSDL. This can be a try-and-error aproach.
imp = Import('http://schemas.xmlsoap.org/soap/encoding/')
# Below is your targetNamespace presented in WSDL. Remember
# that you can add more namespaces by appending more imp.filter.add
imp.filter.add('http://someURL.asmx')
doctor = ImportDoctor(imp)
client = Client(url, doctor=doctor)
You didn't provide the WSDL you're working with, I suppose you have reasons to
not showing to us... so I think you have to try these possibilities by
yourself. Good luck!
|
Why does weakproxy not always preserve equivalence in python?
Question: MySQLDb uses weak proxy to prevent circular dependencies between cursors and
connections.
But you would expect from the documentation on weakref that you could still
tests for equivalence. Yet:
In [36]: interactive.cursor.connection.thread_id()
Out[36]: 4267758
In [37]: interactive.web_logic.conns.primary.thread_id()
Out[37]: 4267758
In [38]: interactive.cursor.connection == interactive.web_logic.conns.primary
Out[38]: False
In [39]: interactive.cursor.connection
Out[39]: <weakproxy at 0x3881c60 to Connection at 0x94c010>
In [40]: interactive.web_logic.conns.primary
Out[40]: <_mysql.connection open to 'xendb01' at 94c010>
How do I tell if the connections are the same ?
Answer: I've long found `weakref.proxy`'s design and implementation to be somewhat
shaky. Witness...:
>>> import weakref
>>> ob=set(range(23))
>>> rob=weakref.proxy(ob)
>>> rob==ob
False
>>> rob.__eq__(ob)
True
...DEFINITELY peculiar! In practice what I use from `weakref` are weak-key or
sometimes weak-value dictionaries; but `weakref.ref` is sounder than the proxy
wrapper on top of it:
>>> wr=weakref.ref(ob)
>>> wr()==ob
True
The need to "call" the ref to get the object (or None if the object has since
disappeared) unfortunately makes it non-transparent (so a DB API module
couldn't do it while staying compliant to the API). I don't understand why
MySqlDb wants weak cursor->connection referencing at all, but if they do I see
why they felt they had to use proxies rather than refs. However, one pays a
very high price for that transparency!
Btw, the "explicit `__eq__`" trick (or an equivalent one with `__cmp__`,
depending on the type of the underlying object) may help you, even though it's
definitely inelegant!
|
Running doctests through iPython and pseudo-consoles
Question: I've got a fairly basic doctestable file:
class Foo():
"""
>>> 3+2
5
"""
if __name__ in ("__main__", "__console__"):
import doctest
doctest.testmod(verbose=True)
which works as expected when run directly through python.
However, in iPython, I get
1 items had no tests:
__main__
0 tests in 1 items.
0 passed and 0 failed.
Test passed.
Since this is part of a Django project and will need access to all of the
appropriate variables and such that manage.py sets up, I can also run it
through a modified command, which uses code.InteractiveConsole, one result of
which is `__name__` gets set to '`__console__`'.
With the code above, I get the same result as with iPython. I tried changing
the last line to this:
this = __import__(__name__)
doctest.testmod(this, verbose=True)
and I get an ImportError on `__console__`, which makes sense, I guess. This
has no effect on either python or ipython.
So, I'd like to be able to run doctests successfully through all three of
these methods, especially the InteractiveConsole one, since I expect to be
needing Django pony magic fairly soon.
Just for clarification, this is what I'm expecting:
Trying:
3+2
Expecting:
5
ok
1 items had no tests:
__main__
1 items passed all tests:
1 tests in __main__.Foo
1 tests in 2 items.
1 passed and 0 failed.
Test passed.
Answer: The root problem is that `ipython` plays weird tricks with `__main__` (through
its own `FakeModule` module) so that, by the time `doctest` is introspecting
that "alleged module" through its `__dict__`, `Foo` is _NOT_ there -- so
doctest doesn't recurse into it.
Here's one solution:
class Foo():
"""
>>> 3+2
5
"""
if __name__ in ("__main__", "__console__"):
import doctest, inspect, sys
m = sys.modules['__main__']
m.__test__ = dict((n,v) for (n,v) in globals().items()
if inspect.isclass(v))
doctest.testmod(verbose=True)
This DOES produce, as requested:
$ ipython dot.py
Trying:
3+2
Expecting:
5
ok
1 items had no tests:
__main__
1 items passed all tests:
1 tests in __main__.__test__.Foo
1 tests in 2 items.
1 passed and 0 failed.
Test passed.
Python 2.5.1 (r251:54863, Feb 6 2009, 19:02:12)
[[ snip snip ]]
In [1]:
Just setting global `__test__` doesn't work, again because setting it as a
global of what you're thinking of as `__main__` does NOT actually place it in
the `__dict__` of the actual object that gets recovered by `m =
sys.modules['__main__']`, and the latter is exactly the expression `doctest`
is using internally (actually it uses `sys.modules.get`, but the extra
precaution is not necessary here since we do know that `__main__` exists in
`sys.modules`... it's just NOT the object you expect it to be!-).
Also, just setting `m.__test__ = globals()` directly does not work either, for
a different reason: `doctest` checks that the values in `__test__` are
strings, functions, classes, or modules, and without some selection you cannot
guarantee that `globals()` will satisfy that condition (in fact it won't).
Here I'm selecting just classes, if you also want functions or whatnot you can
use an `or` in the `if` clause in the genexp within the `dict` call.
I don't know exactly how you're running a Django shell that's able to execute
your script (as I believe `python manage.py shell` doesn't accept arguments,
you must be doing something else, and I can't guess exactly what!-), but a
similar approach should help (whether your Django shell is using ipython, the
default when available, or plain Python): appropriately setting `__test__` in
the object you obtain as `sys.modules['__main__']` (or `__console__`, if
that's what you're then passing on to doctest.testmod, I guess) should work,
as it mimics what doctest will then be doing internally to locate your test
strings.
And, to conclude, a philosophical reflection on design, architecture,
simplicity, transparency, and "black magic"...:
All of this effort is basically what's needed to defeat the "black magic" that
ipython (and maybe Django, though it may be simply delegating that part to
ipython) is doing on your behalf for your "convenience"... any time at which
two frameworks (or more;-) are independently doing each its own brand of black
magic, interoperability may suddenly require substantial effort and become
anything BUT convenient;-).
I'm not saying that the same convenience could have been provided (by any one
or more of ipython, django and/or doctests) _without_ black magic,
introspection, fake modules, and so on; the designers and maintainers of each
of those frameworks are superb engineers, and I expect they've done their
homework thoroughly, and are performing only the minimum amount of black magic
that's indispensable to deliver the amount of user convenience they decided
they needed. Nevertheless, even in such a situation, "black magic" suddenly
turns from a dream of convenience to a nightmare of debugging as soon as you
want to do something even marginally outside what the framework's author had
conceived.
OK, maybe in this case not quite a nightmare, but I do notice that this
question has been open a while and even with the lure of the bounty it didn't
get many answers yet -- though you now do have two answers to pick from, mine
using the `__test__` special feature of doctest, @codeape's using the peculiar
`__IP.magic_run` feature of ironpython. I prefer mine because it does not rely
on anything internal or undocumented -- `__test__` IS a documented feature of
doctest, while `__IP`, with those two looming leading underscores, scream
"deep internals, don't touch" to me;-)... if it breaks at the next point
release I wouldn't be at all surprised. Still, matter of taste -- that answer
may arguably be considered more "convenient".
But, this is exactly my point: convenience may come at an enormous price in
terms of giving up simplicity, transparency, and/or avoidance of
internal/undocumented/unstable features; so, as a lesson for all of us, the
least black magic &c we can get away with (even at the price of giving up an
epsilon of convenience here and there), the happier we'll all be in the long
run (and the happier we'll make other developers that need to leverage our
current efforts in the future).
|
python variable scope
Question: I have started to learn about python and is currently reading through a script
written by someone else. I noticed that globals are scattered throughout the
script (and I don't like it).. Besides that, I also noticed that when I have
code like this
def some_function():
foo.some_method()
# some other code
if __name__ == '__main__' :
foo = Some_Object()
some_function()
even though I don't pass in foo into some_function(), but some_function is
still able to manipulate foo (??!). I don't quite like this although it is
somewhat similar to Javascript closure (?). I would like to know whether it is
possible to stop some_function() from accessing foo if foo is not passed in as
a function argument? Or this is the preferred way in python??! (I'm using
python 2.5 under ubuntu hardy at the moment)
Answer: That script has really serious issues with style and organization -- for
example, if somebody imports it they have to somehow divine the fact that they
have to set `thescript.foo` to an instance of `Some_Object` before calling
`some_function`... yeurgh!-)
It's unfortunate that you're having to learn Python from a badly written
script, but I'm not sure I understand your question. Variable scope in Python
is locals (including arguments), nonlocals (i.e., locals of surrounding
functions, for nested functions), globals, builtins.
Is what you want to stop access to globals? `some_function.func_globals` is
read-only, but you could make a new function with empty globals:
import new
f=new.function(some_function.func_code, {})
now calling `f()` will given an exception `NameError: global name 'foo' is not
defined`. You could set this back in the module with the name `some_function`,
or even do it systematically via a decorator, e.g.:
def noglobal(f):
return new.function(f.func_code, {})
...
@noglobal
def some_function(): ...
this will guarantee the exception happens whenever `some_function` is called.
I'm not clear on what benefit you expect to derive from that, though. Maybe
you can clarify...?
|
Share Python Interpreter in Apache Prefork / WSGI
Question: I am attempting to run a Python application within Apache (prefork) with WSGI
in such a way that a single Python interpreter will be used. This is necessary
since the application uses thread synchronization to prevent race conditions
from occurring. Since Apache prefork spawns multiple processes, the code winds
up not being shared between the interpreters and thus the thread
synchronization is irrelevant (i.e. each thread only sees it own locks which
have no bearing on the other processes).
Here is the setup:
* Apache 2.0 (prefork)
* WSGI
* Python 2.5
Here is the relevant Apache configuration:
WSGIApplicationGroup %{GLOBAL}
<VirtualHost _default_:80>
WSGIScriptAlias / /var/convergedsecurity/apache/osvm.wsgi
Alias /admin_media/ /var/www/html/admin_media/
<Directory /var/www/html/admin_media>
Order deny,allow
Allow from all
</Directory>
Alias /media/ /var/www/html/media/
<Directory /var/www/html/media>
Order deny,allow
Allow from all
</Directory>
</VirtualHost>
Here is what I tried so far (none of which worked):
1. Adding [WSGIApplicationGroup %{GLOBAL}](http://code.google.com/p/modwsgi/wiki/ConfigurationDirectives#WSGIApplicationGroup)
2. Specifying **WSGIDaemonProcess** and **WSGIProcessGroup** within the virtual host:
WSGIDaemonProcess osvm threads=50
WSGIProcessGroup osvm
Is there no way to force Apache prefork to use a single Python interpreter
with WSGI? The documents seem to imply you can with the WSGIDaemonProcess and
WSGIApplicationGroup options but Apache still creates a separate Python
interpreter for each process.
Answer: You can't have the WSGI application run in embedded mode on UNIX systems,
whether it be prefork or worker MPM, as there will indeed be multiple
processes. See:
<http://code.google.com/p/modwsgi/wiki/ProcessesAndThreading>
Creating a daemon process group consisting of single process and delegating
WSGI application to that should achieve what you want. You shouldn't even need
to use WSGIApplicationGroup if it is only one mounted WSGI application you are
talking about. If you want to be absolutely sure though, you can also set it.
Thus configuration within VirtualHost would be:
WSGIDaemonProcess osvm
WSGIProcessGroup osvm
WSGIApplicationGroup %{GLOBAL}
WSGIScriptAlias / /var/convergedsecurity/apache/osvm.wsgi
Although 'processes=1' for WSGIDaemonProcess makes it explicit that one
process is created, don't provide the option though and just let it default to
one process. Any use of 'processes' option, even if for one process will see
'wsgi.multiprocess' set to True.
Rather than use your actual WSGI application, I would suggest you test with
the following simple test program.
import cStringIO
import os
def application(environ, start_response):
headers = []
headers.append(('Content-Type', 'text/plain'))
write = start_response('200 OK', headers)
input = environ['wsgi.input']
output = cStringIO.StringIO()
print >> output, "PID: %s" % os.getpid()
print >> output
keys = environ.keys()
keys.sort()
for key in keys:
print >> output, '%s: %s' % (key, repr(environ[key]))
print >> output
output.write(input.read(int(environ.get('CONTENT_LENGTH', '0'))))
return [output.getvalue()]
In the output of that, the PID value should always be the same. The
wsgi.multiprocess flag should be False. The mod_wsgi.process_group value
should be what ever you called the daemon process group. And the
mod_wsgi.application_group should be an empty string.
If this isn't what you are seeing, ensure you actually restarted Apache after
making configuration changes. Also add:
LogLevel debug
to Apache configuration for VirtualHost. Doing that will cause mod_wsgi to log
a lot more messages in Apache error log about process creation and script
loading, including details of process group and application group things are
happening for.
For other information on debugging, see:
<http://code.google.com/p/modwsgi/wiki/DebuggingTechniques>
If still problems, suggest you go to the mod_wsgi mailing list on Google
Groups.
|
Using Regexp to replace math expression inside Latex File
Question: I am trying to replace characters inside a math environment with their
boldface versions. Unfortunately, these characters occur inside the rest of
the text, as well.
My text:
text text text text Gtext G G text ....
\begin{align}
f&=gG \\
G &= tG
\end{align}
text textG text $G$ text.
* * *
Every G inside \begin{align} \end{align} and between the dollar signs $G$
shall be replaced with
\mathbf{G}.
The others shall remain untouched.
I appreciate every idea :)
Thank you
BIG EDIT: So far, I have a working Program (Python), thanks to the advice and
some other findings in stackoverflow.
But the program replaces f.e **_\quad_** to **_\q"replace"ad_**. if I want to
replace all the **_"u"_** s with **_"replace"_**.
from tempfile import mkstemp
from shutil import move
from os import remove, close
import shutil
def replace(file, outputfile, pattern, subst, boundary1, boundary2):
#Create temp file
fh, abs_path = mkstemp()
newfile="tempfile.tmp"
new_file = open(newfile,'w')
old_file = open(file)
inAlign=False
for line in old_file:
if boundary1 in line:
inAlign = True
if inAlign:
print line
print line.replace(pattern, subst)
new_file.write(line.replace(pattern, subst))
else:
new_file.write(line)
if boundary2 in line:
inAlign = False;
#close temp file
new_file.close()
close(fh)
old_file.close()
shutil.move(newfile,outputfile)
replace("texfile.tex","texfile_copy.tex","G", "\\mathbf{G}", "\\begin{align}", "\\end{align}")
Hopefully I got the formatting right...
Answer: This will be hard-to-impossible with regexes alone. What language are you
using? It it's perl, there's a module LaTeX::TOM that will help you out a
great deal.
However, if you know that your `\begin` and `\end` tags are always on their
own line, the following pseudocode would work:
foreach (line in file)
if line.matches( /\\begin{align}/ )
inAlign = true
end
if inAlign
line.replace( /(G)/\\mathbf{$1}/ )
else
line.replace( /\$(G)\$/\$\\mathbf{$1}\$/ )
end
if line.matches( /\\end{align}/ )
inAlign = false;
end
end
|
Multiprocessing debug techniques
Question: I'm having trouble debugging a multi-process application (specifically using a
process pool in python's multiprocessing module). I have an apparent deadlock
and I do not know what is causing it. The stack trace is not sufficient to
describe the issue, as it only displays code in the multiprocessing module.
Are there any python tools, or otherwise general techniques used to debug
deadlocks?
Answer: Yah, debugging deadlocks is fun. You can set the logging level to be higher --
see [the Python
documentation](http://docs.python.org/library/multiprocessing.html) for a
description of it, but really quickly:
import multiprocessing, logging
logger = multiprocessing.log_to_stderr()
logger.setLevel(multiprocessing.SUBDEBUG)
Also, add logging for anything in your code that deals with a resource or
whatnot that might be in contention. Finally, shot in the dark: spawning off
child processes during an import might cause a problem.
|
How does this Python code work?
Question: I don't know if it is feasable to paste all of the code here but I am looking
at the code in [this git
repo](http://github.com/cloudkick/libcloud/tree/master).
If you look at the example they do:
ec2 = EC2('access key id', 'secret key')
...but there is no EC2 class. However, it looks like in libcloud\providers.py
there is a dict that maps the EC2 to the EC2NodeDriver found in
libcloud\drivers\ec2.py. The correct mapping is calculated by
get_driver(provider), but that method doesn't appear to be called anywhere.
I am new to python, obviously, but not to programming. I'm not even sure what
I should be looking up in the docs to figure this out. Thanks.
Answer: `example.py` includes an `import` statement that reads:
from libcloud.drivers import EC2, Slicehost, Rackspace
This means that the `EC2` class is imported from the `libcloud.drivers`
module. However, in this case, `libcloud.drivers` is actually a _package_ (a
Python _package_ contains _modules_), which means that `EC2` should be defined
in a file `__init__.py` in `libcloud/drivers/`, but it's not. Which means that
in this specific case, their example code is actually wrong. (I downloaded the
code and got an import error when running `example.py`, and as you can see,
the file `libcloud/drivers/__init__.py` does not contain any definitions at
all, least of all an `EC2` definition.)
|
Cubic root of the negative number on python
Question: Can someone help me to find a solution on how to calculate a cubic root of the
negative number using python?
>>> math.pow(-3, float(1)/3)
nan
it does not work. Cubic root of the negative number is negative number. Any
solutions?
Answer: A simple use of [De Moivre's
formula](http://en.wikipedia.org/wiki/De%5FMoivre%27s%5Fformula), is
sufficient to show that the cube root of a value, regardless of sign, is a
multi-valued function. That means, for any input value, there will be three
solutions. Most of the solutions presented to far only return the principle
root. A solution that returns all valid roots, and explicitly tests for non-
complex special cases, is shown below.
import numpy
import math
def cuberoot( z ):
z = complex(z)
x = z.real
y = z.imag
mag = abs(z)
arg = math.atan2(y,x)
return [ mag**(1./3) * numpy.exp( 1j*(arg+2*n*math.pi)/3 ) for n in range(1,4) ]
**Edit:** As requested, in cases where it is inappropriate to have dependency
on numpy, the following code does the same thing.
def cuberoot( z ):
z = complex(z)
x = z.real
y = z.imag
mag = abs(z)
arg = math.atan2(y,x)
resMag = mag**(1./3)
resArg = [ (arg+2*math.pi*n)/3. for n in range(1,4) ]
return [ resMag*(math.cos(a) + math.sin(a)*1j) for a in resArg ]
|
How can I impersonate the current user with IronPython?
Question: I am trying to manage an IIS7 installation remotely using the
Microsoft.Web.Administration library.
I'm doing this in IronPython:
import Microsoft.Web.Administration
from Microsoft.Web.Administration import ServerManager
manager = ServerManager.OpenRemote("RemoteServerName")
for site in manager.Sites:
print "Site: %(site)s" % { 'site' : site.Name }
On the last line as it attempts to communicate with the remote server I get
the following error:
> Retrieving the COM class factory for remote component with CLSID
> {2B72133B-3F5B-4602-8952-803546CE3344} from machine devdealernetsvr failed
> due to the following error: 80070005.
