[Python-Dev] unexpected import behaviour

[Michael Foord]

On 31/07/2010 16:07, Nick Coghlan wrote:
> On Sat, Jul 31, 2010 at 3:57 PM, Daniel Waterworth
> <da.waterworth at gmail.com>  wrote:
>    
>> @Nick: I suppose the simplest way to detect re-importation in the
>> general case, is to store a set of hashes of files that have been
>> imported. When a user tries to import a file where it's hash is
>> already in the set, a warning is generated. It's simpler than trying
>> to figure out all the different ways that a file can be imported, and
>> will also detect copied files. This is less infrastructure than you
>> were suggesting, but it's not a perfect solution.
>>      
> Hashing every file on import would definitely be more overhead than
> just checking __file__ values (since we already calculate the latter,
> and regardless of how a file is imported, it needs to end up in
> sys.modules eventually). Besides, importing the same code under
> different names happens in several places in our own test suite (we
> use it to check that code behaviour doesn't change just because we
> import it differently), so we can hardly disable that behaviour.
>
> That said, I really don't think catching such a rare error is worth
> *any* runtime overhead. Just making "__main__" and the real module
> name refer to the same object in sys.modules is a different matter,
> but I'm not confident enough that I fully grasp the implications to do
> it without gathering feedback from a wider audience.
>
>    

Some people workaround the potential for bugs caused by __main__ 
reimporting itself by doing it *deliberately*. Glyf even recommends it 
as good practise. ;-)

http://glyf.livejournal.com/60326.html

So - the fix you suggest would *break* this code. Raising a warning 
wouldn't... (and would eventually make this workaround unnecessary.)

Michael
> Cheers,
> Nick.
>
>    


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