Why won't python 2.7 define a class without an inherited object will not have a __mro__ method?

Question

I am working on Mac OS X Yosemite with python 2.7.9.

Here is what I tried:

• define 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__'
  

• 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 difference between these two definitions? I found that in python 3 in the edition without an object there is a method __mro__.

Answer 1

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

Answer 2

__mro__ , Python object.

Python 3 . object . , Python 2 .