My research into the error lead me to believe that I do not have the proper
credentials against the remote machine and so I would like to impersonate a
user that does.
I was hard pressed to find a way to do this with IronPython. Any help is much
appreciated.
Answer: (this doesn't necessarily answer the question but it does fix the problem)
It turned out my application worked fine. My setup was the problem. :(
To fix my issue I needed to:
1. Log into the remote server.
2. In IIS enable remote management (it's just a check box)
3. In the Services snap-in, start the remote management service and set it to automatic
Thanks Anthony!
|
Python generating Python
Question: I have a group of objects which I am creating a class for that I want to store
each object as its own text file. I would really like to store it as a Python
class definition which subclasses the main class I am creating. So, I did some
poking around and found a Python Code Generator on effbot.org. I did some
experimenting with it and here's what I came up with:
#
# a Python code generator backend
#
# fredrik lundh, march 1998
#
# [email protected]
# http://www.pythonware.com
#
# Code taken from http://effbot.org/zone/python-code-generator.htm
import sys, string
class CodeGeneratorBackend:
def begin(self, tab="\t"):
self.code = []
self.tab = tab
self.level = 0
def end(self):
return string.join(self.code, "")
def write(self, string):
self.code.append(self.tab * self.level + string)
def indent(self):
self.level = self.level + 1
def dedent(self):
if self.level == 0:
raise SyntaxError, "internal error in code generator"
self.level = self.level - 1
class Point():
"""Defines a Point. Has x and y."""
def __init__(self, x, y):
self.x = x
self.y = y
def dump_self(self, filename):
self.c = CodeGeneratorBackend()
self.c.begin(tab=" ")
self.c.write("class {0}{1}Point()\n".format(self.x,self.y))
self.c.indent()
self.c.write('"""Defines a Point. Has x and y"""\n')
self.c.write('def __init__(self, x={0}, y={1}):\n'.format(self.x, self.y))
self.c.indent()
self.c.write('self.x = {0}\n'.format(self.x))
self.c.write('self.y = {0}\n'.format(self.y))
self.c.dedent()
self.c.dedent()
f = open(filename,'w')
f.write(self.c.end())
f.close()
if __name__ == "__main__":
p = Point(3,4)
p.dump_self('demo.py')
That feels really ugly, is there a cleaner/better/more pythonic way to do
this? Please note, this is not the class I actually intend to do this with,
this is a small class I can easily mock up in not too many lines. Also, the
subclasses don't need to have the generating function in them, if I need that
again, I can just call the code generator from the superclass.
Answer: We use Jinja2 to fill in a template. It's much simpler.
The template looks a lot like Python code with a few `{{something}}`
replacements in it.
|
How to count possibilities in python lists
Question: Given a list like this:
num = [1, 2, 3, 4, 5]
There are 10 three-element combinations:
[123, 124, 125, 134, 135, 145, 234, 235, 245, 345]
How can I generate this list?
Answer: Use
[itertools.combinations](http://docs.python.org/library/itertools.html#itertools.combinations):
import itertools
num = [1, 2, 3, 4, 5]
combinations = []
for combination in itertools.combinations(num, 3):
combinations.append(int("".join(str(i) for i in combination)))
# => [123, 124, 125, 134, 135, 145, 234, 235, 245, 345]
print len(combinations)
# => 10
**Edit**
You can skip int(), join(), and str() if you are only interested in the number
of combinations. itertools.combinations() gives you tuples that may be good
enough.
|
File open: Is this bad Python style?
Question: To read contents of a file:
data = open(filename, "r").read()
The open file immediately stops being referenced anywhere, so the file object
will eventually close... and it shouldn't affect other programs using it,
since the file is only open for reading, not writing.
EDIT: This has actually bitten me in a project I wrote - it prompted me to ask
[this](http://stackoverflow.com/questions/2023608/check-what-files-are-open-
in-python/2023791#2023791) question. File objects are cleaned up only when you
run out of memory, not when you run out of file handles. So if you do this too
often, you could end up running out of file descriptors and causing your IO
attempts at opening files to throw exceptions.
Answer: Just for the record: This is only slightly longer, and closes the file
immediately:
from __future__ import with_statement
with open(filename, "r") as f:
data = f.read()
|
How to extract a string between 2 other strings in python?
Question: Like if I have a string like `str1 = "IWantToMasterPython"`
If I want to extract `"Py"` from the above string. I write:
extractedString = foo("Master","thon")
I want to do all this because i am trying to extract lyrics from an html page.
The lyrics are written like `<div class = "lyricbox"> ....lyrics goes
here....</div>`.
Any suggestions on how can I implement.
Answer: The solution is to use a regexp:
import re
r = re.compile('Master(.*?)thon')
m = r.search(str1)
if m:
lyrics = m.group(1)
|
Code organization in Python: Where is a good place to put obscure methods?
Question: I have a class called `Path` for which there are defined about 10 methods, in
a dedicated module `Path.py`. Recently I had a need to write 5 more methods
for `Path`, however these new methods are quite obscure and technical and 90%
of the time they are irrelevant.
Where would be a good place to put them so their context is clear? Of course I
can just put them with the class definition, but I don't like that because I
like to keep the important things separate from the obscure things.
Currently I have these methods as functions that are defined in a separate
module, just to keep things separate, but it would be better to have them as
bound methods. (Currently they take the `Path` instance as an explicit
parameter.)
Does anyone have a suggestion?
Answer: If the method is relevant to the Path - no matter how obscure - I think it
should reside within the class itself.
If you have multiple places where you have path-related functionality, it
leads to problems. For example, if you want to check if some functionality
already exists, how will a new programmer know to check the other, less
obvious places?
I think a good practice might be to order functions by importance. As you may
have heard, some suggest putting public members of classes first, and
private/protected ones after. You could consider putting the common methods in
your class higher than the obscure ones.
|
Serving file download with python
Question: Hey gang, I'm trying to convert a legacy php script over to python and not
having much luck.
The intent of the script is to serve up a file while concealing it's origin.
Here's what's working in php:
<?php
$filepath = "foo.mp3";
$filesize = filesize($filepath);
header("Pragma: no-cache");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
// force download dialog
//header("Content-Type: application/force-download");
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header('Content-Disposition: attachment;filename="'.$filepath.'"');
header("Content-Transfer-Encoding: binary");
#header('Content-Type: audio/mpeg3');
header('Content-Length: '.$filesize);
@readfile($filepath);
exit(0);
?>
When I do the equivilent in Python, I get a download that is zero bytes.
Here's what I'm trying:
#!/usr/bin/env python
# encoding: utf-8
import sys
import os
import cgitb; cgitb.enable()
filepath = "foo.mp3"
filesize = os.path.getsize(filepath)
print "Prama: no-cache"
print "Expires: 0"
print "Cache-Control: must-revalidate, post-check=0, pre-check=0"
print "Content-Type: application/octet-stream"
print "Content-Type: application/download"
print 'Content-Disposition: attachment;filename="'+filepath+'"'
print "Content-Transfer-Encoding: binary"
print 'Content-Length: '+str(filesize)
print #required blank line
open(filepath,"rb").read()
Can anyone please help me?
Answer: Well, maybe it's just me missing something, but... You are actually not
writing the contents of the file to stdout. You are just reading it into
memory, so it will never show up on the other side of the TCP connection...
Try:
sys.stdout.write(open(filepath,"rb").read())
sys.stdout.flush()
Depending on the file size, it might be better to read the file in chunks,
like so:
chunk_size = 4096
handle = open(filepath, "rb")
while True:
buffer = handle.read(chunk_size)
if buffer:
sys.stdout.write(buffer)
else:
break
Another thing to be aware of: writing binary data to stdout may cause Python
to choke due to encoding issues. This depends on the Python version you are
using.
|
PIL: Image resizing : Algorithm similar to firefox's
Question: I'm getting about the same _bad looking_ resizing from all the 4 algorithms of
PIL
>>> data = utils.fetch("http://wavestock.com/images/beta-icon.gif")
>>> image = Image.open(StringIO.StringIO(data)); image.save("/home/ptarjan/www/tmp/metaward/original.png")
>>>
>>> image = Image.open(StringIO.StringIO(data)); image.resize((36,36), Image.ANTIALIAS).save("/home/ptarjan/www/tmp/metaward/antialias.png")
>>> image = Image.open(StringIO.StringIO(data)); image.resize((36,36), Image.BILINEAR).save("/home/ptarjan/www/tmp/metaward/bilinear.png")
>>> image = Image.open(StringIO.StringIO(data)); image.resize((36,36), Image.BICUBIC).save("/home/ptarjan/www/tmp/metaward/bicubic.png")
>>> image = Image.open(StringIO.StringIO(data)); image.resize((36,36), Image.NEAREST).save("/home/ptarjan/www/tmp/metaward/nearest.png")
>>>
>>> image = Image.open(StringIO.StringIO(data)); image.thumbnail((36,36), Image.ANTIALIAS); image.save("/home/ptarjan/www/tmp/metaward/antialias-thumb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.thumbnail((36,36), Image.BILINEAR); image.save("/home/ptarjan/www/tmp/metaward/bilinear-thumb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.thumbnail((36,36), Image.BICUBIC); image.save("/home/ptarjan/www/tmp/metaward/bicubic-thumb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.thumbnail((36,36), Image.NEAREST); image.save("/home/ptarjan/www/tmp/metaward/nearest-thumb.png")
>>>
>>> image = Image.open(StringIO.StringIO(data)); image.convert("RGB").resize((36,36), Image.ANTIALIAS).save("/home/ptarjan/www/tmp/metaward/antialias-rgb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.convert("RGB").resize((36,36), Image.BILINEAR).save("/home/ptarjan/www/tmp/metaward/bilinear-rgb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.convert("RGB").resize((36,36), Image.BICUBIC).save("/home/ptarjan/www/tmp/metaward/bicubic-rgb.png")
>>> image = Image.open(StringIO.StringIO(data)); image.convert("RGB").resize((36,36), Image.NEAREST).save("/home/ptarjan/www/tmp/metaward/nearest-rgb.png")
But the results look much worse that just resizing in firefox.
<http://paulisageek.com/tmp/metaward/images.html>
How can I get a similar effect to the firefox result using PIL (or another
python image library)?
**EDIT** : Hover your mouse to see what each image is
It looks like the RGB and then ANTIALIS looks the best. Any other
recommendations?
For reference, this is the one that looked the best :
>>> image = Image.open(StringIO.StringIO(data));
>>> image.convert("RGB").resize((36,36), Image.ANTIALIAS)
Answer: I resized the "original" with Python and found the same results as you did. I
also resized the "original" with GIMP and I got the same (if not inferior)
quality. This made me suspect that Firefox cheats. Possibly it converts to RGB
("original" mode is indexed color). Thus the following code:
import Image
im=Image.open("beta-icon.gif")
im = im.convert("RGB")
im=im.resize((36,36), Image.ANTIALIAS)
im.save("q5.png")
The result is almost as good as that of Firefox.
|
Python: Automatically initialize instance variables?
Question: I have a python class that looks like this:
class Process:
def __init__(self, PID, PPID, cmd, FDs, reachable, user):
followed by:
self.PID=PID
self.PPID=PPID
self.cmd=cmd
...
Is there any way to autoinitialize these instance variables, like C++'s
initialization list? It would spare lots of redundant code.
Answer: **Edit: extended the solution to honor default arguments also**
Here is the complete solution:
from functools import wraps
import inspect
def initializer(func):
"""
Automatically assigns the parameters.
>>> class process:
... @initializer
... def __init__(self, cmd, reachable=False, user='root'):
... pass
>>> p = process('halt', True)
>>> p.cmd, p.reachable, p.user
('halt', True, 'root')
"""
names, varargs, keywords, defaults = inspect.getargspec(func)
@wraps(func)
def wrapper(self, *args, **kargs):
for name, arg in list(zip(names[1:], args)) + list(kargs.items()):
setattr(self, name, arg)
for name, default in zip(reversed(names), reversed(defaults)):
if not hasattr(self, name):
setattr(self, name, default)
func(self, *args, **kargs)
return wrapper
* * *
**Edit: Adam asked me to extend the solution to support keyword arguments**
from functools import wraps
import inspect
def initializer(fun):
names, varargs, keywords, defaults = inspect.getargspec(fun)
@wraps(fun)
def wrapper(self, *args, **kargs):
for name, arg in zip(names[1:], args) + kargs.items():
setattr(self, name, arg)
fun(self, *args, **kargs)
return wrapper
* * *
You can use a decorator:
from functools import wraps
import inspect
def initializer(fun):
names, varargs, keywords, defaults = inspect.getargspec(fun)
@wraps(fun)
def wrapper(self, *args):
for name, arg in zip(names[1:], args):
setattr(self, name, arg)
fun(self, *args)
return wrapper
class process:
@initializer
def __init__(self, PID, PPID, cmd, FDs, reachable, user):
pass
Output:
>>> c = process(1, 2, 3, 4, 5, 6)
>>> c.PID
1
>>> dir(c)
['FDs', 'PID', 'PPID', '__doc__', '__init__', '__module__', 'cmd', 'reachable', 'user'
|
Application Structure for GUI & Functions
Question: I'm starting a basic application using Python and PyQt and could use some
experienced insight. Here's the structure I was thinking. This is
understandably subjective, but is there a better way?
myApp/GUI/__init__.py
mainWindow.py
subWindow1.py
subWindow2.py
myApp/Logic/__init__.py
setOfMethods1.py
setOfMethods2.py
mainWindow imports subWindows
mainWindow imports Logic module
Answer: ## MVC
It looks like you have been reading about model-view-controller.
Separating the UI from the back end is a good idea. It will make runnings
tests and debugging just the logic side easier, and the internal structure
will be more modular.
I'm not certain it makes as much sense to split the UI into the currently
anticipated windows, though. I might just let the UI part grow and factor for
common code.
|
apt like column output - python library
Question: Debian's apt tool outputs results in uniform width columns. For instance, try
running "aptitude search svn" .. and all names appear in the first column of
the same width.
Now if you resize the terminal, the column width is adjusted accordingly.
Is there a Python library that enables one to do this? Note that the library
has to be aware of the terminal width and take a table as input - which could
be, for instance, `[('rapidsvn', 'A GUI client for subversion'), ...]` .. and
you may also specify a max-width for the first column (or any column). Also
note how the string in the second column below is trimmed if exceeds the
terminal width .. thus not introducing the undesired second line.
$ aptitude search svn
[...]
p python-svn-dbg - A(nother) Python interface to Subversion (d
v python2.5-svn -
v python2.6-svn -
p rapidsvn - A GUI client for subversion
p statsvn - SVN repository statistics
p svn-arch-mirror - one-way mirroring from Subversion to Arch r
p svn-autoreleasedeb - Automatically release/upload debian package
p svn-buildpackage - helper programs to maintain Debian packages
p svn-load - An enhanced import facility for Subversion
p svn-workbench - A Workbench for Subversion
p svnmailer - extensible Subversion commit notification t
p websvn - interface for subversion repositories writt
$
**EDIT** : (in response to Alex's answer below) ... the output will be similar
to 'aptitude search' in that 1) only the last column (which is the only column
with the longest string in a row) is to be trimmed, 2) there are typically 2-4
columns only, but the last column ("description") is expected to take at least
half the terminal width. 3) all rows contain equal number of columns, 4) all
entries are strings only
Answer: **Update** : The `colprint` routine is now available in the
[applib](http://code.activestate.com/pypm/applib/) Python library [hosted in
GitHub](https://github.com/ActiveState/applib/blob/master/applib/textui.py#L213).
Here's the complete program for those of you interested:
# This function was written by Alex Martelli
# http://stackoverflow.com/questions/1396820/
def colprint(table, totwidth=None):
"""Print the table in terminal taking care of wrapping/alignment
- `table`: A table of strings. Elements must not be `None`
- `totwidth`: If None, console width is used
"""
if not table: return
if totwidth is None:
totwidth = find_console_width()
totwidth -= 1 # for not printing an extra empty line on windows
numcols = max(len(row) for row in table)
# ensure all rows have >= numcols columns, maybe empty
padded = [row+numcols*('',) for row in table]
# compute col widths, including separating space (except for last one)
widths = [ 1 + max(len(x) for x in column) for column in zip(*padded)]
widths[-1] -= 1
# drop or truncate columns from the right in order to fit
while sum(widths) > totwidth:
mustlose = sum(widths) - totwidth
if widths[-1] <= mustlose:
del widths[-1]
else:
widths[-1] -= mustlose
break
# and finally, the output phase!
for row in padded:
print(''.join([u'%*s' % (-w, i[:w])
for w, i in zip(widths, row)]))
def find_console_width():
if sys.platform.startswith('win'):
return _find_windows_console_width()
else:
return _find_unix_console_width()
def _find_unix_console_width():
"""Return the width of the Unix terminal
If `stdout` is not a real terminal, return the default value (80)
"""
import termios, fcntl, struct, sys
# fcntl.ioctl will fail if stdout is not a tty
if not sys.stdout.isatty():
return 80
s = struct.pack("HHHH", 0, 0, 0, 0)
fd_stdout = sys.stdout.fileno()
size = fcntl.ioctl(fd_stdout, termios.TIOCGWINSZ, s)
height, width = struct.unpack("HHHH", size)[:2]
return width
def _find_windows_console_width():
"""Return the width of the Windows console
If the width cannot be determined, return the default value (80)
"""
# http://code.activestate.com/recipes/440694/
from ctypes import windll, create_string_buffer
STDIN, STDOUT, STDERR = -10, -11, -12
h = windll.kernel32.GetStdHandle(STDERR)
csbi = create_string_buffer(22)
res = windll.kernel32.GetConsoleScreenBufferInfo(h, csbi)
if res:
import struct
(bufx, bufy, curx, cury, wattr,
left, top, right, bottom,
maxx, maxy) = struct.unpack("hhhhHhhhhhh", csbi.raw)
sizex = right - left + 1
sizey = bottom - top + 1
else:
sizex, sizey = 80, 25
return sizex
|
Downloading file using IE from python
Question: I'm trying to download file with Python using IE:
from win32com.client import DispatchWithEvents
class EventHandler(object):
def OnDownloadBegin(self):
pass
ie = DispatchWithEvents("InternetExplorer.Application", EventHandler)
ie.Visible = 0
ie.Navigate('http://website/file.xml')
After this, I'm getting a window asking the user where to save the file. How
can I save this file automatically from python?
I need to **use some browser** , not urllib or mechanize, because **before
downloading file I need to interact with some ajax functionality**.
