"isinstance" question

[Gabriel Genellina]

En Tue, 22 Jun 2010 23:45:07 -0300, John Nagle <nagle at animats.com>  
escribió:

>    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'>
>
> Without a constructor, we get an "instance".  With a constructor,
> we get an "object", one which is not an InstanceType.

That's not the relevant difference. In the first case, you don't inherit  
 from object; in the second one, you do.

foo is a "classic" class (or "old-style" class); x is an instance of foo,  
its *type* is InstanceType, its *class* is foo. All instances of any other  
classic class have the same type (InstanceType).

bar is a "new-style" class, b is an instance of bar, its type is bar, its  
class is bar. class and type are equivalent for new style classes; things  
are a lot more regular and predictable. In Python 3.x classic classes are  
gone.

-- 
Gabriel Genellina