Python __eq__ Magic Method

To customize the behavior of the equality operator x == y, override the __eq__() dunder method in your class definition. Python internally calls x.__eq__(y) to compare two objects using x == y. If the __eq__() method is not defined, Python will use the is operator per default that checks for two arbitrary objects whether they reside on the same memory address.

Syntax

__eq__(self, other)

To use the equal to operator on custom objects, define the __eq__() “dunder” magic method that takes two arguments: self and other. You can then use attributes of the custom objects to determine if one is equal to the other. It should return a Boolean True or False.

Let’s have a look at an example next.

Example

In the following code, you check if a Person is equal to another Person by using the age attribute as a decision criterion:

class Person:
    def __init__(self, age):
        self.age = age

    def __eq__(self, other):
        return self.age == other.age



alice = Person(18)
bob = Person(19)
carl = Person(18)

print(alice == bob)
# False

print(alice == carl)
# True

Because Alice is 18 years old and Bob is 19 years old, the result of alice == bob is False. But the result of alice == carl evaluates to True as both have the same age.

Background Video

Default Implementation of __eq__

Per default, the __eq__() dunder method is implemented using the is identity operator. Identity operators are used to check whether two values or variables reside at the same memory location, i.e., refer to the same object in memory.

Because the fallback identity operator is defined for each object, you can also check equality for any two object.

The following example shows that you can compare custom persons using the equality operator ==, even without defining the __eq__ method. Internally, Python uses the identity operator:

class Person:
    def __init__(self, age):
        self.age = age



alice = Person(18)
bob = Person(19)
carl = Person(18)

print(alice == bob)
# False

print(alice == carl)
# False

Background Video Identity Operator

To understand the identity operator, feel free to watch the following background video:

Commutativity of Equality ==

The output of x == y and y == x may be different because the former calls x.__eq__(y) and the latter calls y.__eq__(x). If x and y have different definitions of the dunder method __eq__(), the operation becomes non-commutative.

You can see this in the following example:

class Person:
    def __eq__(self, other):
        return False


class Human:
    def __eq__(self, other):
        return True


alice = Person()
bob = Human()


print(alice == bob)
# False

print(bob == alice)
# True

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