Answer: This works for me as long as the IE dialogs are in the foreground and the
downloaded file does not already exist in the "Save As" directory:
import time
import threading
import win32ui, win32gui, win32com, pythoncom, win32con
from win32com.client import Dispatch
class IeThread(threading.Thread):
def run(self):
pythoncom.CoInitialize()
ie = Dispatch("InternetExplorer.Application")
ie.Visible = 0
ie.Navigate('http://website/file.xml')
def PushButton(handle, label):
if win32gui.GetWindowText(handle) == label:
win32gui.SendMessage(handle, win32con.BM_CLICK, None, None)
return True
IeThread().start()
time.sleep(3) # wait until IE is started
wnd = win32ui.GetForegroundWindow()
if wnd.GetWindowText() == "File Download - Security Warning":
win32gui.EnumChildWindows(wnd.GetSafeHwnd(), PushButton, "&Save");
time.sleep(1)
wnd = win32ui.GetForegroundWindow()
if wnd.GetWindowText() == "Save As":
win32gui.EnumChildWindows(wnd.GetSafeHwnd(), PushButton, "&Save");
|
Python: copy.deepcopy produces an error
Question: I have been using this copy method for quite a while, in lots of classes that
needed it.
class population (list):
def __init__ (self):
pass
def copy(self):
return copy.deepcopy(self)
It has suddenly started producing this error:
File "C:\Python26\lib\copy.py", line 338, in _reconstruct
state = deepcopy(state, memo)
File "C:\Python26\lib\copy.py", line 162, in deepcopy
y = copier(x, memo)
File "C:\Python26\lib\copy.py", line 255, in _deepcopy_dict
y[deepcopy(key, memo)] = deepcopy(value, memo)
File "C:\Python26\lib\copy.py", line 189, in deepcopy
y = _reconstruct(x, rv, 1, memo)
File "C:\Python26\lib\copy.py", line 323, in _reconstruct
y = callable(*args)
File "C:\Python26\lib\copy_reg.py", line 93, in __newobj__
return cls.__new__(cls, *args)
TypeError: object.__new__(generator) is not safe, use generator.__new__()
>>>
the lines which include the references to lines 338, 162, 255, 189 were
repeated quite a few times before the 'line 338' that I copied here.
Answer: Are you cloning a generator? [Generators can't be
cloned.](https://mail.python.org/pipermail/python-list/2009-March/539759.html)
Copying answer by Gabriel Genellina here:
* * *
There is no way of "cloning" a generator:
py> g = (i for i in [1,2,3])
py> type(g)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'generator' instances
py> g.gi_code = code
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: readonly attribute
py> import copy
py> copy.copy(g)
Traceback (most recent call last):
...
TypeError: object.__new__(generator) is not safe, use generator.__new__()
py> type(g).__new__
<built-in method __new__ of type object at 0x1E1CA560>
You can do that with a generator function because it acts as a "generator
factory", building a new generator when called. Even using the Python C
API, to create a generator one needs a frame object -- and there is no way
to create a frame object "on the fly" that I know of :(
py> import ctypes
py> PyGen_New = ctypes.pythonapi.PyGen_New
py> PyGen_New.argtypes = [ctypes.py_object]
py> PyGen_New.restype = ctypes.py_object
py> g = (i for i in [1,2,3])
py> g2 = PyGen_New(g.gi_frame)
py> g2.gi_code is g.gi_code
True
py> g2.gi_frame is g.gi_frame
True
py> g.next()
1
py> g2.next()
2
g and g2 share the same execution frame, so they're not independent. There
is no easy way to create a new frame in Python:
py> type(g.gi_frame)()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: cannot create 'frame' instances
One could try using PyFrame_New -- but that's way too magic for my taste...
|
Python decorate a class to change parent object type
Question: Suppose you have two classes X & Y. You want to decorate those classes by
adding attributes to the class to produce new classes X1 and Y1.
For example:
class X1(X):
new_attribute = 'something'
class Y1(Y):
new_attribute = 'something'
*new_attribute* will always be the same for both X1 and Y1. X & Y are not related in any meaningful way, except that multiple inheritance is not possible. There are a set of other attributes as well, but this is degenerate to illustrate.
I feel like I'm overcomplicating this, but I had thought to use a decorator,
somewhat likeso:
def _xywrap(cls):
class _xy(cls):
new_attribute = 'something'
return _xy
@_xywrap(X)
class X1():
pass
@_xywrap(Y)
class Y1():
pass
It feels like I'm missing a fairly common pattern, and I'd be much obliged for
thoughts, input and feedback.
Thank you for reading.
Brian
**EDIT:** Example:
Here is a relevant extract that may illuminate. The common classes are as
follows:
from google.appengine.ext import db
# I'm including PermittedUserProperty because it may have pertinent side-effects
# (albeit unlikely), which is documented here: [How can you limit access to a
# GAE instance to the current user][1].
class _AccessBase:
users_permitted = PermittedUserProperty()
owner = db.ReferenceProperty(User)
class AccessModel(db.Model, _AccessBase):
pass
class AccessExpando(db.Expando, _AccessBase):
pass
# the order of _AccessBase/db.* doesn't seem to resolve the issue
class AccessPolyModel(_AccessBase, polymodel.PolyModel):
pass
Here's a sub-document:
class Thing(AccessExpando):
it = db.StringProperty()
Sometimes Thing will have the following properties:
Thing { it: ... }
And other times:
Thing { it: ..., users_permitted:..., owner:... }
I've been unable to figure out why Thing would sometimes have its
_AccessParent properties, and other times not.
Answer: Use 3-arguments [type](http://docs.python.org/library/functions.html?#type):
def makeSomeNicelyDecoratedSubclass(someclass):
return type('MyNiceName', (someclass,), {'new_attribute':'something'})
This is indeed, as you surmised, a reasonably popular idiom.
**Edit** : in the general case if someclass has a custom metaclass you may
need to extract and use it (with a 1-argument `type`) in lieu of `type`
itself, to preserve it (this may be the case for your Django and App Engine
models):
def makeSomeNicelyDecoratedSubclass(someclass):
mcl = type(someclass)
return mcl('MyNiceName', (someclass,), {'new_attribute':'something'})
This also works where the simpler version above does (since in simple cases
w/no custom metaclasses `type(someclass) is type`).
|
Showing an image from console in Python
Question: What is the easiest way to show a `.jpg` or `.gif` image from Python console?
I've got a Python console program that is checking a data set which contains
links to images stored locally. How should I write the script so that it would
display images pop-up graphical windows?
Answer: Using the awesome [Pillow](https://pypi.python.org/pypi/Pillow/2.7.0) library:
>>> from PIL import Image
>>> img = Image.open('test.png')
>>> img.show()
This will open the image in your default image viewer.
|
To Ruby or not to Ruby
Question: I know that this is a difficult question to answer, but I thought I would try
anwyays....
I am just starting at a new company where they have a minimal existing code
base. It probably has a month of man effort invested at this point. It is
currently written in Ruby.
It is also currently using Ruby on Rails -- but mostly just for the sake of
testing out the Ruby code.
The ultimate goal of the code is actually to drive a backend to a site that
will be written in php (could be a backend to Drupal, Echo, etc...).
I have no experience with Ruby, so I would tend to want to go with a language
I know better (like Python), but am not willing to rule Ruby out for no
reason.
If you are not going to use Ruby for a Rails project, is it still worth it?
Will I be better off going with Python or some other language?
How do the libraries stack up?
Thanks!!!
Answer: My advice would depend on your own goals, which might look like this... you
might want to ask yourself (or score each of these from 1-10) if you prefer
to:
1. learn a new language you might use in future? = Ruby
2. deepen your Python skills by using it for everything (say [Django](http://www.djangoproject.com/) or [Web.Py](http://webpy.org/)) = Python
3. move the [Ruby testing away from Rails](http://ruby-toolbox.com/categories/testing%5Fframeworks.html) = Ruby
Other questions you could ask yourself to help the decision might be:
1. is speed important? Do some tests in the various languages. (If Ruby, then use [Ruby 1.9](http://rads.stackoverflow.com/amzn/click/0596516177) and get [the other Ruby book](http://rads.stackoverflow.com/amzn/click/0672328844)).
2. is integration important? If so, why use a PHP front end?
3. is your connection to the language community important? If so, choose on 'community feel'.
4. is there a lot of backend text processing? (Perl?)
5. do you want to use an ORM or write SQL? = look at Ruby and Python [lightweight frameworks](http://www.linux-mag.com/id/7324).
I don't think the libraries will be an issue, since (I'm pretty sure that)
libraries for popular languages cover all common tasks.
If you can score all the above from 1-10 it may help isolate a preferred
direction...
Then, as I see it, the issue breaks down into 3 things:
1. what language do you most like to code in (work should be enjoyable)?
2. can the front end and back end be generated in a single language?
3. do you want to use a framework or a ready-made CMS for the front end?
It's worth looking at the origins of languages: [PHP was originally
announced](http://groups.google.ch/group/comp.infosystems.www.authoring.cgi/msg/cc7d43454d64d133?oe=UTF-8&output=gplain)
as an extension of [SSI](http://vr-zone.com/manual/en/howto/ssi.html), Ruby
tries to take the best of Perl, Smalltalk and Lisp but has elements of a
C/Java-like syntax, Perl is intimately connected to Unix and everywhere,
although usually invisible to end users (despite some [very good Perl web
frameworks](http://mark.stosberg.com/blog/2008/12/titanium-a-new-release-and-
more.html)). You already know about Python.
As for frameworks and CMSs, a trawl through the
distinctions/limitations/features might also help. It is too easy to install a
PHP CMS (fine for a site with a well-defined purpose) but then find yourself
hampered in acres of impenetrable code when you want do do something it can't
do out of the box. A framework in the backend language will enable you to hook
the back and front ends together more easily.
|
How to extract and then refer to variables defined in a python module?
Question: I'm trying to build a simple environment check script for my firm's test
environment. My goal is to be able to ping each of the hosts defined for a
given test environment instance. The hosts are defined in a file like this:
#!/usr/bin/env python
host_ip = '192.168.100.10'
router_ip = '192.168.100.254'
fs_ip = '192.168.200.10'
How can I obtain all of these values in a way that is iterable (i.e. I need to
loop through and ping each ip address)?
I've looked at local() and vars(), but trying do something like this:
for key, value in vars():
print key, value
generates this error:
ValueError: too many values to unpack
I have been able to extract the names of all variables by checking
`dir(local_variables)` for values that don't contain a '__' string, but then I
have a list of strings, and I can't figure out how to get from the string to
the value of the same-named variable.
Answer: First off, I strongly recommend not doing it that way. Instead, do:
hosts = {
"host_ip": '192.168.100.10',
"router_ip": '192.168.100.254',
"fs_ip": '192.168.200.10',
}
Then you can simply import the module and reference it normally--this gives an
ordinary, standard way to access this data from any Python code:
import config
for host, ip in config.hosts.iteritems():
...
If you do access variables directly, you're going to get a bunch of stuff you
don't want: the builtins (`__builtins__`, `__package__`, etc); anything that
was imported while setting up the other variables, etc.
You'll also want to make sure that the context you're running in is different
from the one whose variables you're iterating over, or you'll be creating new
variables in locals() (or vars(), or globals()) while you're iterating over
it, and you'll get `"RuntimeError: dictionary changed size during iteration"`.
|
Just installed QtOpenGL but cannot import it (from Python)
Question: I just installed it with apt-get on debian linux with
apt-get install libqt4-opengl
the rest of PyQt4 is available, but I cant get to this new module.
from PyQt4 import QtOpenGL
raises ImportError. any idea what to do?
Answer: Did you forget to install the Python bindings?
apt-get install python-qt4-gl
|
Accessing a Python variable in a list
Question: I think this is probably something really simple, but I'd appreciate a hint:
I am using a python list to hold some some database insert statements:
list = [ "table_to_insert_to" ],["column1","column2"],[getValue.value1],["value2"]]
The problem is one of the values isn't evaluated until runtime-- so before the
page even gets run, it breaks when it tries to import the function.
How do you handle this?
Answer: You've just pointed out one (out of a zillion) problems with global variables:
not using global variables is the best solution to this problem and many
others. If you still mistakenly believe you must use a global variable, put a
placeholder (e.g. `None`) in the place where the value you don't yet know will
go, and assign the right value there when it's finally known.
|
How to install MySQLdb package? (ImportError: No module named setuptools)
Question: I am trying to install MySQLdb package. I found the source code
[here](https://sourceforge.net/projects/mysql-python/files/).
I did the following:
gunzip MySQL-python-1.2.3c1.tar.gz
tar xvf MySQL-python-1.2.3c1.tar
cd MySQL-python-1.2.3c1
python setup.py build
As the result I got the following:
Traceback (most recent call last):
File "setup.py", line 5, in ?
from setuptools import setup, Extension
ImportError: No module named setuptools
Does anybody knows how to solve this problem? By the way, if I am able to do
the described step, I will need to do the following:
sudo python setup.py install
And I have no system-administrator-rights. Do I still have a chance to install
MySQLdb?
Thank you.
Answer: After trying many suggestions, simply using `sudo apt-get install python-
mysqldb` worked for me.
More info: [Getting "Error loading MySQLdb module: No module named MySQLdb" \-
have tried previously posted
solutions](http://stackoverflow.com/questions/2952187/getting-error-loading-
mysqldb-module-no-module-named-mysqldb-have-tried-pre)
|
How to set ignorecase flag for part of regular expression in Python?
Question: Is it possible to implement in Python something like this simple one:
#!/usr/bin/perl
my $a = 'Use HELLO1 code';
if($a =~ /(?i:use)\s+([A-Z0-9]+)\s+(?i:code)/){
print "$1\n";
}
Letters of token in the middle of string are always capital. Letters of the
rest of words can have any case (USE, use, Use, CODE, code, Code and so on)
Answer: As far as I could find, the python regular expression engine does not support
partial ignore-case. Here is a solution using a case-insensitive regular
expression, which then tests if the token is uppercase afterward.
#! /usr/bin/env python
import re
token_re = re.compile(r'use\s+([a-z0-9]+)\s+code', re.IGNORECASE)
def find_token(s):
m = token_re.search(s)
if m is not None:
token = m.group(1)
if token.isupper():
return token
if __name__ == '__main__':
for s in ['Use HELLO1 code',
'USE hello1 CODE',
'this does not match',
]:
print s, '->',
print find_token(s)
Here is the program's output:
Use HELLO1 code -> HELLO1
USE hello1 CODE -> None
this does not match -> None
|
Python Eval: What's wrong with this code?
Question: I'm trying to write a very simple Python utility for personal use that counts
the number of lines in a text file for which a predicate specified at the
command line is true. Here's the code:
import sys
pred = sys.argv[2]
if sys.argv[1] == "stdin" :
handle = sys.stdin
else :
handle = open(sys.argv[1])
result = 0
for line in handle :
eval('result += 1 if ' + pred + ' else 0')
print result
When I run it using `python count.py myFile.txt "int(line) == 0"`, I get the
following error:
File "c:/pycode/count.py", line 10, in <module>
eval('toAdd = 1 if ' + pred + ' else 0')
File "<string>", line 1
toAdd = 1 if int(line) == 0 else 0
This looks like perfectly valid Python code to me (though I've never used
Python's eval before, so I don't know what its quirks, if any, are). Please
tell me how I can fix this to make it work.
Answer: Try using exec instead of eval. The difference between the 2 is explained
[here](http://www.ibiblio.org/g2swap/byteofpython/read/exec-statement.html)
|
Can Python encode a string to match ASP.NET membership provider's EncodePassword
Question: I'm working on a Python script to create hashed strings from an existing
system similar to that of ASP.NET's MembershipProvider. Using Python, is there
a way to take a hexadecimal string and convert it back to a binary and then do
a base64 encoding, somehow treating the original string as Unicode. Let's try
some code. I'm looking to re-encode a hashed password so that the hashes would
be equal in Python and ASP.NET/C#:
import base64
import sha
import binascii
def EncodePassword(password):
# strings are currently stored as hex
hex_hashed_password = sha.sha(password).hexdigest()
# attempt to convert hex to base64
bin_hashed_password = binascii.unhexlify(hex_hashed_password)
return base64.standard_b64encode(bin_hashed_password)
print EncodePassword("password")
# W6ph5Mm5Pz8GgiULbPgzG37mj9g=
The ASP.NET MembershipProvider users this method to encode:
static string EncodePassword(string pass)
{
byte[] bytes = Encoding.Unicode.GetBytes(pass);
//bytes = Encoding.ASCII.GetBytes(pass);
byte[] inArray = null;
HashAlgorithm algorithm = HashAlgorithm.Create("SHA1");
inArray = algorithm.ComputeHash(bytes);
return Convert.ToBase64String(inArray);
}
string s = EncodePassword("password");
// 6Pl/upEE0epQR5SObftn+s2fW3M=
That doesn't match. But, when I run it with the password encoded with ASCII
encoding, it matches, so the Unicode part of the .NET method is what's the
difference.
> W6ph5Mm5Pz8GgiULbPgzG37mj9g=
Is there a way in the python script to get an output to match the default .NET
version?
Answer: This is the trick:
> Encoding.Unicode
“Unicode” encoding is confusing Microsoft-speak for UTF-16LE (specifically,
without any BOM). Encode the string to that before hashing and you get the
right answer:
>>> import hashlib
>>> p= u'password'
>>> hashlib.sha1(p.encode('utf-16le')).digest().encode('base64')
'6Pl/upEE0epQR5SObftn+s2fW3M=\n'
|
Python serializable objects json
Question:
class gpagelet:
"""
Holds 1) the pagelet xpath, which is a string
2) the list of pagelet shingles, list
"""
def __init__(self, parent):
if not isinstance( parent, gwebpage):
raise Exception("Parent must be an instance of gwebpage")
self.parent = parent # This must be a gwebpage instance
self.xpath = None # String
self.visibleShingles = [] # list of tuples
self.invisibleShingles = [] # list of tuples
self.urls = [] # list of string
class gwebpage:
"""
Holds all the datastructure after the results have been parsed
holds: 1) lists of gpagelets
2) loc, string, location of the file that represents it
"""
def __init__(self, url):
self.url = url # Str
self.netloc = False # Str
self.gpagelets = [] # gpagelets instance
self.page_key = "" # str
Is there a way for me to make my class json serializable? The thing that I am
worried is the recursive reference.
Answer: Write your own encoder and decoder, which can be very simple like `return
__dict__`
e.g. here is a encoder to dump totally recursive tree structure, you can
enhance it or use as it is for your own purpose
import json
class Tree(object):
def __init__(self, name, childTrees=None):
self.name = name
if childTrees is None:
childTrees = []
self.childTrees = childTrees
class MyEncoder(json.JSONEncoder):
def default(self, obj):
if not isinstance(obj, Tree):
return super(MyEncoder, self).default(obj)
return obj.__dict__
c1 = Tree("c1")
c2 = Tree("c2")
t = Tree("t",[c1,c2])
print json.dumps(t, cls=MyEncoder)
it prints
{"childTrees": [{"childTrees": [], "name": "c1"}, {"childTrees": [], "name": "c2"}], "name": "t"}
you can similarly write a decoder but there you will somehow need to identify
is it is your object or not, so may be you can put a type too if needed.
|
How can I protect myself from a zip bomb?
Question: I just read about [zip bombs](http://en.wikipedia.org/wiki/Zip%5Fbomb), i.e.
zip files that contain very large amount of highly compressible data
(00000000000000000...).
When opened they fill the server's disk.
How can I detect a zip file is a zip bomb **before** unzipping it?
**UPDATE** Can you tell me how is this done in Python or Java?
Answer: Try this in Python:
import zipfile
z = zipfile.ZipFile('c:/a_zip_file')
print 'total files size=', sum(e.file_size for e in z.infolist())
z.close()
|
How do I get this program to start over in python?
