[Python-Dev] avoiding accidental shadowing of top-level libraries by the main module

[Michael Foord]

On 12/07/2010 22:59, Antoine Pitrou wrote:
> On Mon, 12 Jul 2010 17:47:31 -0400
> Fred Drake<fdrake at acm.org>  wrote:
>
>    
>> On Mon, Jul 12, 2010 at 5:42 PM, Michael Foord
>> <fuzzyman at voidspace.org.uk>  wrote:
>>      
>>> I'm sure Brett will love this idea, but if it was impossible to reimport the
>>> script being executed as __main__ with a different name it would solve these
>>> problems.
>>>        
>> Indeed!  And I'd be quite content with such a solution, since I
>> consider scripts and modules to be distinct.
>>      
> Except that modules can often be executed as scripts...
>    

Allowing a module-executed-as-script to be in sys.modules twice with 
different names and classes is still a recipe for problems. For example 
see this blog entry from Phil Hassey, an experienced Python developer, 
caused by this issue:

http://www.philhassey.com/blog/2009/07/16/oddball-python-252-import-issue/

For scripts that need to be in sys.modules with their "real" name (e.g. 
modules executed with python -m) the following trivial workaround is 
possible:

if __name__ == '__main__':
sys.modules[real_name] = sys.modules['__main__']

That avoids the problem of error messages like:

TypeError: unbound method foo() must be called with A instance as first 
argument (got A instance instead)

(Two different classes with the same name created - one from __main__ 
and one from real_name.) Use cases for *genuinely* reimporting the same 
module with different names (as different module objects rather than 
aliases) are relatively rare, and the problem of modules *accidentally* 
reimporting themselves not that rare.

All the best,

Michael




> Antoine.
>
>
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