"isinstance" question

[Thomas Jollans]

On 06/23/2010 08:39 AM, Satish Eerpini wrote:
> 
> 
>           I want to test whether an object is an instance of any
>         user-defined
>         class.  "isinstance" is less helpful than one would expect.
> 
>          >>> import types
>          >>> class foo() : # define dummy class
>         ...     pass
>         ...
>          >>> x = foo()
>          >>>
>          >>> type(x)
>         <type 'instance'>
>          >>>
>          >>> isinstance(x, types.ClassType)
>         False
>          >>> isinstance(x, types.InstanceType)
>         True
>          >>> foo
>         <class __main__.foo at 0x004A2BD0>
>          >>> x
>         <__main__.foo instance at 0x020080A8>
> 
>         So far, so good. x is an InstanceType.  But let's try a
>         class with a constructor:
> 
>          >>> class bar(object) :
>         ...    def __init__(self, val) :
>         ...      self.val = val
>         ...
>          >>> b = bar(100)
>          >>> b
>         <__main__.bar object at 0x01FF50D0>
>          >>> isinstance(b, types.InstanceType)
>         False
>          >>> isinstance(b, types.ClassType)
>         False
>          >>>>>> bar
>         <class '__main__.bar'>
> 
> well the same code on my side returns true when you run isinstance(b,
> types.InstanceType) even when the class has a constructor. Why is there
> a difference in the output when we are both using Cython 2.6 ?? (2.6.4
> to be exact)

I assume you mean CPython, not Cython.

As Gabriel said, the difference is that bar is a subclass of object. If
isinstance(bar(100), types.InstanceType), then you did not subclass
object, and get an old-style class.