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if module was marked as not existing by setting sys.modules [fullname] to None, then pkgutil.get_loader (fullname) will throw AttributeError.
Example:
#! /usr/bin/evn python
import unittest
import pkgutil
def main ():
 pkgutil.get_loader ('unittest.functools')
if __name__ == '__main__':
 main ()
Patch is attached

So I'm no pkgutil expert, but at least in the case of None in sys.modules, that triggers at least an ImportError in __import__ if that is come across.

Main use case of pkgutil.get_loader is to determine if its possible to load module and either return loader or None. But it throws exception more over it is AttributeErrror exception.

I'm not yet sure the proposed fix in the patch is the right approach (I need to look at the surrounding code), but I believe Pavel's right that get_loader() should be returning None in this case instead of throwing an exception.

Update as of Python 3.4: pkgutil.get_loader() still throws AttributeError for this case, but importlib.util.find_spec() returns None as expected.

This function is broken again in version 3.4
The way it should look is:
Python 2.7.6 (default, Feb 26 2014, 12:07:17) 
[GCC 4.8.2 20140206 (prerelease)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkgutil
>>> pkgutil.get_loader('no_such_module') # returns None
>>> 
How it really looks:
Python 3.4.0 (default, Apr 27 2014, 23:33:09) 
[GCC 4.8.2 20140206 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import pkgutil
>>> pkgutil.get_loader('no_such_module')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib/python3.4/pkgutil.py", line 467, in get_loader
 return find_loader(fullname)
 File "/usr/lib/python3.4/pkgutil.py", line 488, in find_loader
 return spec.loader
AttributeError: 'NoneType' object has no attribute 'loader'
>>>
find_loader is at fault (change "return spec.loader" -> "return spec and spec.loader"). Thanks.

I'll take a look the next time I have some Python time (in a week or two) and make sure this gets dealt with.

New changeset 660c82192c69 by Brett Cannon in branch '3.4':
Issue #14710: Fix both pkgutil.find_loader() and get_loader() to not
http://hg.python.org/cpython/rev/660c82192c69
New changeset 1adc8eb362f0 by Brett Cannon in branch 'default':
Merge for issue #14710
http://hg.python.org/cpython/rev/1adc8eb362f0 

Thanks for the bug report, Pavel. It turned out pkgutil.find_loader is broken as well as pkgutil.get_loader in different ways.