Class changes in circular imports when __name__ == '__main__'

Carl Banks pavlovevidence at gmail.com
Mon Sep 6 01:09:12 CEST 2010


On Sep 5, 1:19 pm, Spencer Pearson <speeze.pear... at gmail.com> wrote:
> Hi! I'm writing a package with several files in it, and I've found
> that "isinstance" doesn't work the way I expect under certain
> circumstances.
>
> Short example: here are two files.
> # fileone.py
> import filetwo
>
> class AClass( object ):
>   pass
>
> if __name__ == '__main__':
>   a = AClass()
>   filetwo.is_aclass( a )
>
> # filetwo.py
>
> import fileone
>
> def is_aclass( a ):
>   print "The argument is", ("" if isinstance(a, fileone.AClass) else
> "not"), "an instance of fileone.AClass"
>
> If you run fileone.py, it will tell you that "The argument is not an
> instance of fileone.AClass", which seems strange to me, given that the
> fileone module is the one that CREATES the object with its own AClass
> class. And if you replace "if __name__ == '__main__'" with "def
> main()", start Python, import fileone, and call fileone.main(), it
> tells you that the argument IS an instance of AClass.
>
> So, the module's name change to __main__ when you run it on its own...
> well, it looks like it puts all of the things defined in fileone in
> the __main__ namespace INSTEAD of in the fileone module's namespace,
> and then when filetwo imports fileone, the class is created again,
> this time as fileone.AClass, and though it's identical in function to
> __main__.AClass, one "is not" the other.

Correct.  Python always treats the main script as a module called
__main__.  If you then try to import the main script file from another
module, Python will actually import it again with whatever its usual
name is.

This is easily one of the most confusing and unfortunate aspects of
Python.


> Is this kind of doubled-back 'isinstance' inherently sinful? I mean, I
> could solve this problem by giving all of my classes "classname"
> attributes or something, but maybe it's just a sign that I shouldn't
> have to do this in the first place.

Even if there are better ways than isinstance, the weird behavior of
__main__ shouldn't be the reason not to use it.

My recommendation for most programmers is to treat Python files either
as scripts (which you start Python interpreter with) or modules (which
you import from within Python); never both.  Store most functionality
in modules and keep startup scripts small.  If you do this, the weird
semantics of __main__ is a moot point.

If you want to be able to run a module as a script while avoiding side
effects due to it being named __main__, the easiest thing to do is to
put something like the following boilerplate at the top of the module
(this causes the module to rename itself).

import sys
if __name__ == '__main__':
    is_main = True # since you're overwriting __name__ you'll need
this later
    __name__ = 'foo'
    sys.modules['foo'] = sys.modules['__main__']
else:
    is_main = False


All of this gets a lot more complicated when packages are involved.


Carl Banks



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