Question: I believe the word is "recurse" instead of 'start over.' I've created this
program to hone my multiplication skills in the morning. I can get it to give
me a multiplication problem, but how do I get it to ask me another one?
from random import randint
print 'Good Morning Pete!'
X = randint(0, 10)
Y = randint(0, 10)
A = X * Y
Z = int(raw_input('%i * %i = ? ' % (X, Y)))
count = 0
if Z == A:
count += 1
print 'Good Job!'
else:
print 'Sorry!'
if count == '10':
print 'Time to kill \'em'
how do I get it to spit out a new problem for me to solve? I'm a beginner.
Thanks all!
Answer: Pete, you wouldn't need _recursion_ in this case, but merely a _loop_.
I suggest you put the bulk of the logic of this program (the part that asks
the multiplcation problem and check your answer), into a function. Say One
Problem(). This function could return 0 if you answered wrong, 1 if you
answered correctly and -1 if you entered some key indicating that you want to
stop. (BTW, this function is introduced to help you structure the program,
make it more readeable but it is not needed for introducing a loop. you could
well keep all this stuff inside the loop. Also, you should know that there are
other loop constructs in python, for exampe while loops.)
Then you'd just need in your main section something like that :
GoodReplyCtr = 0
for i in range(0, 10): # or 100 or 1000 if you feel ambitious...
cc = OneProblem()
if cc < 0:
break
GoodReplyCtr += cc
print(GoodReplyCtr)
The concept of recursion (again not needed here), is when a function calls
itself. This is a common practice when navigating graphs (like say the
directory structure on you drive C:), or with some mathematical problems. We
typically do not need to cover recursion early in the learning of computer
languages concepts, but once you have a good mastery of things, you may find
it quite useful (and challenging at time ;-) )
Keep at it! Math and python are cool.
**Edit** : One last trick:
You may find that you need to work on some multiplication tables more than
other. Rather than using randint you can use the random's module
random.choice() method to favor some numbers or to eliminate others. for
example
import random
X = random.choice((2, 3, 4, 6, 7, 8, 9, 7, 9)) # see, no 0, 1,or 5 but more chance to get 7 or 9
|
multiline checkbox in wxpython
Question: I'm working with wxpython (2.8) with python 2.5. is it possible to force a
wx.CheckBox to display its label on multiple lines? I'd like to be able to do
the same as wx.StaticText.Wrap(width)
See the attached example: the wx.CheckBox is 200 px wide, but it's label does
not fit in this space.
Any help is really appreciated! Thanks a lot Mauro
#example starts here
import wx
class MyFrame(wx.Frame):
def __init__(self):
wx.Frame.__init__(self, None, title="Hello World", size=
(300,200))
self.panel = wx.Panel(self, -1)
myVSizer = wx.BoxSizer(wx.VERTICAL)
#instantiating a checkbox 200 px wide. but the label is too
long
cb = wx.CheckBox(self.panel, -1, label="This is a very very
long label for 200 pixel wide cb!", size =wx.Size(200, -1))
myVSizer.Add( cb, 1)
self.panel.SetSizer(myVSizer)
myVSizer.Layout()
app = wx.App(redirect=True)
top = MyFrame()
top.Show()
app.MainLoop()
Answer: what about something like this? Flex! (I've made it a radio button to show
that it still behaves like one)
import wx
import textwrap
class MultilineRadioButton(wx.RadioButton):
def __init__(self, parent, id=-1, label=wx.EmptyString, wrap=10, pos=wx.DefaultPosition, size=wx.DefaultSize, style=0, validator=wx.DefaultValidator, name=wx.RadioButtonNameStr):
wx.RadioButton.__init__(self,parent,id,'',pos,size,style,validator,name)
self._label = label
self._wrap = wrap
lines = self._label.split('\n')
self._wrappedLabel = []
for line in lines:
self._wrappedLabel.extend(textwrap.wrap(line,self._wrap))
self._textHOffset = 20
dc = wx.ClientDC(self)
font = wx.SystemSettings.GetFont(wx.SYS_DEFAULT_GUI_FONT)
dc.SetFont(font)
maxWidth = 0
totalHeight = 0
lineHeight = 0
for line in self._wrappedLabel:
width, height = dc.GetTextExtent(line)
maxWidth = max(maxWidth,width)
lineHeight = height
totalHeight += lineHeight
self._textHeight = totalHeight
self.SetInitialSize(wx.Size(self._textHOffset + maxWidth,totalHeight))
self.Bind(wx.EVT_PAINT, self.OnPaint)
def OnPaint(self, event):
dc = wx.PaintDC(self)
self.Draw(dc)
self.RefreshRect(wx.Rect(0,0,self._textHOffset,self.GetSize().height))
event.Skip()
def Draw(self, dc):
dc.Clear()
font = wx.SystemSettings.GetFont(wx.SYS_DEFAULT_GUI_FONT)
dc.SetFont(font)
height = self.GetSize().height
if height > self._textHeight:
offset = height / 2 - self._textHeight / 2
else:
offset = 0
for line in self._wrappedLabel:
width, height = dc.GetTextExtent(line)
dc.DrawText(line,self._textHOffset,offset)
offset += height
class HFrame(wx.Frame):
def __init__(self,pos=wx.DefaultPosition):
wx.Frame.__init__(self,None,title="Hello World",size=wx.Size(600,400),pos=pos)
self.panel = wx.Panel(self,-1)
sizer = wx.BoxSizer(wx.HORIZONTAL)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
self.panel.SetSizer(sizer)
sizer.Layout()
class VFrame(wx.Frame):
def __init__(self,pos=wx.DefaultPosition):
wx.Frame.__init__(self,None,title="Hello World",size=wx.Size(600,400),pos=pos)
self.panel = wx.Panel(self,-1)
sizer = wx.BoxSizer(wx.VERTICAL)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
cb = RadioButton(self.panel,-1,label="This is a very very long label for the control!",wrap=10)
sizer.Add(cb,1)
self.panel.SetSizer(sizer)
sizer.Layout()
app = wx.App(redirect=False)
htop = HFrame(pos=wx.Point(0,50))
htop.Show()
vtop = VFrame(pos=wx.Point(650,50))
vtop.Show()
app.MainLoop()
|
Python CreateFile Cannot Find PhysicalMemory
Question: I am trying to access the Physical Memory of a Windows 2000 system (trying to
do this without a memory dumping tool). My understanding is that I need to do
this using the CreateFile function to create a handle. I have used an older
version of [win32dd](http://www.msuiche.net/2008/06/14/capture-memory-under-
win2k3-or-vista-with-win32dd/) to help me through this. Other documentation on
the web points me to using either "\Device\PhysicalMemory" or
"\\\\.\PhysicalMemory". Unfortunately, I get the same error for each.
Traceback (most recent call last):
File "testHandles.py", line 101, in (module)
File "testHandles.py", line 72, in createFileHandle
pywintypes.error: (3, 'CreateFile', 'The system cannot find the path specified.')
Actually, the error number returned is different for each run
\\\\.\PhysicalMemory == 3 and \Device\PhysicalMemory == 2. Review of pywin32,
win32file, createfile, pyhandle, and pywintypes did not produce information as
to the different return values.
Here is my code. I am using py2exe to get this working on Windows 2000 (and
yes it compiles successfully). I realize that I might also have a problem with
DeviceIoControl but right now I am concentrating on CreateFile.
# testHandles.py
import ctypes
import socket
import struct
import sys
import win32file
import pywintypes
def createFileHandle():
outLoc = pywintypes.Unicode("C:\\Documents and Settings\\Administrator\\My Documents\\pymemdump_dotPM.dd")
handleLoc = pywintypes.Unicode("\\\\.\\PhysicalMemory")
#handleLoc = pywintypes.Unicode("\\Device\\PhysicalMemory")
placeHolder = 0
BytesReturned = 0
# Device = CreateFile(L"\\\\.\\win32dd", GENERIC_ALL, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
# CreateFile(fileName, desiredAccess , shareMode , attributes , creationDisposition , flagsAndAttributes , hTemplateFile )
#hMemHandle = win32file.CreateFile(handleLoc, GENERIC_ALL, SHARE_READ, None, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, None)
hMemHandle = win32file.CreateFile(handleLoc, win32file.GENERIC_READ, win32file.FILE_SHARE_READ, None, win32file.OPEN_EXISTING, win32file.FILE_ATTRIBUTE_NORMAL, None)
print "hMemHandle: %s" % hMemHandle
if (hMemHandle == NO_ERROR):
print "Could not build hMemHandle"
sys.exit()
# We send destination path to the driver.
#if (!DeviceIoControl(hMemHandle, 0x19880922, outLoc, (ULONG)(wcslen(outLoc) + 1) * sizeof(TCHAR), NULL, 0, &BytesReturned, NULL))
if (ctypes.windll.Kernel32.DeviceIoControl(hMemHandle, 0x19880922, outLoc, 5, NULL, 0, BytesReturned, NULL)):
print "Error: DeviceIoControl(), Cannot send IOCTL.\n"
else:
print "[win32dd] Physical memory dumped. You can now check %s.\n" % outLoc
# Dump memory
createFileHandle()
Thank you, Cutaway
Answer: I don't believe it's possible to access the physical memory object from user
mode land in Windows. As your [win32dd
link](http://www.msuiche.net/2008/06/14/capture-memory-under-win2k3-or-vista-
with-win32dd/) suggests, you will need to do it from kernel mode.
|
Why can't I import the 'math' library when embedding python in c?
Question: I'm using the example in python's 2.6 docs to begin a foray into embedding
some python in C. The [example
C-code](http://docs.python.org/extending/embedding.html#pure-embedding) does
not allow me to execute the following 1 line script:
import math
Using line:
./tmp.exe tmp foo bar
it complains
Traceback (most recent call last):
File "/home/rbroger1/scripts/tmp.py", line 1, in <module>
import math
ImportError: [...]/python/2.6.2/lib/python2.6/lib-dynload/math.so: undefined symbol: PyInt_FromLong
When I do `nm` on my generated binary (tmp.exe) it shows
0000000000420d30 T PyInt_FromLong
The function seems to be defined, so why can't the shared object find the
function?
Answer: I'm using Python 2.6, and I successfully compiled and ran that same example
code that you listed, without changing anything in the source.
$ gcc python.c -I/usr/include/python2.6/ /usr/lib/libpython2.6.so
$ ./a.out random randint 1 100
Result of call: 39
$ ./a.out random randint 1 100
Result of call: 57
I specifically chose the `random` module because it does have `from math
import log,...` so it is certainly importing the `math` module as well.
Your issue is probably due to how you're linking; see [this forum
post](http://objectmix.com/python/311970-weird-embedding-problem.html) for a
similar issue someone else had. I can't find the links again, but it seems
like there are some common issues when trying to link against Python's static
library then importing modules that require a dynamic library.
|
Eclipse+PyDev+GAE memcache error
Question: I've started using Eclipe+PyDev as an environment for developing my first app
for Google App Engine. Eclipse is configured according to [this
tutorial](http://code.google.com/appengine/articles/eclipse.html).
Everything was working until I start to use memcache. PyDev reports the errors
and I don't know how to fix it:
Error: Undefined variable from import: get
How to fix this? Sure, it is only PyDev checker problem. Code is correct and
run on GAE.
UPDATE:
1. I'm using PyDev 1.5.0 but experienced the same with 1.4.8.
2. My PYTHONPATH includes (set in Project Properties/PyDev - PYTHONPATH):
* `C:\Program Files\Google\google_appengine`
* `C:\Program Files\Google\google_appengine\lib\django`
* `C:\Program Files\Google\google_appengine\lib\webob`
* `C:\Program Files\Google\google_appengine\lib\yaml\lib`
UPDATE 2:
I took a look at `C:\Program
Files\Google\google_appengine\google\appengine\api\memcache\__init__.py` and
found `get()` is not declared as `memcache` module function. They use the
following trick to do that (I didn't hear about such possibility):
_CLIENT = None
def setup_client(client_obj):
"""Sets the Client object instance to use for all module-level methods.
Use this method if you want to have customer persistent_id() or
persistent_load() functions associated with your client.
Args:
client_obj: Instance of the memcache.Client object.
"""
global _CLIENT
var_dict = globals()
_CLIENT = client_obj
var_dict['set_servers'] = _CLIENT.set_servers
var_dict['disconnect_all'] = _CLIENT.disconnect_all
var_dict['forget_dead_hosts'] = _CLIENT.forget_dead_hosts
var_dict['debuglog'] = _CLIENT.debuglog
var_dict['get'] = _CLIENT.get
var_dict['get_multi'] = _CLIENT.get_multi
var_dict['set'] = _CLIENT.set
var_dict['set_multi'] = _CLIENT.set_multi
var_dict['add'] = _CLIENT.add
var_dict['add_multi'] = _CLIENT.add_multi
var_dict['replace'] = _CLIENT.replace
var_dict['replace_multi'] = _CLIENT.replace_multi
var_dict['delete'] = _CLIENT.delete
var_dict['delete_multi'] = _CLIENT.delete_multi
var_dict['incr'] = _CLIENT.incr
var_dict['decr'] = _CLIENT.decr
var_dict['flush_all'] = _CLIENT.flush_all
var_dict['get_stats'] = _CLIENT.get_stats
setup_client(Client())
Hmm... Any idea how to force PyDev to recognize that?
Answer: There is a cleaner solution: Try adding GAE's memcache to your forced
builtins.
In your PyDev->Interpreter-Python->ForcedBuiltins window, add the
"google.appengine.api.memcache" entry and apply.
Double-click on the memcache errors to check them back, they disappear!
Please make sure that system pythonpath includes google APE install directory.
|
Why is the WindowsError while deleting the temporary file?
Question: 1. I have created a temporary file.
2. Added some data to the file created.
3. Saved it and then trying to delete it.
But I am getting `WindowsError`. I have closed the file after editing it. How
do I check which other process is accessing the file.
C:\Documents and Settings\Administrator>python
Python 2.6.1 (r261:67517, Dec 4 2008, 16:51:00) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import tempfile
>>> __, filename = tempfile.mkstemp()
>>> print filename
c:\docume~1\admini~1\locals~1\temp\tmpm5clkb
>>> fptr = open(filename, "wb")
>>> fptr.write("Hello World!")
>>> fptr.close()
>>> import os
>>> os.remove(filename)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
WindowsError: [Error 32] The process cannot access the file because it is being used by
another process: 'c:\\docume~1\\admini~1\\locals~1\\temp\\tmpm5clkb'
Answer: From the
[documentation](http://docs.python.org/library/tempfile.html#tempfile.mkstemp):
> mkstemp() returns a tuple containing an OS-level handle to an open file (as
> would be returned by os.open()) and the absolute pathname of that file, in
> that order. New in version 2.3.
So, `mkstemp` returns both the OS file handle to _and_ the filename of the
temporary file. When you re-open the temp file, the original returned file
handle is still open (no-one stops you from opening twice or more the same
file in your program).
If you want to operate on that OS file handle as a python file object, you
can:
>>> __, filename = tempfile.mkstemp()
>>> fptr= os.fdopen(__)
and then continue with your normal code.
|
Twisted network client with multiprocessing workers?
Question: So, I've got an application that uses Twisted + Stomper as a STOMP client
which farms out work to a multiprocessing.Pool of workers.
This appears to work ok when I just use a python script to fire this up, which
(simplified) looks something like this:
# stompclient.py
logging.config.fileConfig(config_path)
logger = logging.getLogger(__name__)
# Add observer to make Twisted log via python
twisted.python.log.PythonLoggingObserver().start()
# initialize the process pool. (child processes get forked off immediately)
pool = multiprocessing.Pool(processes=processes)
StompClientFactory.username = username
StompClientFactory.password = password
StompClientFactory.destination = destination
reactor.connectTCP(host, port, StompClientFactory())
reactor.run()
As this gets packaged for deployment, I thought I would take advantage of the
twistd script and run this from a tac file.
Here's my very-similar-looking tac file:
# stompclient.tac
logging.config.fileConfig(config_path)
logger = logging.getLogger(__name__)
# Add observer to make Twisted log via python
twisted.python.log.PythonLoggingObserver().start()
# initialize the process pool. (child processes get forked off immediately)
pool = multiprocessing.Pool(processes=processes)
StompClientFactory.username = username
StompClientFactory.password = password
StompClientFactory.destination = destination
application = service.Application('myapp')
service = internet.TCPClient(host, port, StompClientFactory())
service.setServiceParent(application)
For the sake of illustration, I have collapsed or changed a few details;
hopefully they were not the essence of the problem. For example, my app has a
plugin system, the pool is initialized by a separate method, and then work is
delegated to the pool using pool.apply_async() passing one of my plugin's
process() methods.
So, if I run the script (stompclient.py), everything works as expected.
It also appears to work OK if I run twist in non-daemon mode (-n):
twistd -noy stompclient.tac
however, it does _not_ work when I run in daemon mode:
twistd -oy stompclient.tac
The application appears to start up OK, but when it attempts to fork off work,
it just hangs. By "hangs", I mean that it appears that the child process is
never asked to do anything and the parent (that called pool.apply_async())
just sits there waiting for the response to return.
I'm sure that I'm doing something stupid with Twisted + multiprocessing, but
I'm really hoping that someone can explain to my the flaw in my approach.
Thanks in advance!
Answer: Since the difference between your working invocation and your non-working
invocation is only the "-n" option, it seems most likely that the problem is
caused by the daemonization process (which "-n" prevents from happening).
On POSIX, one of the steps involved in daemonization is forking and having the
parent exit. Among of things, this has the consequence of having your code run
in a different process than the one in which the .tac file was evaluated. This
also re-arranges the child/parent relationship of processes which were started
in the .tac file - as your pool of multiprocessing processes were.
The multiprocessing pool's processes start off with a parent of the twistd
process you start. However, when that process exits as part of daemonization,
their parent becomes the system init process. This may cause some problems,
although probably not the hanging problem you described. There are probably
other similarly low-level implementation details which normally allow the
multiprocessing module to work but which are disrupted by the daemonization
process.
Fortunately, avoiding this strange interaction should be straightforward.
Twisted's service APIs allow you to run code after daemonization has
completed. If you use these APIs, then you can delay the initialization of the
multiprocessing module's process pool until after daemonization and hopefully
avoid the problem. Here's an example of what that might look like:
from twisted.application.service import Service
class MultiprocessingService(Service):
def startService(self):
self.pool = multiprocessing.Pool(processes=processes)
MultiprocessingService().setServiceParent(application)
Now, separately, you may also run into problems relating to clean up of the
multiprocessing module's child processes, or possibly issues with processes
created with Twisted's process creation API, reactor.spawnProcess. This is
because part of dealing with child processes correctly generally involves
handling the SIGCHLD signal. Twisted and multiprocessing aren't going to be
cooperating in this regard, though, so one of them is going to get notified of
all children exiting and the other will never be notified. If you don't use
Twisted's API for creating child processes at all, then this may be okay for
you - but you might want to check to make sure any signal handler the
multiprocessing module tries to install actually "wins" and doesn't get
replaced by Twisted's own handler.
|
tail -f in python with no time.sleep
Question: I need to emulate "tail -f" in python, but I don't want to use time.sleep in
the reading loop. I want something more elegant like some kind of blocking
read, or select.select with timeout, but python 2.6 "select" documentation
specifically says: "it cannot be used on regular files to determine whether a
file has grown since it was last read." Any other way? In a few days if no
solution is given I will read tail's C source code to try to figure it out. I
hope they don't use sleep, hehe Thanks.
