Q:

Getting the remaining arguments in argparse

I want to get all the remaining unused arguments at once. How do I do it?
parser.add_argument('-i', action='store', dest='i', default='i.log')
parser.add_argument('-o', action='store', dest='o', default='o.log')

A:

Use parse_known_args():
args, unknownargs = parser.parse_known_args()

A:

Use argparse.REMAINDER:
parser.add_argument('rest', nargs=argparse.REMAINDER)

Example:
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-i', action='store', dest='i', default='i.log')
parser.add_argument('-o', action='store', dest='o', default='o.log')
parser.add_argument('rest', nargs=argparse.REMAINDER)
parser.parse_args(['hello', 'world'])
>>> Namespace(i='i.log', o='o.log', rest=['hello', 'world'])

A:

I went and coded up the three suggestions given as answers in this thread. The test code appears at the bottom of this answer. Conclusion: The best all-around answer so far is nargs=REMAINDER, but it might really depend on your use case.
Here are the differences I observed:
(1) nargs=REMAINDER wins the user-friendliness test.
$ python test.py --help
Using nargs=*          : usage: test.py [-h] [-i I] [otherthings [otherthings ...]]
Using nargs=REMAINDER  : usage: test.py [-h] [-i I] ...
Using parse_known_args : usage: test.py [-h] [-i I]

(2) nargs=* quietly filters out the first -- argument on the command line, which seems bad. On the other hand, all methods respect -- as a way to say "please don't parse any more of these strings as known args."
$ ./test.py hello -- -- cruel -- -- world
Using nargs=*          : ['hello', '--', 'cruel', '--', '--', 'world']
Using nargs=REMAINDER  : ['hello', '--', '--', 'cruel', '--', '--', 'world']
Using parse_known_args : ['hello', '--', '--', 'cruel', '--', '--', 'world']

$ ./test.py -i foo -- -i bar
Using nargs=*          : ['-i', 'bar']
Using nargs=REMAINDER  : ['--', '-i', 'bar']
Using parse_known_args : ['--', '-i', 'bar']

(3) Any method except parse_known_args dies if it tries to parse a string beginning with - and it turns out not to be valid.
$ python test.py -c hello
Using nargs=*          : "unrecognized arguments: -c" and SystemExit
Using nargs=REMAINDER  : "unrecognized arguments: -c" and SystemExit
Using parse_known_args : ['-c', 'hello']

(4) nargs=REMAINDER completely stops parsing when it hits the first unknown argument. parse_known_args will gobble up arguments that are "known", no matter where they appear on the line (and will die if they look malformed).
$ python test.py hello -c world
Using nargs=*          : "unrecognized arguments: -c world" and SystemExit
Using nargs=REMAINDER  : ['hello', '-c', 'world']
Using parse_known_args : ['hello', '-c', 'world']

$ python test.py hello -i world
Using nargs=*          : ['hello']
Using nargs=REMAINDER  : ['hello', '-i', 'world']
Using parse_known_args : ['hello']

$ python test.py hello -i
Using nargs=*          : "error: argument -i: expected one argument" and SystemExit
Using nargs=REMAINDER  : ['hello', '-i']
Using parse_known_args : "error: argument -i: expected one argument" and SystemExit

Here's my test code.
#!/usr/bin/env python

import argparse
import sys

def using_asterisk(argv):
    parser = argparse.ArgumentParser()
    parser.add_argument('-i', dest='i', default='i.log')
    parser.add_argument('otherthings', nargs='*')
    try:
        options = parser.parse_args(argv)
        return options.otherthings
    except BaseException as e:
        return e

def using_REMAINDER(argv):
    parser = argparse.ArgumentParser()
    parser.add_argument('-i', dest='i', default='i.log')
    parser.add_argument('otherthings', nargs=argparse.REMAINDER)
    try:
        options = parser.parse_args(argv)
        return options.otherthings
    except BaseException as e:
        return e

def using_parse_known_args(argv):
    parser = argparse.ArgumentParser()
    parser.add_argument('-i', dest='i', default='i.log')
    try:
        options, rest = parser.parse_known_args(argv)
        return rest
    except BaseException as e:
        return e

if __name__ == '__main__':
    print 'Using nargs=*          : %r' % using_asterisk(sys.argv[1:])
    print 'Using nargs=REMAINDER  : %r' % using_REMAINDER(sys.argv[1:])
    print 'Using parse_known_args : %r' % using_parse_known_args(sys.argv[1:])

