python - How int() object uses "==" operator without __eq__() method in python2?

Question

Recently I read the "Fluent python" and understood how == operator works with python objects, using __eq__() method. But how it works with int instances in python2?

>>> a = 1
>>> b = 1
>>> a == b
True
>>> a.__eq__(b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute '__eq__'

in python3 all a.__eq__(b) returns True

Answer 1

Python prefers to use rich comparison functions (__eq__, __lt__, __ne__, etc.), but if those don't exist, it falls back to using a single comparison function (__cmp__, removed in Python 3):

These are the so-called “rich comparison” methods, and are called for comparison operators in preference to __cmp__() below.

The Python 2 integer type doesn't implement a rich comparison function:

PyTypeObject PyInt_Type = {
    ...
    (cmpfunc)int_compare,                       /* tp_compare */
    ...
    0,                                          /* tp_richcompare */

In Python 3, the integer type (now a long) implements only a rich comparison function, since Python 3 dropped support for __cmp__:

PyTypeObject PyLong_Type = {
    ...
    long_richcompare,                           /* tp_richcompare */

This is why (123).__eq__ doesn't exist. Instead, Python 2 falls back to (123).__cmp__ when testing the equality of two integers:

>>> (1).__eq__(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute '__eq__'
>>> (1).__cmp__(2)
-1