MarioR
Answer: (update) Either use FS monitors tools
* For [linux](http://pyinotify.sourceforge.net/)
* For [Windows](http://timgolden.me.uk/python/win32%5Fhow%5Fdo%5Fi/watch%5Fdirectory%5Ffor%5Fchanges.html)
* For [Mac](http://pypi.python.org/pypi/pyobjc-framework-FSEvents/2.2b2)
Or a single sleep usage (which I would you consider as much more elegant).
import time
def follow(thefile):
thefile.seek(0,2) # Go to the end of the file
while True:
line = thefile.readline()
if not line:
time.sleep(0.1) # Sleep briefly
continue
yield line
logfile = open("access-log")
loglines = follow(logfile)
for line in loglines:
print line
|
Why I cannot build a chain of methods? (method1.method2.method3)
Question: If I have the following code:
import sqlite
sqlite.connect('tmp.db').cursor().close()
I get the following error message:
Traceback (most recent call last):
File "searchengine2.py", line 13, in ?
sqlite.connect('tmp.db').cursor().close()
File "/usr/lib64/python2.4/site-packages/sqlite/main.py", line 280, in close
if self.con and self.con.closed:
ReferenceError: weakly-referenced object no longer exists
However, if I modify the code in the following way:
import sqlite
x1 = sqlite.connect('tmp.db')
x2 = x1.cursor()
x3 = x2.close()
everything is fine. Why?
Answer: Apparently the `cursor` keeps a weak reference to the connection (`self.con`).
Because you chain the functions, the connection you've instantiated is out of
scope as soon as you instantiate the cursor -- nothing holds a _strong_
reference to the connection anymore, and the connection is eligible for
garbage collection.
Therefore, by the time you try to `close` the cursor (which in turn tries to
`close` the connection), the connection is already out of scope and may have
been garbage collected -- if it was, then the connection is already closed.
There's no way to get around this without modifying the cursor's source to
make it hold a strong reference to the connection, and there's no easy way to
tell how many problems that might introduce if you do it that way. (Good)
designers don't arbitrarily make weak references without a good reason.
(Hopefully you understand weak references from [your last
question](http://stackoverflow.com/questions/1482141/what-does-it-mean-weakly-
referenced-object-no-longer-exists).)
|
`xrange(2**100)` -> OverflowError: long int too large to convert to int
Question: `xrange` function doesn't work for large integers:
>>> N = 10**100
>>> xrange(N)
Traceback (most recent call last):
...
OverflowError: long int too large to convert to int
>>> xrange(N, N+10)
Traceback (most recent call last):
...
OverflowError: long int too large to convert to int
Python 3.x:
>>> N = 10**100
>>> r = range(N)
>>> r = range(N, N+10)
>>> len(r)
10
Is there a backport of py3k builtin `range()` function for Python 2.x?
### Edit
I'm looking for a complete implementation of "lazy" `range()`, not just a
partial implementation of some of its functionality.
Answer: I believe there is no backport (Py 3's completely removed the int/long
distinction, after all, but in 2.* it's here to stay;-) but it's not hard to
hack your own, e.g....:
import operator
def wowrange(start, stop, step=1):
if step == 0:
raise ValueError('step must be != 0')
elif step < 0:
proceed = operator.gt
else:
proceed = operator.lt
while proceed(start, stop):
yield start
start += step
**Edit** it appears the OP doesn't just want looping (the normal purpose of
xrange, and range in Py3), but also `len` and the `in` operator (the latter
does work on the above generator, but slowly -- optimizations are possible).
For such richness a class is better...:
import operator
class wowrange(object):
def __init__(self, start, stop=None, step=1):
if step == 0: raise ValueError('step must be != 0')
if stop is None: start, stop = 0, start
if step < 0:
self.proceed = operator.gt
self.l = (stop-start+step+1)//step
else:
self.proceed = operator.lt
self.l = (stop-start+step-1)//step
self.lo = min(start, stop)
self.start, self.stop, self.step = start, stop, step
def __iter__(self):
start = self.start
while self.proceed(start, self.stop):
yield start
start += self.step
def __len__(self):
return self.l
def __contains__(self, x):
if x == self.stop:
return False
if self.proceed(x, self.start):
return False
if self.proceed(self.stop, x):
return False
return (x-self.lo) % self.step == 0
I wouldn't be surprised if there's an off-by-one or similar glitch lurking
here, but, I hope this helps!
**Edit** again: I see indexing is ALSO required. Is it just too hard to write
your own `__getitem__`? I guess it is, so here it, too, is, served on a silver
plate...:
def __getitem__(self, i):
if i < 0:
i += self.l
if i < 0: raise IndexError
elif if i >= self.l:
raise IndexError
return self.start + i * self.step
I don't know if 3.0 `range` supports slicing (`xrange` in recent `2.*`
releases doesn't -- it used to, but that was removed because the complication
was ridiculous and prone to bugs), but I guess I do have to draw a line in the
sand somewhere, so I'm not going to add it;-).
|
Python Script to find instances of a set of strings in a set of files
Question: I have a file which I use to centralize all strings used in my application.
Lets call it Strings.txt;
TITLE="Title"
T_AND_C="Accept my terms and conditions please"
START_BUTTON="Start"
BACK_BUTTON="Back"
...
This helps me with I18n, the issue is that my application is now a lot larger
and has evolved. As such a lot of these strings are probably not used anymore.
I want to eliminate the ones that have gone and tidy up the file.
I want to write a python script, using regular expressions I can get all of
the string aliases but how can I search all files in a Java package hierarchy
for an instance of a string? If there is a reason I use use perl or bash then
let me know as I can but I'd prefer to stick to one scripting language.
Please ask for clarification if this doesn't make sense, hopefully this is
straightforward, I just haven't used python much.
Thanks in advance,
Gav
Answer: Assuming the files are of reasonable size (as source files will be) so you can
easily read them in memory, and that you're looking for the parts in quotes
right of the = signs:
import collections
files_by_str = collections.defaultdict(list)
thestrings = []
with open('Strings.txt') as f:
for line in f:
text = line.split('=', 1)[1]
text = text.strip().replace('"', '')
thestrings.append(text)
import os
for root, dirs, files in os.walk('/top/dir/of/interest'):
for name in files:
path = os.path.join(root, name)
with open(path) as f:
data = f.read()
for text in thestrings:
if text in data:
files_by_str[text].append(path)
break
This gives you a dict with the texts (those that are present in 1+ files,
only), as keys, and lists of the paths to the files containing them as values.
If you care only about a yes/no answer to the question "is this text present
somewhere", and don't care where, you can save some memory by keeping only a
set instead of the defaultdict; but I think that often knowing what files
contained each text will be useful, so I suggest this more complete version.
|
Combining Dictionaries Of Lists In Python
Question: I have a very large collection of (p, q) tuples that I would like to convert
into a dictionary of lists where the first item in each tuple is a key that
indexes a list that contains q.
Example:
Original List: (1, 2), (1, 3), (2, 3)
Resultant Dictionary: {1:[2, 3], 2:[3]}
Furthermore, I would like to efficiently combine these dictionaries.
Example:
Original Dictionaries: {1:[2, 3], 2:[3]}, {1:[4], 3:[1]}
Resultant Dictionary: {1:[2, 3, 4], 2:[3], 3:[1]}
These operations reside within an inner loop, so I would prefer that they be
as fast as possible.
Thanks in advance
Answer: If the list of tuples is sorted, `itertools.groupby`, as suggested by
@gnibbler, is not a bad alternative to `defaultdict`, but it needs to be used
differently than he suggested:
import itertools
import operator
def lot_to_dict(lot):
key = operator.itemgetter(0)
# if lot's not sorted, you also need...:
# lot = sorted(lot, key=key)
# NOT in-place lot.sort to avoid changing it!
grob = itertools.groupby(lot, key)
return dict((k, [v[1] for v in itr]) for k, itr in grob)
For "merging" dicts of lists into a new d.o.l...:
def merge_dols(dol1, dol2):
keys = set(dol1).union(dol2)
no = []
return dict((k, dol1.get(k, no) + dol2.get(k, no)) for k in keys)
I'm giving `[]` a nickname `no` to avoid uselessly constructing a lot of empty
lists, given that performance is important. If the sets of the dols' keys
overlap only modestly, faster would be:
def merge_dols(dol1, dol2):
result = dict(dol1, **dol2)
result.update((k, dol1[k] + dol2[k])
for k in set(dol1).intersection(dol2))
return result
since this uses list-catenation only for overlapping keys -- so, if those are
few, it will be faster.
|
Web Service client in Python using ZSI - "Classless struct didn't get dictionary"
Question: I am trying to write a sample client in Python using ZSI for a simple Web
Service. The Web Service WSDL is following:
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<wsdl:definitions xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" xmlns:tns="http://www.example.org/test/" xmlns:wsdl="http://schemas.xmlsoap.org/wsdl/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" name="test" targetNamespace="http://www.example.org/test/">
<wsdl:message name="NewOperationRequest">
<wsdl:part name="NewOperationRequest" type="xsd:string"/>
</wsdl:message>
<wsdl:message name="NewOperationResponse">
<wsdl:part name="NewOperationResponse" type="xsd:string"/>
</wsdl:message>
<wsdl:portType name="test">
<wsdl:operation name="NewOperation">
<wsdl:input message="tns:NewOperationRequest"/>
<wsdl:output message="tns:NewOperationResponse"/>
</wsdl:operation>
</wsdl:portType>
<wsdl:binding name="testSOAP" type="tns:test">
<soap:binding style="rpc" transport="http://schemas.xmlsoap.org/soap/http"/>
<wsdl:operation name="NewOperation">
<soap:operation soapAction="http://www.example.org/test/NewOperation"/>
<wsdl:input>
<soap:body namespace="http://www.example.org/test/" use="literal"/>
</wsdl:input>
<wsdl:output>
<soap:body namespace="http://www.example.org/test/" use="literal"/>
</wsdl:output>
</wsdl:operation>
</wsdl:binding>
<wsdl:service name="test">
<wsdl:port binding="tns:testSOAP" name="testSOAP">
<soap:address location="http://localhost/test"/>
</wsdl:port>
</wsdl:service>
</wsdl:definitions>
Every time I run following code:
from ZSI.ServiceProxy import ServiceProxy
service = ServiceProxy('test.wsdl')
service.NewOperation('test')
I receive:
(...)
/var/lib/python-support/python2.5/ZSI/TCcompound.pyc in cb(self, elt, sw, pyobj, name, **kw)
345 f = lambda attr: pyobj.get(attr)
346 if TypeCode.typechecks and type(d) != types.DictType:
--> 347 raise TypeError("Classless struct didn't get dictionary")
348
349 indx, lenofwhat = 0, len(self.ofwhat)
TypeError: Classless struct didn't get dictionary
I have searched Google for this error and I found couple posts describing
similar problem but with no answer. Do you know was is wrong here? Is there an
error in the WSDL, do I miss something in the code or there is a bug in ZSI?
Thank you in advance for you help :-)
Answer: Finally, I have found the solution.
I should run like this:
from ZSI.ServiceProxy import ServiceProxy
service = ServiceProxy('test.wsdl')
service.NewOperation(NewOperationRequest='test')
The reason of the problem was that the name of the parameter was missing
(sic!) - silly error ;-)
|
Parsing out data using BeautifulSoup in Python
Question: I am attempting to use BeautifulSoup to parse through a DOM tree and extract
the names of authors. Below is a snippet of HTML to show the structure of the
code I'm going to scrape.
<html>
<body>
<div class="list-authors">
<span class="descriptor">Authors:</span>
<a href="/find/astro-ph/1/au:+Lin_D/0/1/0/all/0/1">Dacheng Lin</a>,
<a href="/find/astro-ph/1/au:+Remillard_R/0/1/0/all/0/1">Ronald A. Remillard</a>,
<a href="/find/astro-ph/1/au:+Homan_J/0/1/0/all/0/1">Jeroen Homan</a>
</div>
<div class="list-authors">
<span class="descriptor">Authors:</span>
<a href="/find/astro-ph/1/au:+Kosovichev_A/0/1/0/all/0/1">A.G. Kosovichev</a>
</div>
<!--There are many other div tags with this structure-->
</body>
</html>
My point of confusion is that when I do soup.find, it finds the first
occurrence of the div tag that I'm searching for. After that, I search for all
'a' link tags. At this stage, how do I extract the authors names from each of
the link tags and print them out? Is there a way to do it using BeautifulSoup
or do I need to use Regex? How do I continue iterating over every other other
div tag and extract the authors names?
import re
import urllib2,sys
from BeautifulSoup import BeautifulSoup, NavigableString
html = urllib2.urlopen(address).read()
soup = BeautifulSoup(html)
try:
authordiv = soup.find('div', attrs={'class': 'list-authors'})
links=tds.findAll('a')
for link in links:
print ''.join(link[0].contents)
#Iterate through entire page and print authors
except IOError:
print 'IO error'
Answer: just use findAll for the divs link you do for the links
for authordiv in soup.findAll('div', attrs={'class': 'list-authors'}):
|
How to pick certain elements of x-tuple returned by a function?
Question: I am a newbie to Python. Consider the function `str.partition()` which returns
a 3-tuple. If I am interested in only elements 0 and 2 of this tuple, what is
the best way to pick only certain elements out of such a tuple?
I can currently do either:
# Introduces "part1" variable, which is useless
(part0, part1, part2) = str.partition(' ')
Or:
# Multiple calls and statements, again redundancy
part0 = str.partition(' ')[0]
part2 = str.partition(' ')[2]
I would like to be able to do _something_ like this, but cannot:
(part0, , part2) = str.partition(' ')
# Or:
(part0, part2) = str.partition(' ')[0, 2]
Answer: Underscore is often used as a name for stuff you do not need, so something
like this would work:
part0, _, part2 = str.partition(' ')
In this particular case, you could do this, but it isn't a pretty solution:
part0, part2 = str.partition(' ')[::2]
A more esoteric solution:
from operator import itemgetter
part0, part2 = itemgetter(0, 2)(str.partition(' '))
|
How to use export with Python on Linux
Question: I need to make an export like this in Python :
# export MY_DATA="my_export"
I've tried to do :
# -*- python-mode -*-
# -*- coding: utf-8 -*-
import os
os.system('export MY_DATA="my_export"')
But when I list export, "MY_DATA" not appear :
# export
How I can do an export with Python without saving "my_export" into a file ?
Answer: `export` is a command that you give directly to the shell (e.g. `bash`), to
tell it to add or modify one of its environment variables. You can't change
your shell's environment from a child process (such as Python), it's just not
possible.
Here's what's happening with you try `os.system('export
MY_DATA="my_export"')`...
/bin/bash process, command `python yourscript.py` forks python subprocess
|_
/usr/bin/python process, command `os.system()` forks /bin/sh subprocess
|_
/bin/sh process, command `export ...` changes local environment
When the bottom-most `/bin/sh` subprocess finishes running your `export ...`
command, then it's discarded, along with the environment that you have just
changed.
|
how to run both python 2.6 and 3.0 on the same windows XP box?
Question: What kind of setup do people use to run both python 2.6 and python 3.0 on the
same windows machine?
Answer: No problem, each version is installed in its own directory. On my Windows box,
I have `C:\Python26\` and `C:\Python31\`. The _Start Menu_ items are also
distinct. Just use the standard installers from the Python Programming
Language [Official Website](http://www.python.org/download/), or the ready-to-
install distributions from
[ActiveState](http://www.activestate.com/activepython/).
A direct way to select the wanted version is to name it explicitly on the
command line.
C:\> C:\Python25\python ver.py
2.5.4 (r254:67916, Dec 23 2008, 15:10:54) [MSC v.1310 32 bit (Intel)]
C:\> C:\Python31\python ver.py
3.1.1 (r311:74483, Aug 17 2009, 17:02:12) [MSC v.1500 32 bit (Intel)]
Where `ver.py` is:
import sys
print (sys.version)
|
map string position to line number in regex output
Question: I'm working on a "grep-like" utility in Python for searching Oracle source
code files. Coding standards have changed over time, so trying to find
something like "all deletes from table a.foo" could span multiple lines, or
not, depending on the age of that piece of code:
s = """-- multiline DDL statement
DELETE
a.foo f
WHERE
f.bar = 'XYZ';
DELETE a.foo f
WHERE f.bar = 'ABC';
DELETE a.foo WHERE bar = 'PDQ';
"""
import re
p = re.compile( r'\bDELETE\b.+?a\.foo', re.MULTILINE | re.DOTALL )
for m in re.finditer( p, s ):
print s[ m.start() : m.end() ]
This outputs:
DELETE
a.foo
DELETE a.foo
DELETE a.foo
What I want:
[2] DELETE
[3] a.foo
[7] DELETE a.foo
[10] DELETE a.foo
Is there a quick/simple/builtin way to map string indices to line numbers?
Answer:
lineno = s.count("\n",0,m.start())+1
|
How to print a list in Python "nicely"
Question: In PHP, I can do this:
echo '<pre>'
print_r($array);
echo '</pre>'
In Python, I currently just do this:
print the_list
However, this will cause a big jumbo of data. Is there any way to print it
nicely into a readable tree? (with indents)?
Answer:
from pprint import pprint
pprint(the_list)
|
Does anyone know a way to scramble the elements in a list?
Question:
thelist = ['a','b','c','d']
How I can to scramble them in Python?
Answer:
>>> import random
>>> thelist = ['a', 'b', 'c', 'd']
>>> random.shuffle(thelist)
>>> thelist
['d', 'a', 'c', 'b']
Your result will (hopefully!) vary.
|
How do I change my float into a two decimal number with a comma as a decimal point separator in python?
Question: I have a float: 1.2333333
How do I change it into a two decimal number with a comma as a decimal point
separator, eg 1,23?
Answer: The [locale module](http://docs.python.org/library/locale.html) can help you
with reading and writing numbers in the locale's format.
>>> import locale
>>> locale.setlocale(locale.LC_ALL, "")
'sv_SE.UTF-8'
>>> locale.format("%f", 2.2)
'2,200000'
>>> locale.format("%g", 2.2)
'2,2'
>>> locale.atof("3,1415926")
3.1415926000000001
|
How to read lines from a file into a multidimensional array (or an array of lists) in python
Question: I have a file with a format similar to this:
a,3,4,2,1
3,2,1,a,2
I want to read the file and create an array of lists
in a way that:
array[0] = ['a','3','4','2','1']
array[1] = ['3','2','1','a','2']
How can I do that?
So far I am stuck with:
f = open('./urls-eu.csv', 'r')
for line in f:
arr = line.split(',')
print arr
I am really new to python.
Answer: Batteries included:
>>> import csv
>>> array = list( csv.reader( open( r'./urls-eu.csv' ) ) )
>>> array[0]
['a', '3', '4', '2', '1']
>>> array[1]
['3', '2', '1', 'a', '2']
|
Why Does List Argument in Python Behave Like ByRef?
Question: This may be for most languages in general, but I'm not sure. I'm a beginner at
Python and have always worked on copies of lists in C# and VB. But in Python
whenever I pass a list as an argument and enumerate through using a "for i in
range," and then change the value of the list argument, the input values
actually changes the original list. I thought Python was supposed to pass
arguments by value by default so that once the function is finished I still
have the original values from before I called the function. What am I missing?
