Why does defining a class in Python 2.7, without inheriting an object, not result in a mro method?

Question

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

Here is what I have tried:

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__'

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.

http://stackoverflow.com/questions/54867/what-is-the-difference-between-old-style-and-new-style-classes-in-python — Holloway, Mar 10 '15 at 15:46

score 6 · Accepted Answer · edited May 23 '17 at 12:01

6

__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.

edited May 23 '17 at 12:01

Community

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1

answered Mar 10 '15 at 15:47

jwodder

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score 1 · Answer 2 · answered Mar 10 '15 at 15:47

1

The __mro__ is only defined for new style classes, those that inherit from Python object.

In Python 3, all classes are new-style classes. They inherit from object implicitly. In other words, old-style Python 2 classes have been removed from the language.

answered Mar 10 '15 at 15:47

Martijn Pieters

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

2 Answers2

Why does defining a class in Python 2.7, without inheriting an object, not result in a mro method?