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$python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:09:56) 
[GCC 4.4.3] on linux2
>>> class Old: pass
>>> class New(object): pass
>>> o = Old()
>>> n = New()
>>> isinstance(o, object)
True
This is it, basically. Is it a bug or a feature?
More tests :
>>> isinstance(o, Old)
True
>>> isinstance(o, New)
False
>>> isinstance(n, Old)
False
>>> isinstance(o, int)
False
Please note that some unimportant output was deleted from above.
PS. If this is a feature, how do I detect an old-style class then ?

In addition:
>>> issubclass(Old, object)
False

Yes, everything is a object. issubclass, though, works differently for old-style and new-style classes.

>>> help(isinstance)
isinstance(...)
 isinstance(object, class-or-type-or-tuple) -> bool
 
 Return whether an object is an instance of a class or of a subclass thereof.
 (...)
So are the old-style class instances descendants of the object?
I feel like I am missing something (except for the fact that you have closed the bug).

This is a result of how old-style classes are implemented.
If you look at type(Old()), you can see that it isn't Old, but "instance".
(And "instance" is a subclass of object again.)
"issubclass" for old-style classes doesn't check type(o) but o.__class__, which are different: the former is "instance" and the latter your class. That is one reason we removed old-style classes in Python 3...

I do not mean to reopen the bug (there are supposedly much more important things to work on in Python). 
But just for the record, let me state that I feel like there is some misleading inconsistency here:
- by definition, a new style class is "Any class which inherits from object" ( see http://docs.python.org/glossary.html#term-new-style-class ) ;
- to support this statement, new classes are indeed explicitly defined in the form "NewClass(object)" ;
- now isinstance(), that is supposed to "return whether an object is an instance of a class or of a subclass thereof" (see help(isinstance)), returns True for old-style objects.
It also seems reasonable if the descendants of a class will inherit its powers, which -- in the case of the old-style classes -- they obviously don't.
Furthermore, I personally see no /point/ in returning True for isinstance(Old(), object): as it is quite misleading, one could easily have made it returning e.g. None as well.
As I completely accept the fact it's a feature -- ( may be slightly confusing, and probably also useless -- but ... hey, nobody's perfect ) -- should I take then calling
issubclass(obj.__class__, object) 
to be the official way to distinguish between the new-style and the old-style classes?

2012年4月26日 Q <report@bugs.python.org>:
> issubclass(obj.__class__, object)
>
> to be the official way to distinguish between the new-style and the old-style classes?
Just do type(cls) is types.ClassType.

thanks, that's rather convenient