Difference between <type 'classobj'>, <type 'object'>?

python, python-2.7

Solution

First note that in Python 2.x, the syntax:

class XXX:
    # whatever

will yield an "old style" class. You can read more about "old style" vs "new style" classes in the fine manual, on this doc page and in the Python wiki

Then, in your case, you're not asking for the mro of class `a` but for the mro of the type of class `a` (which is what `type(a)` returns), in this case the `classobj` type. But you shouldn't worry about old-style classes, `classobj` etc unless you have to maintain or deal with legacy code. For the record, most of Python's standard OO features (including `super()`, descriptors etc) won't work properly on old-style classes, so don't use them, and don't try to understand stuff pertaining to the "new" (well, about 10 years old by now ) object model using old-style classes...

TL;DR: you want:

class A(object):
    # this is a new-style class

Maybe I am a bit dizzy today... But I guess the classobj just means class a, right?

Wrong. `classobj` is a kind of a special type which serves as metaclass for old-style classes. The metaclass being the class of class (since classes are objects, they are instances of a class).

So the above output just says the method resolution order is first class a then type object.

No, it says that the mro for the `classobj` type is first `classobj` then `object`.

If you want the mro of class `a` then just ask for it - and then you'll find out why you shouldn't assess new-style features on old-style classes:

>>> class a: pass
... 
>>> a.__mro__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class a has no attribute '__mro__'

Now compare with a new-style class:

>>> class Foo(object): pass
... 
>>> Foo.__mro__
(<class '__main__.Foo'>, <type 'object'>)
>>> 

Problem

Original source