isinstance() bug

[Sidharth Kuruvila]

"Michal Vitecek" <fuf at mageo.cz> wrote in message
news:mailman.907.1075296157.12720.python-list at python.org...
> hello,
>
>  please consider the following situation:
>
>  under the current directory there's a subdirectory 'package' with two
>  files: __init__.py and module.py
>
>     ./package:
>         __init__.py
>         module.py
>
>  module.py contains:
>
>  class A(object):
>     pass
>
>  aModule = A()
>
>
>  now, let's do:
>
>  current directory:
>
>     >>> import package.module
>     >>> type(package.module.aModule)
>     <class 'package.module.A'>
>     >>> isinstance(package.module.aModule, package.module.A) # so far good
>     1
>     >>> a = package.module.A()
>     >>> isinstance(a, package.module.A) # so far good
>     >>> import sys
>     >>> sys.path.append('package')
>     >>> import module
>     >>> a = module.A()
>     >>> isinstance(a, package.module.A) # will return 0 !!!
>     0
>     >>> isinstance(package.module.aModule, module.A) # will return 0 !!!
>     0
>     >>>
>
>  how is it possible that it IS important how you imported a class
>  definition for isinstance() to work? it's insane!
>
> --
> fuf (fuf at mageo.cz)
>

its got to do with how python imports modules.
both imports are treated as seperate modules because the relative paths are
diferent.
python is a dynamic language it figures out where modules are located at run
time, it would be expensive to check all paths to see if they ended up at
the same file.