Is subclassing from object the same as defining type as metaclass?

Question

This is an old-style class:

class OldStyle:
    pass

This is a new-style class:

class NewStyle(object):
    pass

This is also a new-style class:

class NewStyle2:
    __metaclass__ = type

Is there any difference whatsoever between NewStyle and NewStyle2?

I have the impression that the only effect of inheriting from object is actually to define the type metaclass, but I cannot find any confirmation of that, other than that I do not see any difference.

Answer 1

Pretty much yes, there's no difference between NewStyle and NewStyle2. Both are of type type while OldStyle of type classobj.

If you subclass from object, the __class__ of object (meaning type) is going to be used; if you supply a __metaclass__ that is going to get picked up.

If nothing is supplied as __metaclass__ and you don't inherit from object, Py_ClassType is assigned as the metaclass for you.

In all cases, metaclass.__new__ is going to get called. For Py_ClassType.__new__ it follows the semantics defined (I've never examined them, really) and for type.__new__ it makes sure to pack object in the bases of your class.

Of course, a similar effect is achieved by:

cls = type("NewStyle3", (), {})

where a call is immediately made to type; it's just a bigger hassle :-)