Q:

How to unimport a python module which is already imported?

I'm quite new with NumPy/SciPy. But these days, I've started using it very actively for numerical calculation instead of using Matlab. 
For some simple calculations, I do just in the interactive mode rather than writing a script. In this case, are there any ways to unimport some modules which was already imported?  Unimporting might not needed when I write python programs, but in the interactive mode, it is needed.

A:

There's no way to unload something once you've imported it. Python keeps a copy of the module in a cache, so the next time you import it it won't have to reload and reinitialize it again.
If all you need is to lose access to it, you can use del:
import package
del package

If you've made a change to a package and you want to see the updates, you can reload it. Note that this won't work in some cases, for example if the imported package also needs to reload a package it depends on. You should read the relevant documentation before relying on this.
For Python versions up to 2.7, reload is a built-in function:
reload(package)

For Python versions 3.0 to 3.3 you can use imp.reload:
import imp
imp.reload(package)

For Python versions 3.4 and up you can use importlib.reload:
import importlib
importlib.reload(package)

A:

While you shouldn't worry about "unimporting" a module in Python, you can normally just simply decrement the reference to the imported module or function using del:
>>> import requests
>>> dir()
['__builtins__', '__doc__', '__name__', '__package__', 'readline', 'requests', 'rlcompleter']
>>> del requests
>>> dir()
['__builtins__', '__doc__', '__name__', '__package__', 'readline', 'rlcompleter']
>>>

Note that I'd advise just not worrying about this as the overhead of an unused import is near trivial -- traversing one extra entry in sys.modules is nothing compared to the false security del some_module will give you (consider if the __init__  does some setup or you ran from X import *).

A:

When you import a module, Python looks if this module is in sys.modules dictionary. If not, it runs the module, and puts the resulting module object in sys.modules.
So, to unimport a module, one might use
import sys
sys.modules.pop('your_module_name_here')

The next time it is imported, python will re-execute the module. However, in all other loaded modules that have imported the module, the old code will still be used, as they still hold the reference to the old module object. reload function explained in other answers helps with that situation by updating the module object rather than creating a brand new one. But it doesn't help with from your_module import some_function style of imports.