Thanks!
Answer: Python does pass arguments by value but the value you are receiving is a copy
of the reference (incidentally this is the exact same way that C#, VB.NET, and
Java behave as well).
This is the important thing to remember:
> _Objects are not passed by reference - object references are passed by
> value_.
Since you have a copy of the reference, any operation on what that reference
points to will be just as if you were holding the original reference itself.
|
(strongly vs weakly) typed AND (statically vs dynamically) typed languages and Moore's law
Question: I do not know how many faces this problem. If I do programming in
weakly/dynamically typed language like python,php,javascript for few days I
lose touch with strongly typed languages like c++,Java,.net. I recently heard
languages like python and ruby which people loved programming in.
It is very easy programming in weakly/dynamically typed languages, but there
is a danger of losing touch with languages like c++,Java. Processors now
becoming very powerful and according to [Moore's
law](http://en.wikipedia.org/wiki/Moore%27s%5Flaw) it will increase speed with
time exponentially. So efficiency may not be issue as similar thing happened
when we shifted from embedded to high level languages like c++,java.
* So is the world shifting toward weakly/dynamically typed languages?
* Will the weakly/dynamically typed languages replace strongly typed languages in future?
* Are there any fields in which strongly typed language is a must use and cannot be replaced current time as well as near future?
Answer: First, Moore's law is only an empirical observation. Sooner or later, the laws
of physics will mean that it is no longer possible to keep increase
uniprocessor speed. Moore's law is not a useful predictor of the future in the
medium to long term, and possibly not even in the short term.
Second, strongly and weakly typed languages are EQUALLY affected by Moores
law.
Third, Moore's law is about uniprocessors. We're well into a world where
increases in raw computing power are coming through multi-processing, but
there aren't the software tools (e.g. languages) around yet that help the
average Joe programmer to write programs that take advantage of multi-
processing. However, functional languages offer more promise in this area than
procedural ones.
Fourth, I think you are really comparing statically typed versus dynamically
typed languages. (The terms "strongly typed" and "weakly typed" have become so
confused due to conflicting definitions that they are no longer meaningful.)
I guess your argument is that Moore's law means that efficiency matters less,
so we can "get away with" using less efficient computation paradigms; e.g.
dynamically typed languages. (And if we are talking about interactive tasks,
the computer only needs to keep up with the user's speed of asking for things
and mentally processing the answers.)
The flip side of that argument is that people are wanting their computers to
do more compute intensive things; e.g. each generation of computer games
requires more power to do the graphics. Online business wants to do more
things (e.g. serve more web requests) faster with hardware that is cheaper to
run. In short, there are lots of situations where efficiency does matter, and
this will always be the case.
So what you find is that in places where speeds is important, we tend to use
efficient computing techniques, and where it is unimportant we use techniques
that minimize software development and maintenance costs.
* * *
**UPDATE**
On rereading my answer, I missed something. If we taken it as read that Moores
law is breaking down, and that future increases in computing "power" will come
in the form of more cores, etcetera, then there will be an increasing role for
functional languages.
Anyone who has tried to exploit parallelism in an imperative or OO language
will recognize that it is a tricky problem, fraught with pitfalls. By
contrast, in a pure functional language, parallelism is much simpler. Since
the state of data structures doesn't change, you don't need to worry about
threads synchronizing over the use of the data structures. Furthermore, it is
simpler for the compiler or runtime system of the language to spot that a
particular part of your program could be done in parallel ... and just do it.
Or at a higher level, the FP language IDE (or whatever) could find / suggest
opportunities for large scale transformations to aid parallel execution.
IMO, this is what is behind the (slow) rise in popularity of functional
languages ...
|
Python 3.1.1 with --enable-shared : will not build any extensions
Question: Summary: Building Python 3.1 on RHEL 5.3 64 bit with `--enable-shared` fails
to compile all extensions. Building "normal" works fine without any problems.
**Please note** that this question may seem to blur the line between
programming and system administration. However, I believe that because it has
to deal directly with getting language support in place, and it very much has
to do with supporting the process of programming, that I would cross-post it
here. Also at: <http://serverfault.com/questions/73196/python-3-1-1-with-
enable-shared-will-not-build-any-extensions>. Thank you!
**Problem:**
Building Python 3.1 on RHEL 5.3 64 bit with `--enable-shared` fails to compile
all extensions. Building "normal" works fine without any problems.
I can build python 3.1 just fine, but when built as a shared library, it emits
many warnings (see below), and refuses to build any of the `c` based modules.
Despite this failure, I can still build mod_wsgi 3.0c5 against it, and run it
under apache. Needless to say, the functionality of Python is greatly
reduced...
Interesting to note that Python 3.2a0 (from svn) compiles fine with --enable-
shared, and mod_wsgi compiles fine against it. But when starting apache, I
get:
`Cannot load /etc/httpd/modules/mod_wsgi.so into server:
/etc/httpd/modules/mod_wsgi.so: undefined symbol: PyCObject_FromVoidPtr`
The project that this is for is a long-term project, so I'm okay with alpha
quality software if needed. Here are some more details on the problem.
**Host:**
* Dell PowerEdge
* Intel Xenon
* RHEL 5.3 64bit
* Nothing "special"
**Build:**
* Python 3.1.1 source distribution
* Works fine with `./configure`
* Does not work fine with `./configure --enable-shared`
(`export CFLAGS="-fPIC"` has been done)
**make output**
* * *
`gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-
prototypes -I. -IInclude -I./Include -fPIC -DPy_BUILD_CORE -c
./Modules/_weakref.c -o Modules/_weakref.o`
* * *
`building 'bz2' extension gcc -pthread -fPIC -fno-strict-aliasing -DNDEBUG -g
-fwrapv -O3 -Wall -Wstrict-prototypes -I. -I./Include -I/usr/local/include
-IInclude -I/home/build/RPMBUILD/BUILD/Python-3.1.1 -c
/home/build/RPMBUILD/BUILD/Python-3.1.1/Modules/bz2module.c -o
build/temp.linux-x86_64-3.1/home/build/RPMBUILD/BUILD/Python-3.1.1/Modules/bz2module.o
gcc -pthread -shared -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall
-Wstrict-prototypes
build/temp.linux-x86_64-3.1/home/build/RPMBUILD/BUILD/Python-3.1.1/Modules/bz2module.o
-L/usr/local/lib -L. -lbz2 -lpython3.1 -o build/lib.linux-x86_64-3.1/bz2.so
/usr/bin/ld: /usr/local/lib/libpython3.1.a(abstract.o): relocation R_X86_64_32
against 'a local symbol' can not be used when making a shared object;
recompile with -fPIC`
* * *
Failed to build these modules:
_bisect _codecs_cn _codecs_hk
_codecs_iso2022 _codecs_jp _codecs_kr
_codecs_tw _collections _csv
_ctypes _ctypes_test _curses
_curses_panel _dbm _elementtree
_gdbm _hashlib _heapq
_json _lsprof _multibytecodec
_multiprocessing _pickle _random
_socket _sqlite3 _ssl
_struct _testcapi array
atexit audioop binascii
bz2 cmath crypt
datetime fcntl grp
itertools math mmap
nis operator ossaudiodev
parser pyexpat readline
resource select spwd
syslog termios time
unicodedata zlib
Answer: Something is wrong with your build environment. It is picking up a
libpython3.1.a from `/usr/local/lib`; this confuses the error messages. It
tries linking with that library, which fails - however, it shouldn't have
tried that in the first place, since it should have used the libpython that it
just built. I recommend taking the Python 3.1 installation in `/usr/local` out
of the way.
You don't show in your output whether a libpython3.1.so.1.0 was created in the
build tree; it would be important to find out whether it exists, how it was
linked, and what symbols it has exported.
|
Python for C++ or Java Programmer
Question: I have a background in C++ and Java and Objective C programming, but i am
finding it hard to learn python, basically where its "Main Function" or from
where the program start executing. So is there any tutorial/book which can
teach python to people who have background in C++ or Java. Basically something
which can show if how you were doing this in C++ and how this is done in
Python.
OK i think i did not put the question heading or question right, basically i
was confused about the "Main" Function, otherwise other things are quite
obvious from python official documentation except this concept.
Thanks to all
Answer: When you run a script through the Python interpreter (or import that script
from another script), **it actually executes all the code from beginning to
end** \-- in _that_ sense, **there is no "entry point" to a Python script**.
So to work around this, Python automatically creates a `__name__` variable and
fills it with the value `"__main__"` when you are running a script by itself
(as opposed to something else importing that script). That's why you'll see
many scripts like:
def foo():
print "Hello!"
if __name__ == "__main__":
foo()
where all the function/class definitions are at the top, and there is a
similar if-statement as the last thing in the script. You are guaranteed that
Python will start executing the script from top-to-bottom, so it will read all
of your definitions there. If you wanted, you could intermingle actual
functional code inside all the function definitions.
If this script was named `bar.py`, you could do `python bar.py` at the command
line and you would see the script print out `"Hello!"`.
On the other hand, if you did `import bar` from another Python script, nothing
would print out until you did `bar.foo()`, because `__name__` was no longer
`"__main__"` and the if-statement failed, thus `foo` was never executed.
|
Why doesn't memcache work in my Django?
Question:
from django.core.cache import cache
def testcache():
cache.set('test','I am putting this message in',3333)
print cache.get('test')
It just prints "**None** "
This is in "ps aux":
dovr 2241 0.0 0.8 57824 2144 ? Ssl 04:20 0:00 memcached -d -u root -m 3900 -p 11211
dovr 2247 0.0 3.7 83696 9800 pts/2 S+ 04:20 0:00 python manage.py runserver 0.0.0.0:8000
And this is in my settings.py:
CACHE_BACKEND = 'memcached://MYIPADRESS:11211/'
By the way, it worked before, but not anymore! I don't know why. So weird.
Answer: You can insure that you can reach memcached from your code by logging value
returned from `set()` method. Probably memcached listens on 127.0.0.1 while
you are trying to connect to external interface.
|
Python sqlite3 version
Question:
Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlite3
>>> sqlite3.version
'2.4.1'
Questions:
* Why is the version of the sqlite**3** module **'2.4.1'**
* Whats the reason behind bundling such an old sqlite with Python? The sqlite releaselog says **2002 Mar 13 (2.4.1)**.
Answer:
Python 2.5.1
>>> import sqlite3
>>> sqlite3.version
'2.3.2'
>>> sqlite3.sqlite_version
'3.3.4'
version - pysqlite version
sqlite_version - sqlite version
|
python: interact with the session in cgi scripts
Question: Can python cgi scripts write and read data to the session? If so how? Is there
a high-level API or must I roll my own classes?
Answer: There's no "_session_ " on `cgi`. You must roll your own session handling code
if you're using raw `cgi`.
Basically, sessions work by creating a unique cookie number and sending it on
a response header to the client, and then checking for this cookie on every
connection. Store the session data somewhere on the server (memory, database,
disk) and use the cookie number as a key to retrieve it on every request made
by the client.
However `cgi` is not how you develop applications for the web in python. Use
[`wsgi`](http://wsgi.org/). Use a web framework.
Here's a quick example using [cherrypy](http://cherrypy.org/).
`cherrypy.tools.sessions` is a cherrypy tool that handles cookie
setting/retrieving and association with data automatically:
import cherrypy
class HelloSessionWorld(object):
@cherrypy.tools.sessions()
def index(self):
if 'data' in cherrypy.session:
return "You have a cookie! It says: %r" % cherrypy.session['data']
else:
return "You don't have a cookie. <a href='getcookie'>Get one</a>."
index.exposed = True
@cherrypy.tools.sessions()
def getcookie(self):
cherrypy.session['data'] = 'Hello World'
return "Done. Please <a href='..'>return</a> to see it"
getcookie.exposed = True
application = cherrypy.tree.mount(HelloSessionWorld(), '/')
if __name__ == '__main__':
cherrypy.quickstart(application)
Note that this code is a `wsgi` application, in the sense that you can publish
it to any `wsgi`-enabled web server (apache has
[`mod_wsgi`](http://code.google.com/p/modwsgi)). Also, cherrypy has its own
`wsgi` server, so you can just run the code with python and it will start
serving on [`http://localhost:8080/`](http://localhost:8080/)
|
How can I build the Boost.Python example on Ubuntu 9.10?
Question: I am using Ubuntu 9.10 beta, whose repositories contain boost 1.38. I would
like to build the hello-world example. I followed the instructions here
(<http://www.boost.org/doc/libs/1%5F40%5F0/libs/python/doc/tutorial/doc/html/python/hello.html>),
found the example project, and issued the "bjam" command. I have installed
bjam and boost-build.
I get the following output:
Jamroot:18: in modules.load
rule python-extension unknown in module Jamfile</usr/share/doc/libboost1.38-doc/examples/libs/python/example>.
/usr/share/boost-build/build/project.jam:312: in load-jamfile
/usr/share/boost-build/build/project.jam:68: in load
/usr/share/boost-build/build/project.jam:170: in project.find
/usr/share/boost-build/build-system.jam:248: in load
/usr/share/boost-build/kernel/modules.jam:261: in import
/usr/share/boost-build/kernel/bootstrap.jam:132: in boost-build
/usr/share/doc/libboost1.38-doc/examples/libs/python/example/boost-build.jam:7: in module scope
I do not know enough about Boost (this is an exploratory exercise for myself)
to understand why the python-extension macro in the included Jamroot is not
valid. I am running this example from the install directory, so I have not
altered the Jamroot's use-project setting.
As a side question, if I were to just willy-nilly start a project in an
arbitrary directory, how would I write my jamroot?
Answer: The problem comes from using Ubuntu package instead of boost compiled from
source. You have to edit you Jamroot to say it to use global libboost-python,
instead of looking for lib in relative boost source tree.
Summarily you should have these lines at the beginning of your Jamroot:
using python ;
lib libboost_python : : <name>boost_python ;
project : requirements <library>libboost_python ;
It was reported as a bug on Debian and corrected at least on lenny with
libboost-python1.40 ...mostly. The example in libboost_python still refers to
boost_python-mt instead of boost_python, but /usr/lib/libboost_python.so
exists but not /usr/lib/libboost_python-mt.so.
Hopefully Ubuntu will soon have the same fix and the next user won't stumble
on this... I know the answer to your question because I **did** had the exact
same problem not long ago.
|
In Python, how do I transform a string into a file?
Question: There is a read-only library function that takes a file as an argument. But I
have a string.
How do I convert a string to a file, that if you read the file it will return
this string? I don't want to write to disk.
Answer: The `StringIO` module:
>>> import StringIO
>>> f = StringIO.StringIO("foo")
>>> f.read()
'foo'
The `cStringIO` module has the same interface, and is faster, but can't deal
with Unicode strings that have non-ASCII characters.
[StringIO documentation](http://docs.python.org/library/stringio.html)
|
How do I request data securely via Google OAuth?
Question: Until recently users of my site were able to import data from Google, via
OAuth. However, recently they have received the warning below, in a yellow
box, when authorising (although the import still works).
I've also noticed this same warning on Facebook's GMail authenticator!
What's changed / am I missing?
> This website is registered with Google to make authorization requests, but
> has not been configured to send requests securely.
>
> If you grant access but you did not initiate this request at www.foo.com, it
> may be possible for other users of www.foo.com to access your data. We
> recommend you deny access unless you are certain that you initiated this
> request directly with www.foo.com.
(The site is written in Zope/Python, but the step/documentation I'm missing is
more important)
Answer: Did you try Googling the error message? Doing so took me to [this
page](http://code.google.com/apis/accounts/docs/RegistrationForWebAppsAuto.html),
which states:
> Registered with enhanced security: Registered applications **with a security
> certificate on file** can use secure tokens. The Access Request page removes
> cautions, displaying this message: " Google is not affiliated with , and we
> recommend that you grant access only if you trust the site."
See their docs, step 4, "Upload a security certificate" for more details.
|
syntax error on `If` line
Question: My code:
#!/usr/bin/env python
def Runaaall(aaa):
Objects9(1.0, 2.0)
def Objects9(aaa1, aaa2):
If aaa2 != 0: print aaa1 / aaa2
The error I receive:
$ python test2.py
File "test2.py", line 7
If aaa2 != 0: print aaa1 / aaa2
^
SyntaxError: invalid syntax
I'm at a loss to why this error is happening.
Answer: `if` must be written in lower case.
Furthermore,
* Write function names in lower case (see [PEP 8](http://www.python.org/dev/peps/pep-0008/), the Python style guide).
* Write the body of an `if`-clause on a separate line.
* Though in this case you'll probably not run into trouble, be careful with [comparing floats for equality](http://www.cygnus-software.com/papers/comparingfloats/comparingfloats.htm).
* Since you've just started learning Python, you may want to get acquainted with writing parentheses around the arguments to `print`, since from Python 3 onwards, [print is a function](http://docs.python.org/dev/3.0/whatsnew/3.0.html), not a keyword.
To enforce this syntax in Python 2.6, you can put this at the top of your
file:
from __future__ import print_function
Demonstration:
>>> print 'test'
test
>>> from __future__ import print_function
>>> print 'test'
File "<stdin>", line 1
print 'test'
^
SyntaxError: invalid syntax
>>> print('test')
test
For more on `__future__` imports, see the
[documentation](http://docs.python.org/3.1/library/%5F%5Ffuture%5F%5F.html).
|
Has anyone found that UIWebView fails for some URLS?
Question: I have been working on an iPhone App for a couple of weeks now and one of the
earlier features I added was a _UIWebView_ that loaded context sensitive
Wikipedia page topics. This was pretty trivial to implement and has been
working fine for some time.
Today, I found that functionality had unexpectedly stopped working. I had been
fiddling around at the perimeter of that piece of code and initially assumed
that I had broken something.
I checked all the obvious places, my urls, was the _UIWebView_ still hooked up
in the XIB etc. I didn't find any issues.
Investigating further, I stuck some error handling on my _UIWebViewDelegate_
*didFailLoadWithError* and found I was getting a -999 error:
> **NSURLErrorCancelled**
>
> Returned when an asynchronous load is canceled.
>
> A Web Kit framework delegate will receive this error when it performs a
> cancel operation on a loading resource. Note that an NSURLConnection or
> NSURLDownload delegate will not receive this error if the download is
> canceled.
So this sounds like making a new request (or cancelling) before the original
one has finished. I check my code for anything like this and come up empty.
So I went into my usual downward spiral of paranoia and assumed that Wikipedia
was blocking requests based on UAgent or something and went on something of
wild goose chase attempting to spoof my way back to a happy place. These
attempts were not successful and eventually sanity prevailed. I created a
simple python script to mimic the HTTP request I was making from my APP in the
simulator, to see what Wikipedia was sending back:
string = "GET /wiki/Franklin_D._Roosevelt HTTP/1.1\r\nHost: en.wikipedia.org\r\nUser-Agent: test\r\nReferer: http://en.wikipedia.org/wiki/Franklin_D._Roosevelt\r\nAccept: */*\r\nAccept-Language: en-us\r\n_Accept-Encoding: gzip, deflate\r\nConnection: keep-alive\r\n\r\n"
import socket
s = socket.socket()
s.connect(("en.wikipedia.org",80))
s.send(string)
x = s.recv (1000)
while (x):
print x
x = s.recv (1000)
So I run this guy and discover that Wikipedia is very kindly returning my data
promptly and completely. So what is going on?
