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Is types.InstanceType no longer valid with Python 2.2

Roeland Rengelink r.b.rigilink at chello.nl
Fri Jan 4 18:24:10 EST 2002 
Skip Montanaro wrote:
> 
[snip]
> 
> How can foo be a classic class with no bases, f1 be both an instance of foo
> and of object, but foo not be a subclass of object?  Just to pull all the
> bits together in a simple example, starting from a fresh interpreter prompt:
> 
>     >>> class foo: pass
>     ...
>     >>> f1 = foo()
>     >>> f1.__class__
>     <class __main__.foo at 0x821b964>
>     >>> f1.__class__.__bases__
>     ()
>     >>> isinstance(f1, foo)
>     1
>     >>> isinstance(f1, object)
>     1
>     >>> issubclass(foo, object)
>     0
> 
> I thought I understood this stuff, but my brain is about to explode, so I'll
> have to stop here.
> 


That surprised me a lot too. However, I noticed that:

>>> class foo:pass
... 
>>> isinstance(foo(), object)
1
>>> issubclass(foo, object)
0
>>> issubclass(type(foo()), object)
1
>>> import types
>>> issubclass(types.InstanceType, object)
1
>>> types.InstanceType.__bases__
(<type 'object'>,)

It may be that foo() appears to be an instance of object because
type(foo()) is a subclass of object. not because object is some sort of
hidden baseclass of foo. The surprise then, comes from type(foo()) !=
foo for old style classes.

Since one of the results of healing the type class dichotomy would be
type(foo()) == foo, and hence

isinstance(A(), B) == issubclass(type(A()), B),

I have halfway convinced myself that it does make sense after all.


Hope this helps,

Roeland
-- 
r.b.rigilink at chello.nl

"Half of what I say is nonsense. Unfortunately I don't know which half"
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