[Python-Dev] unexpected import behaviour

[Daniel Waterworth]

On 30 July 2010 18:32, Michael Foord <fuzzyman at voidspace.org.uk> wrote:
> On 30/07/2010 17:59, Oleg Broytman wrote:
>>
>> On Fri, Jul 30, 2010 at 07:26:26AM +0100, Daniel Waterworth wrote:
>>
>>>
>>> @Oleg: ...
>>> This is purely CPython bug-fixing/the discussion of
>>> implementation choices.
>>>
>>
>>    I am not sure it's a bug.
>
> It isn't a bug but it's a very common *cause* of bugs, even for relatively
> experienced Python programmers (this exchange being another case in point).
>
> Michael
>
>> By manipulating sys.path (or symlinks in the
>> FS) one can import the same file as different modules as many times as
>> [s]he wants. Should this be fixed for __main__? I doubt it. Instead of
>> making __main__ a special case follow the rule: don't import the same
>> module under different paths/names.
>>    Make your script simply
>>
>> from test import main
>> main()
>>
>> Oleg.
>>
>
>
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Having thought it through thoroughly, my preference is for a warning.

I don't think it's a good practise to import the __main__ module by
filename, as renaming the file will break the code. I got stung after,
having dropped into a python interpreter shell and imported the
module, I executed a function that uses isinstance.

If a warning showed up after importing the module, explaining the
problem and suggested that I use __import__('__main__') instead, I
would have saved myself a fair amount of time debugging code. This is
another case of "Explicit is better than implicit.".

It also means that code that relies on the current behaviour will not be broken.

@Oleg: yes, but in the __main__ case, it's more difficult to tell that
you are importing something under a different name. I suppose the
proof is in the pudding, can anyone think of a case where someone has
been annoyed that, having imported that same module twice via
symlinks, they have had problems relating to modules being independent
instances?

Thanks,

Daniel

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