isinstance() bug

[Rainer Deyke]

Michal Vitecek wrote:
> Rainer Deyke wrote:
>> Michal Vitecek wrote:
>>>  it's my belief that if i'm instantiating a class from the same
>>>  location no matter how i imported its definition, its instances are
>>>  always of the same class.
>>
>> Your belief is incorrect.  'package.module.A' and 'module.A' are
>> distinct classes.  Modifications to one do not show up in the other.
>> If 'isinstance' reported that an instance of 'package.module.A' is
>> an instance of 'module.A', 'isinstance' would be broken and unusable.
>
>  could you elaborate further why do you consider the current behaviour
>  to be correct? you've only described how it works currently. i'd be
>  interested what's the reasoning behind that.

'isinstance' does the only possible correct thing.  Even without loading the
same file as multiple modules, a module can create many distinct classes
with the same name.  For example:

def create_unique_class():
  class C(object): pass
  return C

a_bunch_of_classes = [create_unique_class() for i in range(8)]

Treating these separate classes as a single class would be just plain wrong.

If there's a problem with the current implementation, it is that it allows
modules in packages to be imported as top level modules.


-- 
Rainer Deyke - rainerd at eldwood.com - http://eldwood.com