Equality of objects in Python

Question

I have a class MyClass, which contains two member variables foo and bar:

class MyClass:
    def __init__(self, foo, bar):
        self.foo = foo
        self.bar = bar

I have two instances of this class, each of which has identical values for foo and bar:

x = MyClass('foo', 'bar')
y = MyClass('foo', 'bar')

However, when I compare them for equality, Python returns False:

>>> x == y
False

How can I make python consider these two objects equal?

Override `__cmp__` method of the class. – Fenikso Apr 27 '12 at 11:16 — Fenikso, Apr 27 '12 at 11:16

ch3ka · Accepted Answer · 2012-04-27T11:24:46.187

11

you have to tell python how exactly you want equality be defined. do so by defining a special method __eq__ like this:

def __eq__(self, other):
    return self.attrfoo == other.attrfoo # change that to your needs

the __cmp__(self, other) is the "old" style to compare instances of classes, and only used when there is no rich comparison special method found. read up on them here: http://docs.python.org/release/2.7/reference/datamodel.html#specialnames

edited Apr 27 '12 at 11:24

answered Apr 27 '12 at 11:16

ch3ka

8

If you define `__eq__`, remember to also define `__ne__` to prevent surprises. – Petr Viktorin Apr 27 '12 at 11:37
Remember to define `__ne__` too: should be written explicitely in the answer. Forgetting it may be really tricky. – zezollo Jul 27 '17 at 13:25

score 7 · Answer 2 · answered Apr 27 '12 at 11:16

7

The standard protocol is to define __cmp__() or __eq__() and __ne__().

If you don't, Python uses object identity ("address") to compare objects.

answered Apr 27 '12 at 11:16

NPE

do you mean python will check whether they are in the same memory? – plasma Aug 06 '20 at 06:07
That's an implementation detail, but for CPython it's like that. – Joe Jan 11 '22 at 10:29

2 Answers2