site.py confusion

George Trojan george.trojan at noaa.gov
Wed Jan 27 19:48:23 CET 2010


Arnaud Delobelle wrote:
> George Trojan <george.trojan at noaa.gov> writes:
> 
>> Inspired by the 'Default path for files' thread I tried to use
>> sitecustomize in my code. What puzzles me is that the site.py's main()
>> is not executed. My sitecustomize.py is
>> def main():
>>     print 'In Main()'
>> main()
>> and the test program is
>> import site
>> #site.main()
>> print 'Hi'
>> The output is
>> $ python try.py
>> Hi
> 
> That's normal as site.py is automatically imported on initialisation.
> So when you do:
> 
>     import site
> 
> the module is not re-executed as it already has been imported.
> Try this:
> 
>     ---- file: foo.py
>     print 'Foo'
>     ---- end
> 
>     --- Interactive session
>     >>> import foo # First time, print statement executed
>     Foo
>     >>> import foo # Second time, print statement not executed
>     >>>
> 
>> When I uncomment the site.main() line the output is
>> $ python try.py
>> In Main()
>> Hi
> 
> Now you explicitely call site.main(), so it executes it!
> 
>> If I change import site to import sitecustomize the output is as
>> above. What gives?
> 
> It's normal, this time it's the first time you import it so its content
> is executed.
> 
> HTH
> 
I understand that importing a module repeatedly does nothing. Also, I 
made a typo in my previous posting - I meant sitecustomize.main(), not 
site.main(). My understanding of the code in site.py is that when the 
module is imported, main() is executed. main() calls execsitecustomize() 
that attempts to import sitecustomize. That action should trigger 
execution of code in sitecustomize.py, which is located in the current 
directory. But that does not work. I changed execsitecustomize() to

def execsitecustomize():
     """Run custom site specific code, if available."""
     try:
         import sitecustomize
     except ImportError:
         pass
         import sys
         print sys.path
         raise

That gave me the explanation why the above happens: when site is 
imported, the current directory is not yet prepended to sys.path.

$ python2.6 -v try.py
...
  ['/usr/local/Python-2.6.3/lib/python26.zip', 
'/usr/local/Python-2.6.3/lib/python2.6', 
'/usr/local/Python-2.6.3/lib/python2.6/plat-linux2', 
'/usr/local/Python-2.6.3/lib/python2.6/lib-tk', 
'/usr/local/Python-2.6.3/lib/python2.6/lib-old', 
'/usr/local/Python-2.6.3/lib/python2.6/lib-dynload', 
'/usr/local/Python-2.6.3/lib/python2.6/site-packages']
'import site' failed; traceback:
Traceback (most recent call last):
   File "/usr/local/Python-2.6.3/lib/python2.6/site.py", line 516, in 
<module>
     main()
   File "/usr/local/Python-2.6.3/lib/python2.6/site.py", line 507, in main
     execsitecustomize()
   File "/usr/local/Python-2.6.3/lib/python2.6/site.py", line 472, in 
execsitecustomize
     import sitecustomize
...

This also explains the recipe http://code.activestate.com/recipes/552729/

I wanted to have library location specific to application without having 
to explicitly change sys.path in all App-Top-Dir/bin/*.py. I thought 
creating bin/sitecustomize.py would do the trick.

I guess the documentation should mention this fact. The comment in 
recipe 552729 is:

Since Python 2.5 the automatic import of the module "sitecustomize.py" 
in the directory of the main program is not supported any more (even if 
the documentation says that it is).

George



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