Chinks started to appear in my every present, paranoid armour "It's always my
fault" and I decide to check out if other iPhone apps can view these URLs. I
post a [tweet](http://twitter.com/redbluething/status/4906373854) with an
ironically amusing (I am easily amused) URL and check out if Tweetie can view
the URL. It can't.
A friend tries it out in Twitterific. Same problem. Works fine in Safari and
the Wikipedia App, but it seems like iPhone apps using a bog standard
_UIWebView_ are having problems with Wikipedia pages.
Just to be completely certain there are no other variables, I created a
[simple test app](http://www.cannonade.net/pics/UIWebViewTest.zip) with just a
UIWebView that loads up <http://en.wikipedia.com>, it fails with the same
error (added that code at the end).
So what do you guys think? Is this just a _UIWebView_ bug? Any Apple gronks
out there know what is going on here?
Have I missed something completely obvious and I am once again sitting on the
train to work without my pants on?
Looking forward to hearing what you think. Keep in mind, this was working fine
yesterday.
**Debrief** (or maybe Post Mortem is more appropriate):
Thanks [Duncan](http://stackoverflow.com/users/75656/duncanwilcox) for the
solution and a pretty clear description of what is going on here.
Looks like the reason I originally saw the error, when I wasn't implementing
the _didFailLoadWithError_ delegate method at all, was that the default
behavior for that method is apparently to clear the _UIWebView_ and
consequently kill the request. When I added my implementation to find out what
was going on, I stuck in some code to write the error to the view and, as
Duncan points out, this is what got me.
Seems like a pretty horrible solution to ignore -999 error codes in the
callback, but I am fine with duct tape.
I tried quite a few apps to test if this was a UIWebView problem (Tweetie,
Twitteriffic etc ...) and they all had the issue. It looks like this might be
a pretty common oversight for developers. Maybe Apple can clean this up in the
next version.
Other interesting point is that when I switched my URLs to use
<http://en.m.wikipedia.com> instead of <http://en.wikipedia.com>, the problem
went away.
// Implement viewDidLoad to do additional setup after loading the view, typically from a nib.
- (void)viewDidLoad {
[super viewDidLoad];
_webView = (UIWebView*)self.view;
_webView.delegate = self;
// load the url
// FAIL
//NSURL * newUrl = [[[NSURL alloc] initWithString:@"http://en.wikipedia.com"] autorelease];
// OK
NSURL * newUrl = [[[NSURL alloc] initWithString:@"http://www.stackoverflow.com"] autorelease];
NSURLRequest * newUrlRequest = [NSURLRequest requestWithURL:newUrl];
[_webView loadRequest:newUrlRequest];
}
// delegate stuff
- (BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)req navigationType:(UIWebViewNavigationType)navigationType { return YES; }
- (void)webViewDidStartLoad:(UIWebView *)wv
{
// starting the load, show the activity indicator in the status bar
[UIApplication sharedApplication].networkActivityIndicatorVisible = YES;
}
- (void)webViewDidFinishLoad:(UIWebView *)webView
{
// finished loading, hide the activity indicator in the status bar
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
}
- (void)webView:(UIWebView *)webView didFailLoadWithError:(NSError *)error
{
NSURLErrorDomain
// load error, hide the activity indicator in the status bar
[UIApplication sharedApplication].networkActivityIndicatorVisible = NO;
[_webView loadHTMLString:[[[NSString alloc] initWithFormat:@"Failed to load page %@", [error localizedDescription]] autorelease] baseURL:nil];
}
Answer: Looks like there's an answer here:
<http://stackoverflow.com/questions/1024748/how-do-i-fix-nsurlerrordomain-
error-999-in-iphone-3-0-os>
which in turn refers to:
<http://www.iphonedevsdk.com/forum/iphone-sdk-development/6280-uiwebview-
didfailloadwitherror-how-get-errors-code-list.html>
Looks like you need a hack like this in the webView:didFailLoadWithError:
delegate:
if ([error code] != NSURLErrorCancelled) {
//show error alert, etc.
}
In essence what's happening is the delegate is getting a "cancelled" (-999)
failure, that might be originated in javascript or perhaps even in a UIWebView
bug.
With your code the webview doesn't show anything at all because you're using
the very same UIWebView to display the error. If you had just NSLog'd the
error you would have seen a failure, but then the page would have loaded just
fine, giving you a hint that the failure is bogus.
|
Running Tests From a Module
Question: I am attempting to run some unit tests in python from what I believe is a
module. I have a directory structure like
TestSuite.py
UnitTests
|__init__.py
|TestConvertStringToNumber.py
In testsuite.py I have
import unittest
import UnitTests
class TestSuite:
def __init__(self):
pass
print "Starting testting"
suite = unittest.TestLoader().loadTestsFromModule(UnitTests)
unittest.TextTestRunner(verbosity=1).run(suite)
Which looks to kick off the testing okay but it doesn't pick up any of the
test in TestConvertNumberToString.py. In that class I have a set of functions
which start with 'test'.
What should I be doing such that running python TestSuite.py actually kicks
off all of my tests in UnitTests?
Answer: Here is some code which will run all the unit tests in a directory:
#!/usr/bin/env python
import unittest
import sys
import os
unit_dir = sys.argv[1] if len(sys.argv) > 1 else '.'
os.chdir(unit_dir)
suite = unittest.TestSuite()
for filename in os.listdir('.'):
if filename.endswith('.py') and filename.startswith('test_'):
modname = filename[:-2]
module = __import__(modname)
suite.addTest(unittest.TestLoader().loadTestsFromModule(module))
unittest.TextTestRunner(verbosity=2).run(suite)
If you call it testsuite.py, then you would run it like this:
testsuite.py UnitTests
|
Apache2 + RewriteMap + Python -- when returning 'NULL', apache hangs
Question: [SOLVED: See solution below.]
I'm having a problem writing a `RewriteMap` program (using Python). I have a
`RewriteMap` directive pointing to a Python script which determines if the
requested URL needs to be redirected elsewhere.
When the script outputs a string terminated by a linebreak, Apache redirects
accordingly. However, when the script outputs `NULL` (with no linebreak),
Apache hangs and subsequent HTTP requests are effectively ignored.
The error log shows no errors. The rewrite log only shows a `pass through`
followed by a `redirect` when successful, then only `pass through` when `NULL`
is returned by the script. Subsequent requests also only show `pass through`.
Additionally, replacing `stdout` with `os.fdopen(sys.stdout.fileno(), 'w', 0)`
to set buffer length to zero did not help.
Any help would be greatly appreciated. Thank you in advance.
### /etc/apache2/httpd.conf
[...]
RewriteLock /tmp/apache_rewrite.lock
### /etc/apache2/sites-available/default
<VirtualHost *:80>
[...]
RewriteEngine on
RewriteLogLevel 1
RewriteLog /var/www/logs/rewrite.log
RewriteMap remap prg:/var/www/remap.py
[...]
</VirtualHost>
### /var/www/webroot/.htaccess
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*_.*) /${remap:$1} [R=301]
### /var/www/remap.py
#!/usr/bin/python
import sys
def getRedirect(str):
new_url = None
# if url needs to be redirected, put this value in new_url
# otherwise new_url remains None
return new_url
while True:
request = sys.stdin.readline().strip()
response = getRedirect(request)
if response:
sys.stdout.write(response + '\n')
else:
sys.stdout.write('NULL')
sys.stdout.flush()
Answer: You have to return a single newline, not 'NULL'.
Apache waits for a newline to know when the URL to be rewrite to ends. If your
script sends no newline, Apache waits forever.
So just change `return ('NULL')` to `return ('NULL\n')`, this will then
redirect to /. If you don't want this to happen, have the program to return
the URL you want when there's no match in the map.
If you want not to redirect when there's no match I would:
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond (${remap:$1}) !NULL
RewriteRule (.*_.*) /%1 [R=301]
Use a match in the RewriteCond (this would work with _NULL_ as well, of
course). But given your problem, this looks like the proper solution.
|
How do I make these relative imports work in Python 3?
Question: I have a directory structure that looks like this:
project/
__init__.py
foo/
__init.py__
first.py
second.py
third.py
plum.py
In `project/foo/__init__.py` I import classes from `first.py`, `second.py` and
`third.py` and put them in `__all__`.
There's a class in `first.py` named `WonderfulThing` which I'd like to use in
`second.py`, and want to import by importing `*` from `foo`. (It's outside of
the scope of this question why I'd like to do so, assume I have a good
reason.)
In `second.py` I've tried `from .foo import *`, `from foo import *` and `from
. import *` and in none of these cases is `WonderfulThing` imported. I also
tried `from ..foo import *`, which raises an error "Attempted relative import
beyond toplevel package".
I've read the docs and the PEP, and I can't work out how to make this work.
Any assistance would be appreciated.
**Clarification/Edit:** It seems like I may have been misunderstanding the way
`__all__` works in packages. I was using it the same as in modules,
from .first import WonderfulThing
__all__ = [ "WonderfulThing" ]
but looking at the docs again it seems to suggest that `__all__` may only be
used in packages to specify the names of modules to be imported by default;
there doesn't seem to be any way to include anything that's not a module.
Is this correct?
**Edit:** A non-wildcard import failed (`cannot import name WonderfulThing`).
Trying `from . import foo` failed, but `import foo` works. Unfortunately,
`dir(foo)` shows nothing.
Answer: Edit: I did misunderstand the question: No `__all__` is not restricted to just
modules.
One question is why you want to do a relative import. There is nothing wrong
with doing `from project.foo import *`, here. Secondly, the `__all__`
restriction on foo won't prevent you from doing `from project.foo.first import
WonderfulThing`, or just `from .first import WonderfulThing`, which still will
be the best way.
And if you really want to import a a lot of things, it's probably best to do
`from project import foo`, and then use the things with `foo.WonderfulThing`
instead for doing an `import *` and then using `WonderfulThing` directly.
However to answer your direct question, to import from the `__init__` file in
second.py you do this:
from . import WonderfulThing
or
from . import *
|
edit text file using Python
Question: I need to update a text file whenever my IP address changes, and then run a
few commands from the shell afterwards.
1. Create variable LASTKNOWN = "212.171.135.53" This is the ip address we have while writing this script.
2. Get the current IP address. It will change on a daily basis.
3. Create variable CURRENT for the new IP.
4. Compare (as strings) CURRENT to LASTKNOWN
5. If they are the same, exit()
6. If they differ,
A. "Copy" the old config file (/etc/ipf.conf) containing LASTKNOWN IP address
into /tmp B. Replace LASTKNOWN with CURRENT in the /tmp/ipf.conf file.
C. Using subprocess "mv /tmp/ipf.conf /etc/ipf.conf"
D. Using subprocess execute, "ipf -Fa -f /etc/ipf.conf"
E. Using subprocess execute, "ipnat -CF -f /etc/ipnat.conf"
7. exit()
I know how to do steps 1 through 6. I fall down on the "file editing" part, A
-> C. I can't tell what module to use or whether I should be editing the file
in place. There are so many ways to do this, I can't decide on the best
approach. I guess I want the most conservative one.
I know how to use subprocess, so you don't need to comment on that.
I don't want to replace entire lines; just a specific dotted quad.
Thanks!
Answer: Another way to simply edit files in place is to use the
[`fileinput`](http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CC8QFjAA&url=http%3A%2F%2Fdocs.python.org%2Flibrary%2Ffileinput.html&ei=TGeZUPCRHOfP0QXQhIHAAw&usg=AFQjCNEmKT3Z55eqWRpCJIDCDyy1ACOGhg)
module:
import fileinput, sys
for line in fileinput.input(["test.txt"], inplace=True):
line = line.replace("car", "truck")
# sys.stdout is redirected to the file
sys.stdout.write(line)
|
Python binary data reading
Question: A urllib2 request receives binary response as below:
00 00 00 01 00 04 41 4D 54 44 00 00 00 00 02 41
97 33 33 41 99 5C 29 41 90 3D 71 41 91 D7 0A 47
0F C6 14 00 00 01 16 6A E0 68 80 41 93 B4 05 41
97 1E B8 41 90 7A E1 41 96 8F 57 46 E6 2E 80 00
00 01 16 7A 53 7C 80 FF FF
Its structure is:
DATA, TYPE, DESCRIPTION
00 00 00 01, 4 bytes, Symbol Count =1
00 04, 2 bytes, Symbol Length = 4
41 4D 54 44, 6 bytes, Symbol = AMTD
00, 1 byte, Error code = 0 (OK)
00 00 00 02, 4 bytes, Bar Count = 2
FIRST BAR
41 97 33 33, 4 bytes, Close = 18.90
41 99 5C 29, 4 bytes, High = 19.17
41 90 3D 71, 4 bytes, Low = 18.03
41 91 D7 0A, 4 bytes, Open = 18.23
47 0F C6 14, 4 bytes, Volume = 3,680,608
00 00 01 16 6A E0 68 80, 8 bytes, Timestamp = November 23,2007
SECOND BAR
41 93 B4 05, 4 bytes, Close = 18.4629
41 97 1E B8, 4 bytes, High = 18.89
41 90 7A E1, 4 bytes, Low = 18.06
41 96 8F 57, 4 bytes, Open = 18.82
46 E6 2E 80, 4 bytes, Volume = 2,946,325
00 00 01 16 7A 53 7C 80, 8 bytes, Timestamp = November 26,2007
TERMINATOR
FF FF, 2 bytes,
How to read binary data like this?
Thanks in advance.
### Update:
I tried struct module on first 6 bytes with following code:
struct.unpack('ih', response.read(6))
(16777216, 1024)
But it should output (1, 4). I take a look at the manual but have no clue what
was wrong.
Answer: So here's my best shot at interpreting the data you're giving...:
import datetime
import struct
class Printable(object):
specials = ()
def __str__(self):
resultlines = []
for pair in self.__dict__.items():
if pair[0] in self.specials: continue
resultlines.append('%10s %s' % pair)
return '\n'.join(resultlines)
head_fmt = '>IH6sBH'
head_struct = struct.Struct(head_fmt)
class Header(Printable):
specials = ('bars',)
def __init__(self, symbol_count, symbol_length,
symbol, error_code, bar_count):
self.__dict__.update(locals())
self.bars = []
del self.self
bar_fmt = '>5fQ'
bar_struct = struct.Struct(bar_fmt)
class Bar(Printable):
specials = ('header',)
def __init__(self, header, close, high, low,
open, volume, timestamp):
self.__dict__.update(locals())
self.header.bars.append(self)
del self.self
self.timestamp /= 1000.0
self.timestamp = datetime.date.fromtimestamp(self.timestamp)
def showdata(data):
terminator = '\xff' * 2
assert data[-2:] == terminator
head_data = head_struct.unpack(data[:head_struct.size])
try:
assert head_data[4] * bar_struct.size + head_struct.size == \
len(data) - len(terminator)
except AssertionError:
print 'data length is %d' % len(data)
print 'head struct size is %d' % head_struct.size
print 'bar struct size is %d' % bar_struct.size
print 'number of bars is %d' % head_data[4]
print 'head data:', head_data
print 'terminator:', terminator
print 'so, something is wrong, since',
print head_data[4] * bar_struct.size + head_struct.size, '!=',
print len(data) - len(terminator)
raise
head = Header(*head_data)
for i in range(head.bar_count):
bar_substr = data[head_struct.size + i * bar_struct.size:
head_struct.size + (i+1) * bar_struct.size]
bar_data = bar_struct.unpack(bar_substr)
Bar(head, *bar_data)
assert len(head.bars) == head.bar_count
print head
for i, x in enumerate(head.bars):
print 'Bar #%s' % i
print x
datas = '''
00 00 00 01 00 04 41 4D 54 44 00 00 00 00 02 41
97 33 33 41 99 5C 29 41 90 3D 71 41 91 D7 0A 47
0F C6 14 00 00 01 16 6A E0 68 80 41 93 B4 05 41
97 1E B8 41 90 7A E1 41 96 8F 57 46 E6 2E 80 00
00 01 16 7A 53 7C 80 FF FF
'''
data = ''.join(chr(int(x, 16)) for x in datas.split())
showdata(data)
this emits:
symbol_count 1
bar_count 2
symbol AMTD
error_code 0
symbol_length 4
Bar #0
volume 36806.078125
timestamp 2007-11-22
high 19.1700000763
low 18.0300006866
close 18.8999996185
open 18.2299995422
Bar #1
volume 29463.25
timestamp 2007-11-25
high 18.8899993896
low 18.0599994659
close 18.4629001617
open 18.8199901581
...which seems to be pretty close to what you want, net of some output
formatting details. Hope this helps!-)
|
How can I get an accurate UTC time with Python?
Question: I wrote a desktop application and was using `datetime.datetime.utcnow()` for
timestamping, however I've recently noticed that some people using the
application get wildly different results than I do when we run the program at
the same time. Is there any way to get the UTC time locally without using
urllib to fetch it from a website?
Answer: Python depends on the underlying operating system to provide an accurate time-
of-day clock. If it isn't doing that, you don't have much choice other than to
bypass the o/s. There's a pure-Python implementation of an NTP client
[here](http://pypi.python.org/pypi/ntplib/). A very simple-minded approach:
>>> import ntplib,datetime
>>> x = ntplib.NTPClient()
>>> datetime.datetime.utcfromtimestamp(x.request('europe.pool.ntp.org').tx_time)
datetime.datetime(2009, 10, 21, 7, 1, 54, 716657)
However, it would not be very nice to be continually hitting on other NTP
servers out there. A good net citizen would use the ntp client library to keep
track of the offset between the o/s system clock and that obtained from the
server and only periodically poll to adjust the time.
|
Python problem executing popen in cron
Question: I use `popen` to execute commands in a Python script, and I call it via cron.
Cron calls out this script but the behavior isn't the same if I call it by
hand.
### Source:
from subprocess import Popen, PIPE
pp = Popen('/usr/bin/which iptables', shell=True, stdout=PIPE)
data = ''
for ln in pp.stdout:
data = data+ln
if data == '':
print 'ko'
else:
print 'ok : '+data
### By hand:
# python /home/user/test.py
> : /sbin/iptables
### By cron (in /tmp/err_cron):
* * * * * /usr/bin/python /home/user/test.py >> /tmp/err_cron
ko
ko
ko
Why does cron not run this script normally?
Answer: Normally when processes are run from cron, the `PATH` is set to a very
restrictive value (the man page for my crontab says `/usr/bin:/bin`). You may
need to add:
PATH=/usr/bin:/bin:/sbin
to the top of your crontab file.
|
Interacting with SVN from appengine
Question: I've got a couple of projects where it would be useful to be able to interact
with an SVN server from appengine.
* Pull specific files from the svn (fairly easy, since there is a web interface which I can grab the data off automatically, but how do I authenticate)
* Commit changes to the svn (this is the really hard/important part)
* Possibly run an SVN server (from an appengine app, I'm guessing this isn't possible)
I would prefer a python solution, but I can survive with java if I must
Answer: you can try using [SVNKit](http://svnkit.com/) with the java runtime
|
How can I grab the color of a pixel on my desktop? (Linux)
Question: I want to grab the color of a pixel with known coordinates on my Linux
desktop.
