Why does defining a class in Python 2.7, without inheriting an object, not result in a __mro__ method? [duplicate]

Question

I am working on Mac OS X v10.10 (Yosemite) with Python 2.7.9.

Here is what I have tried:

1. Define a class

   class A:
       def test(self):
           print "test"
   

   Then run

   A.__mro__
   

   Then I got

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

2. Then I define

   class B(object):
       def test(self):
           print "test"
   

   Then run

   B.__mro__
   

   Then I got

   >>> B.__mro__
   (<class '__main__.B'>, <type 'object'>)
   

What is the different between the two definitions?

I found that in Python 3, the edition without "object" still has the __mro__ method.

Answer 1

__mro__ is only defined for new-style classes. In Python 2, a class is only new-style if it inherits from object (or from a built in type, which in turn inherits from object), while all classes in Python 3 are new-style no matter what.