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.
To use the equal to operator on custom objects, define the
__eq__() “dunder” magic method that takes two arguments:
other. You can then use attributes of the custom objects to determine if one is equal to the other. It should return a Boolean
Let’s have a look at an example next.
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.
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 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
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.