Python __eq__

Summary: in this tutorial, you’ll learn how to use the Python __eq__ method to compare two objects by their values.

Introduction to the Python __eq__ method

Suppose that you have the following Person class with three instance attributes: first_name, last_name, and age:

class Person: def __init__(self, first_name, last_name, age): self.first_name = first_name self.last_name = last_name self.age = age
Code language: Python (python)

And you create two instances of the Person class:

john = Person('John', 'Doe', 25) jane = Person('Jane', 'Doe', 25)
Code language: Python (python)

In this example, the john and jane objects are not the same object. And you can check it using the is operator:

print(john is jane) # False
Code language: Python (python)

Also, when you compare john with jane using the equal operator (==), you’ll get the result of False:

print(john == jane) # False
Code language: Python (python)

Since john and jane have the same age, you want them to be equal. In other words, you want the following expression to return True:

john == jane

To do it, you can implement the __eq__ dunder method in the Person class.

Python automatically calls the __eq__ method of a class when you use the == operator to compare the instances of the class. By default, Python uses the is operator if you don’t provide a specific implementation for the __eq__ method.

The following shows how to implement the __eq__ method in the Person class that returns True if two person objects have the same age:

class Person: def __init__(self, first_name, last_name, age): self.first_name = first_name self.last_name = last_name self.age = age def __eq__(self, other): return self.age == other.age
Code language: Python (python)

Now, if you compare two instances of the Person class with the same age, it returns True:

john = Person('John', 'Doe', 25) jane = Person('Jane', 'Doe', 25) print(john == jane) # True
Code language: Python (python)

And if two instances of the Person class don’t have the same age, the == operator returns False:

john = Person('John', 'Doe', 25) mary = Person('Mary', 'Doe', 27) print(john == mary) # False
Code language: Python (python)

The following compares a Person object with an integer:

john = Person('John', 'Doe', 25) print(john == 20)
Code language: PHP (php)

It returns an error:

AttributeError: 'int' object has no attribute 'age'
Code language: JavaScript (javascript)

To fix this, you can modify the __eq__ method to check if the object is an instance of the Person class before accessing the age attribute.

If the other object isn’t an instance of the Person class, the __eq__ method returns False, like this:

class Person: def __init__(self, first_name, last_name, age): self.first_name = first_name self.last_name = last_name self.age = age def __eq__(self, other): if isinstance(other, Person): return self.age == other.age return False
Code language: Python (python)

And you can now compare an instance of the Person class with an integer or any object of a different type:

john = Person('John', 'Doe', 25) print(john == 20) # False
Code language: Python (python)

Putting it all together.

class Person: def __init__(self, first_name, last_name, age): self.first_name = first_name self.last_name = last_name self.age = age def __eq__(self, other): if isinstance(other, Person): return self.age == other.age return False john = Person('John', 'Doe', 25) jane = Person('Jane', 'Doe', 25) mary = Person('Mary', 'Doe', 27) print(john == jane) # True print(john == mary) # False john = Person('John', 'Doe', 25) print(john == 20) # False
Code language: Python (python)

Summary

  • Implement the Python __eq__ method to define the equality logic for comparing two objects using the equal operator (==)
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