I thought I understood how import worked...

[Roy Smith]

I've been tracking down some weird import problems we've been having with 
django.  Our settings.py file is getting imported twice.  It has some 
non-idempotent code in it, and we blow up on the second import.

I thought modules could not get imported twice.  The first time they get 
imported, they're cached, and the second import just gets you a reference to the 
original.  Playing around, however, I see that it's possible to import a module 
twice if you refer to it by different names.  Here's a small-ish test case which 
demonstrates what I'm talking about (python 2.6.5):

In directory /home/roy/play/import/foo, I've got:

__init__.py  (empty file)
try.py
broken.py


$ cat broken.py
print __file__


$ cat try.py 
import broken
import foo.broken

import sys
for m in sys.modules.items():
    if m[0].endswith('broken'):
        print m


And when I run try.py (with foo as the current directory):

$ PYTHONPATH=/home/roy/play/import python try.py
/home/roy/play/import/foo/broken.pyc
/home/roy/play/import/foo/broken.pyc
('broken', <module 'broken' from '/home/roy/play/import/foo/broken.pyc'>)
('foo.broken', <module 'foo.broken' from '/home/roy/play/import/foo/broken.pyc'>)


So, it appears that you *can* import a module twice, if you refer to it by 
different names!  This is surprising.  It means that having non-idempotent code 
which is executed at import time is a Bad Thing.

It also means that you could have multiple copies of a module's global 
namespace, depending on how your users imported the module.  Which is kind of 
mind-blowing.