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Sat Feb 14 18:18:27 CET 2009 · https://mail.python.org/pipermail/python-dev/2009-February/086108.html

Greg Ewing wrote:
> Georg Brandl wrote:
> 
>> Since I cannot imagine a scenario where you would want to have 
>> non-classes
>> as the arguments of issubclass(),
> 
> I had one today, which is what led me to discover this.
> I'm working on a Python-Ruby bridge that wraps Ruby
> objects and classes in Python objects.
> 
> I wanted to make isinstance() and issubclass() work in
> the expected way when applied to wrappers around Ruby
> classes. The ability to fake things using __classes__
> and __bases__ turned out to be very handy.

The new (in 3.0 and maybe 2.6, but undocumented) special methods 
__instancecheck__ and __subclasscheck__ let one overload the default 
behavior of isinstance and issubclass.  So there is no reason to have 
the default behavior necessarily cover 'unusual' cases.  See
http://www.python.org/dev/peps/pep-3119/#overloading-isinstance-and-issubclass
and http://bugs.python.org/issue5250.

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[Python-Dev] Small misleadingness in docs