Until now, I've used `"import -window SomeWindow -crop 1x1+X+Y /tmp/grab.jpg"`
then extracting the pixel value using Python and
[PIL](http://en.wikipedia.org/wiki/Python%5FImaging%5FLibrary).
This does the job, but since import grabs the whole window before cropping,
it's very slow :(
Are there any clever way to grab the color of only one pixel? I know both
relative (window) and absolute coordinates.
A Python or shell script would be preferable, but if you know some clever
C/[X11](http://en.wikipedia.org/wiki/X%5FWindow%5FSystem) functions, also
please let me know :)
Answer: This does the trick, but requires python-gtk:
import gtk.gdk
import sys
def PixelAt(x, y):
w = gtk.gdk.get_default_root_window()
sz = w.get_size()
pb = gtk.gdk.Pixbuf(gtk.gdk.COLORSPACE_RGB,False,8,sz[0],sz[1])
pb = pb.get_from_drawable(w,w.get_colormap(),0,0,0,0,sz[0],sz[1])
pixel_array = pb.get_pixels_array()
return pixel_array[y][x]
print PixelAt(int(sys.argv[1]), int(sys.argv[2]))
On Ubuntu 9.10, this also requires python-numpy or it segfaults the python
interpreter on the `get_pixels_array` line. Ubuntu 10.04 it still has this
requirement, or it causes an ImportError regarding numpy.core.multiarray.
|
Load different modules without changing the logic file
Question: Suppose I've got 2 different modules which have the uniform(same) interfaces.
The files list like this:
root/
logic.py
sns_api/
__init__.py
facebook/
pyfacebook.py
__init__.py
myspace/
pymyspace.py
__init__.py
And pyfacebook.py and pymyspace.py have the same interfaces, which means:
# in pyfacebook.py
class Facebook:
def __init__(self, a, b):
# do the init
def method1(self, a, b, ...):
# do the logic
# in pymyspace.py
class Myspace:
def __init__(self, a, b):
# do the init
def method1(self, a, b, ...):
# do the logic
Now I have a question. I want to do the logic in logic.py without duplicating
the codes, so I'm wondering how can I just set a flag to show which module I
use and python will load the right codes automatically, which means:
# in logic.py
PLATFORM = "facebook"
# import the right modules in, complete the logic with the current platform
# create the right instance and invoke the right methods
Then I change PLATFORM = 'myspace', the logic will work automatically.
So how can I do this?
I'm wondering whether using the dynamic importing will work, or eval raw
python codes, but seems not a good solution. Or if I can make a uniform
wrapper in
sns_api/__init__.py
Anyone can help?
Answer: With just two i'd do
if platform == 'facebook':
from pyfacebook import FaceBook as Platform
elif platform == 'myspace':
from pymyspace import Myspace as Platform
else:
raise RuntimeError, "not a valid platform"
and use `Platform` in the rest of the code. It's done like this in the
library, see the `os` module.
You can do really dynamic imports using `name =
`[`__import__`](http://docs.python.org/library/functions.html#%5F%5Fimport%5F%5F)`('module')`,
but you probably don't need this.
|
How import Pydev project into interactive console?
Question: Newbie question (I'm just getting started with Python and Pydev):
I've created a project "Playground" with (standard?) src/root sub-folder. In
there I've created example.py.
How do I import my "example" module into Pydev's interactive console? ">>>
import example" gives: "ImportError: No module named example"
Answer: You need to set your PYTHONPATH accordingly (Google search is your friend) or
use *.pth in your installation _site-packages_ directory pointing to your
project path. Don't forget to set your interpreter details with Pydev
(Window->Preferences->Pydev->interpreter).
|
Storing Images on App Engine using Django
Question: I'm trying to upload and save a resized image in a db.BlobProperty field on
Google App Engine using Django.
the relevant part of my view that process the request looks like this:
image = images.resize(request.POST.get('image'), 100, 100)
recipe.large_image = db.Blob(image)
recipe.put()
Which seems like it would be the logical django equivalent of the example in
the docs:
from google.appengine.api import images
class Guestbook(webapp.RequestHandler):
def post(self):
greeting = Greeting()
if users.get_current_user():
greeting.author = users.get_current_user()
greeting.content = self.request.get("content")
avatar = images.resize(self.request.get("img"), 32, 32)
greeting.avatar = db.Blob(avatar)
greeting.put()
self.redirect('/')
(source:
<http://code.google.com/appengine/docs/python/images/usingimages.html#Transform>)
But, I keep getting an error that says: NotImageError / Empty image data.
and refers to this line:
image = images.resize(request.POST.get('image'), 100, 100)
I'm having trouble getting to the image data. Seems like it's not being
uploaded but I can't figure out why. My form has the enctype="multipart/form-
data" and all that. I think something's wrong with how I'm referring to the
image data. "request.POST.get('image')" but I can't figure out how else to
reference it. Any ideas?
Thanks in advance.
Answer: After some guidance from "hcalves" I figured out the problem. First of all,
the default version of Django that comes bundled with App Engine is version
0.96 and how the framework handles uploaded files has changed since then.
However in order to maintain compatibility with older apps you have to
explicitly tell App Engine to use Django 1.1 like this:
from google.appengine.dist import use_library
use_library('django', '1.1')
You can read more about that [in the app engine
docs](http://code.google.com/appengine/docs/python/tools/libraries.html).
Ok, so here's the solution:
from google.appengine.api import images
image = request.FILES['large_image'].read()
recipe.large_image = db.Blob(images.resize(image, 480))
recipe.put()
Then, to serve the dynamic images back again from the datastore, build a
handler for images like this:
from django.http import HttpResponse, HttpResponseRedirect
def recipe_image(request,key_name):
recipe = Recipe.get_by_key_name(key_name)
if recipe.large_image:
image = recipe.large_image
else:
return HttpResponseRedirect("/static/image_not_found.png")
#build your response
response = HttpResponse(image)
# set the content type to png because that's what the Google images api
# stores modified images as by default
response['Content-Type'] = 'image/png'
# set some reasonable cache headers unless you want the image pulled on every request
response['Cache-Control'] = 'max-age=7200'
return response
|
dynamically adding functions to a Python module
Question: Our framework requires wrapping certain functions in some ugly boilerplate
code:
def prefix_myname_suffix(obj):
def actual():
print 'hello world'
obj.register(actual)
return obj
I figured this might be simplified with a decorator:
@register
def myname():
print 'hello world'
However, that turned out to be rather tricky, mainly because the framework
looks for a certain pattern of function names at module level.
I've tried the following within the decorator, to no avail:
current_module = __import__(__name__)
new_name = prefix + func.__name__ + suffix
# method A
current_module[new_name] = func
# method B
func.__name__ = new_name
current_module += func
Any help would be appreciated!
Answer: use either
current_module.new_name = func
or
setattr(current_module, new_name, func)
|
Python - RegExp - Modify text files
Question: Newbie to Python.... help requested with the following task :-)
I have tree of various files, some of them are C source code. I would like to
modify these C files with python script.
The C code has 4 defines -
#define ZR_LOG0(Id, Class, Seveity, Format)
#define ZR_LOG1(Id, Class, Seveity, Format, Attr0)
#define ZR_LOG2(Id, Class, Seveity, Format, Attr0, Attr1)
#define ZR_LOG3(Id, Class, Seveity, Format, Attr0, Attr1, Attr2)
there are various ZR_LOGn lines spread throughout the C source code.
Example: ZR_LOG1 (1, LOG_CLASS_3, LOG_INFO, "hello world %d", 76);
White spaces (spaces, tabs) may appear anywhere in the line (between the
fields).
The python script task is as follow:
1. Replace any 'Id' field (which is an integer type that we don't care about its original value) with sequential counter. (The first 'LOG'... line we'll encounter the 'Id' field will get the value 0, the next one 1, and so on)
2. In a separate output file, for each ZR_LOG line, we'll create an index line in the format _{ NewId, Format },_ For the example above will get:
{ 0, "hello world %d" },
Appreciate your help with it....
* * *
I have started with the following code, you may either look at it or ignore it
altogether.
'''
Created on Oct 25, 2009
@author: Uri Shkolnik
The following version does find & replace LOG Ids for all
C source files in a dir (and below) with sequential counter,
The files are assumed to be UTF-8 encoded.
(which works fine if they are ASCII, because ASCII is a
subset of UTF-8)
It also assemble new index file, composed from all new IDs and format fields
'''
import os, sys, re, shutil
mydir= '/home/uri/proj1'
searched_pattern0 = 'ZR_LOG0'
def search_and_replace(filepath):
''' replaces all string by a regex substitution '''
backupName=filepath+'~re~'
print 'reading:', filepath
input = open(filepath,'rb')
s=unicode(input.read(),'utf-8')
input.close()
m = re.match(ur'''[:space:]ZR_LOG[0-3].*\(.*[0-9]{0,10},LOG_''', s)
print m
def c_files_search(dummy, dirr, filess):
''' search directories for file pattern *.c '''
for child in filess:
if '.c' == os.path.splitext(child)[1] and os.path.isfile(dirr+'/'+child):
filepath = dirr+'/'+child
search_and_replace(filepath)
os.path.walk(mydir, c_files_search, 3)
Answer: A few points:
* You can match whitespace with '\s'.
* The regexp 'capturing groups' are useful here.
So, I would do something like this:
output = ''
counter = 1
for line in lines:
# Match only ZR_LOG lines and capture everything surrounding "Id"
match = re.match('^(.*\sZR_LOG[0-3]\s*\(\s*)' # group(1), before Id
'Id'
'(,.*)$', # group(2), after Id
line)
if match:
# Add everything before Id, the counter value and everything after Id
output += match.group(1) + str(counter) + match.group(2)
counter += 1
# And do extra logging etc.
else:
output += line
|
Using the same handler for multiple wx.TextCtrls?
Question: I'm having a bit of trouble with a panel that has two wxPython TextCtrls in
it. I want either an EVT_CHAR or EVT_KEY_UP handler bound to both controls,
and I want to be able to tell which TextCtrl generated the event. I would
think that event.Id would tell me this, but in the following sample code it's
always 0. Any thoughts? I've only tested this on OS X.
This code simply checks that both TextCtrls have some text in them before
enabling the Done button
import wx
class MyFrame(wx.Frame):
def __init__(self, parent, ID, title):
wx.Frame.__init__(self, parent, ID, title,
wx.DefaultPosition, wx.Size(200, 150))
self.panel = BaseNameEntryPanel(self)
class BaseNameEntryPanel(wx.Panel):
def __init__(self, parent):
wx.Panel.__init__(self, parent, -1)
self.entry = wx.TextCtrl(self, wx.NewId())
self.entry2 = wx.TextCtrl(self, wx.NewId())
self.donebtn = wx.Button(self, wx.NewId(), "Done")
self.donebtn.Disable()
vsizer = wx.BoxSizer(wx.VERTICAL)
vsizer.Add(self.entry, 1, wx.EXPAND|wx.GROW)
vsizer.Add(self.entry2, 1, wx.EXPAND|wx.GROW)
vsizer.Add(self.donebtn, 1, wx.EXPAND|wx.GROW)
self.SetSizer(vsizer)
self.Fit()
self.entry.Bind(wx.EVT_KEY_UP, self.Handle)
self.entry2.Bind(wx.EVT_KEY_UP, self.Handle)
def Handle(self, event):
keycode = event.GetKeyCode()
print keycode, event.Id # <- event.Id is always 0!
def checker(entry):
return bool(entry.GetValue().strip())
self.donebtn.Enable(checker(self.entry) and checker(self.entry2))
class MyApp(wx.App):
def OnInit(self):
frame = MyFrame(None, -1, "Hello from wxPython")
frame.Show(True)
self.SetTopWindow(frame)
return True
app = MyApp(0)
app.MainLoop()
Answer: You could try `event.GetId()` or `event.GetEventObject()` and see if either of
these work.
Another approach to this is to use lambda or functools.partial to effectively
pass a parameter to the handler. So, for example, sub in the lines below into
your program:
self.entry.Bind(wx.EVT_KEY_UP, functools.partial(self.Handle, ob=self.entry))
self.entry2.Bind(wx.EVT_KEY_UP, functools.partial(self.Handle, ob=self.entry2))
def Handle(self, event, ob=None):
print ob
And then ob will be either `entry` or `entry2` depending on which panel is
clicked. But, of course, this shouldn't be necessary, and GetId and
GetEventObject() should both work -- though I don't (yet) have a Mac to try
these on.
|
Terminate a multi-thread python program
Question: How to make a multi-thread python program response to Ctrl+C key event?
**Edit:** The code is like this:
import threading
current = 0
class MyThread(threading.Thread):
def __init__(self, total):
threading.Thread.__init__(self)
self.total = total
def stop(self):
self._Thread__stop()
def run(self):
global current
while current<self.total:
lock = threading.Lock()
lock.acquire()
current+=1
lock.release()
print current
if __name__=='__main__':
threads = []
thread_count = 10
total = 10000
for i in range(0, thread_count):
t = MyThread(total)
t.setDaemon(True)
threads.append(t)
for i in range(0, thread_count):
threads[i].start()
I tried to remove join() on all threads but it still doesn't work. Is it
because the lock segment inside each thread's run() procedure?
**Edit:** The above code is supposed to work but it always interrupted when
current variable was in 5,000-6,000 range and through out the errors as below
Exception in thread Thread-4 (most likely raised during interpreter shutdown):
Traceback (most recent call last):
File "/usr/lib/python2.5/threading.py", line 486, in __bootstrap_inner
File "test.py", line 20, in run
<type 'exceptions.TypeError'>: unsupported operand type(s) for +=: 'NoneType' and 'int'
Exception in thread Thread-2 (most likely raised during interpreter shutdown):
Traceback (most recent call last):
File "/usr/lib/python2.5/threading.py", line 486, in __bootstrap_inner
File "test.py", line 22, in run
Answer: Make every thread except the main one a daemon (`t.daemon = True` in 2.6 or
better, `t.setDaemon(True)` in 2.6 or less, for every thread object `t` before
you start it). That way, when the main thread receives the KeyboardInterrupt,
if it doesn't catch it or catches it but decided to terminate anyway, the
whole process will terminate. See [the
docs](http://docs.python.org/library/threading.html?highlight=daemon#threading.Thread.daemon).
**edit** : having just seen the OP's code (not originally posted) and the
claim that "it doesn't work", it appears I have to add...:
Of course, if you want your main thread to stay responsive (e.g. to
control-C), don't mire it into blocking calls, such as `join`ing another
thread -- especially not totally _useless_ blocking calls, such as `join`ing
**daemon** threads. For example, just change the final loop in the main thread
from the current (utterless and damaging):
for i in range(0, thread_count):
threads[i].join()
to something more sensible like:
while threading.active_count() > 0:
time.sleep(0.1)
if your main has nothing better to do than either for all threads to terminate
on their own, or for a control-C (or other signal) to be received.
Of course, there are many other usable patterns if you'd rather have your
threads not terminate abruptly (as daemonic threads may) -- unless _they_ ,
too, are mired forever in unconditionally-blocking calls, deadlocks, and the
like;-).
|
Unable to query from entities loaded onto the app engine datastore
Question: I am a newbie to python. I am not able to query from the entities- UserDetails
and PhoneBook I loaded to the app engine datastore. I have written this UI
below based on the youtube video by Brett on "Developing and Deploying
applications on GAE" -- shoutout application. Well I just tried to do some
reverse engineering to query from the datastore but failed in every step.
#!/usr/bin/env python
import wsgiref.handlers
from google.appengine.ext import db
from google.appengine.ext import webapp
from google.appengine.ext.webapp import template
import models
class showPhoneBook(db.Model):
""" property to store user_name from UI to persist for the session """
user_name = db.StringProperty(required=True)
class MyHandler(webapp.RequestHandler):
def get(self):
## Query to get the user_id using user_name retrieved from UI ##
p = UserDetails.all().filter('user_name = ', user_name)
result1 = p.get()
for itr1 in result1:
userId = itr.user_id
## Query to get the phone book contacts using user_id retrieved ##
q = PhoneBook.all().filter('user_id = ', userId)
values = {
'phoneBookValues': q
}
self.request.out.write(
template.render('phonebook.html', values))
def post(self):
phoneBookuser = showPhoneBook(
user_name = self.request.get('username'))
phoneBookuser.put()
self.redirect('/')
def main():
app = webapp.WSGIApplication([
(r'.*',MyHandler)], debug=True)
wsgiref.handlers.CGIHandler().run(app)
if __name__ == "__main__":
main()
This is my models.py file where I've defined my UserDetails and PhoneBook
classes,
#!/usr/bin/env python
from google.appengine.ext import db
#Table structure of User Details table
class UserDetails(db.Model):
user_id = db.IntegerProperty(required = True)
user_name = db.StringProperty(required = True)
mobile_number = db.PhoneNumberProperty(required = True)
#Table structure of Phone Book table
class PhoneBook(db.Model):
contact_id = db.IntegerProperty(required=True)
user_id = db.IntegerProperty(required=True)
contact_name = db.StringProperty(required=True)
contact_number = db.PhoneNumberProperty(required=True)
Here are the problems I am facing,
1) I am not able to call `user_name` (retrieved from UI-- `phoneBookuser =
showPhoneBook(user_name = self.request.get('username'))`) in get(self) method
for querying UserDetails to to get the corresponding `user_name`.
2) The code is not able to recognize UserDetails and PhoneBook classes when
importing from models.py file.
3) I tried to define UserDetails and PhoneBook classes in the main.py file
itself, them I get the error at `result1 = p.get()` saying `BadValueError:
Unsupported type for property : <class
'google.appengine.ext.db.PropertiedClass'>`
I have been struggling since 2 weeks to get through the mess I am into but in
vain. Please help me out in straightening out my code('coz I feel what I've
written is a error-prone code all the way).
Answer: I recommend that you read the Python documentation of GAE found
[here](http://code.google.com/appengine/docs/python/gettingstarted/).
Some comments:
1. To use your models found in `models.py`, you either need to use the prefix `models.` (e.g. `models.UserDetails`) or import them using
from models import *
2. in `MyHandler.get()` you don't lookup the `username` get parameter
3. To fetch values corresponding to a query, you do `p.fetch(1)` not `p.get()`
4. You should also read [Reference properties in GAE](http://code.google.com/appengine/docs/python/datastore/entitiesandmodels.html#References) as well. I recommend you having your models as:
class UserDetails(db.Model):
user_name = db.StringProperty(required = True)
mobile_number = db.PhoneNumberProperty(required = True)
#Table structure of Phone Book table
class PhoneBook(db.Model):
user = db.ReferenceProperty(UserDetails)
contact_name = db.StringProperty(required=True)
contact_number = db.PhoneNumberProperty(required=True)
Then your `MyHandler.get()` code will look like:
def get(self):
## Query to get the user_id using user_name retrieved from UI ##
user_name = self.request.get('username')
p = UserDetails.all().filter('user_name = ', user_name)
user = p.fetch(1)[0]
values = {
'phoneBookValues': user.phonebook_set
}
self.response.out.write(template.render('phonebook.html', values))
(Needless to say, you need to handle the case where the username is not found
in the database)
5. I don't quite understand the point of `showPhoneBook` model.
|