[Python-Dev] isinstance() on old-style classes in Py 2.7

Andreas Maier MAIERA at de.ibm.com
Tue Oct 21 18:43:09 CEST 2014

Hi. Today, I ran across this, in Python 2.7.6:

>>> class C:
...   pass
>>> issubclass(C,object)
>>> isinstance(C(),object)
True   <-- ???

The description of isinstance() in Python 2.7 does not reveal this result
(to my reading).

>From a duck-typing perspective, one would also not guess that an instance
of C would be considered an instance of object:

>>> dir(C())
['__doc__', '__module__']
>>> dir(object())
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__',
'__hash__', '__init__', '__new__', '__reduce__
', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',

-> What is the motivation for isinstance(C,object) to return True in Python


Andreas Maier
IBM Senior Technical Staff Member, Systems Management Architecture & Design
IBM Research & Development Laboratory Boeblingen, Germany
maiera at de.ibm.com, +49-7031-16-3654
IBM Deutschland Research & Development GmbH
Vorsitzende des Aufsichtsrats: Martina Koederitz
Geschaeftsfuehrung: Dirk Wittkopp
Sitz der Gesellschaft: Boeblingen
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