[Python-Dev] unexpected import behaviour

[Michael Foord]

On 29/07/2010 07:32, Daniel Waterworth wrote:
> Hi,
>
> I'm not sure if this is a bug or not, I certainly didn't expect it. If
> you create a file called test.py with the following contents,
>    

The issue is that when your code is executed as a script it is run as 
the __main__ module and not as the test module. When you import test you 
then import the same module with a different name - and your Test class 
is recreated (so __main__.Test is then different from test.Test). When 
you import your code as test and it then reimports itself it is only 
created once.

This *is* expected behaviour (not a bug), but it frequently confuses 
even relatively experienced programmers (it can happen by accident and 
cause hard to track down bugs) and I personally think that Python would 
be improved by issuing a warning if a __main__ script reimports itself.

All the best,

Michael Foord

> class Test:
>      pass
>
> def test_1():
>      import test
>      print Test == test.Test
>
> if __name__ == '__main__':
>      test_1()
>
> and then run it ($ python test.py), it'll print False. Now try:
>
> $python
> import test
> test.test_1()
>
> and it'll print True. Is this behaviour expected? What was the
> rationale for it if is?
>
> Thanks,
>
> Daniel
>
>    


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