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
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