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.
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 :-)
