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

[Guido van Rossum]

This is one of the unfortunate effects of the existence of "old-style"
classes in Python 2. The old-style class hierarchy is distinct from the
new-style class hierarchy, but instances of old-style classes are still
objects (since in Python, *everything* is an object).

For new code, and whenever you have an opportunity to refactor old code,
you should use new-style classes, by inheriting your class from object (or
from another class that inherits from object).

On Tue, Oct 21, 2014 at 9:43 AM, Andreas Maier <MAIERA at de.ibm.com> wrote:

>
> Hi. Today, I ran across this, in Python 2.7.6:
>
> >>> class C:
> ...   pass
> ...
> >>> issubclass(C,object)
> False
> >>> 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__',
> '__subclasshook__']
>
> -> What is the motivation for isinstance(C,object) to return True in Python
> 2.7?
>
> Andy
>
> 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
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-- 
--Guido van Rossum (python.org/~guido)
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