Where is new defined in python3?

Question

In Python 2:

>>> class A:
...  pass
... 
>>> A.__new__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: class A has no attribute '__new__'
>>> class A(object):
...  pass
... 
>>> A.__new__
<built-in method __new__ of type object at 0x1062fe2a0>

Conclusion: object contains __new__ and subclasses inherit the method.

in Python 3:

>>> class A:
...  pass
... 
>>> A.__new__
<built-in method __new__ of type object at 0x100229940>

__new__ is a defined method in our class, without any inheritance. How does this work? Where does __new__ "come from"?

Check out `A.__mro__`... – Martijn Pieters Sep 06 '17 at 13:52 — Martijn Pieters, Sep 06 '17 at 13:52

score 5 · Accepted Answer · edited Sep 06 '17 at 13:48

5

In Python 3, if you create a class without adding a parent class it automatically inherits from object. You can't create old style classes anymore like Python 2.

Example:

class A: # gets defaulted to class A(object):
    pass

edited Sep 06 '17 at 13:48

MSeifert

133,177
32
312
322

answered Sep 06 '17 at 13:45

Mohd

5,370
7
18
29

score 1 · Answer 2 · answered Sep 06 '17 at 13:46

All classes in Python3 are subclasses of object as you can see from the mro:

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

class A(object) is still done in some Python 3 code to maintain backward compatibility with Python 2.

Where is __new__ defined in python3?

2 Answers2

Where is new defined in python3?