I thought I understood how import worked...

[Mark Lawrence]

On 07/08/2012 14:18, Roy Smith wrote:
> 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.
>

Maybe not directly applicable to what you're saying, but Brett Cannon 
ought to know something about the import mechanism. I believe he's been 
working on it on and off for several years.  See 
http://docs.python.org/dev/whatsnew/3.3.html for a starter on the gory 
details.

-- 
Cheers.

Mark Lawrence.