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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: weird isinstance/issubclass behavior?
Date: Thu, 29 Nov 2012 16:00:48 -0500
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On 11/29/2012 9:59 AM, lars van gemerden wrote:
> Hi,
>
> I have encountered some strange behavior of isinstance(/issubclass): depending on the import path used for classes i get different output, while the classes i compare are in the same file.
>
> Basically if i import a class as:
>
>      from mod1.mod2 import A
> or:
>      from mod0.mod1.mod2 import A
>
> which both result in importing the same class,

As other said, both import the same abstract class but create two 
different concrete class objects.

 > a call to isinstance(inst, A) in another module can have a different 
output.
> In this module
>      print type(inst), A, isinstance(inst, A), issubclass(type(inst), A)
> gives:
>      <class 'mod0.mod1.mod2.A'> <class 'mod1.mod2.A'> False False

Add print id(type(inst)), id(A) and you will see that they are different 
objects. The isinstance and issubclass calls compare the classes by 
identity (the default meaning of ==) and so False, False are correct.

-- 
Terry Jan Reedy