Difference between \'__eq__\' VS \'is\' VS \'==\' in Python

In Python, object comparison can be performed using three different approaches: the __eq__ method, the is operator, and the == operator. Each serves a distinct purpose in determining equality or identity between objects.

Overview of Comparison Methods

The __eq__() Method

The __eq__ method allows you to define custom equality logic for your classes. When you use the == operator on objects, Python internally calls the __eq__ method.

Example

Here's how to implement custom equality comparison for a Person class ?

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

    def __eq__(self, other):
        if isinstance(other, Person):
            return self.name == other.name and self.age == other.age
        return False

# Create two Person objects with same attributes
p1 = Person("John", 30)
p2 = Person("John", 30)

print(f"p1 == p2: {p1 == p2}")
print(f"p1 is p2: {p1 is p2}")

p1 == p2: True
p1 is p2: False

The 'is' Operator

The is operator checks if two variables point to the same object in memory. It compares object identity, not values.

Example

Here's how is works with different scenarios ?

# Same object reference
a = [1, 2, 3]
b = a
print(f"a is b: {a is b}")

# Different objects with same values
c = [1, 2, 3]
d = [1, 2, 3]
print(f"c is d: {c is d}")
print(f"c == d: {c == d}")

# Small integers are cached
x = 5
y = 5
print(f"x is y: {x is y}")

a is b: True
c is d: False
c == d: True
x is y: True

The == Operator

The == operator compares values of objects. For built-in types, it checks if the values are equal. For custom classes, it calls the __eq__ method.

Example

Here's how == works with different data types ?

# Lists with same values
list1 = [1, 2, 3]
list2 = [1, 2, 3]
print(f"list1 == list2: {list1 == list2}")

# Strings with same content
str1 = "hello"
str2 = "hello"
print(f"str1 == str2: {str1 == str2}")

# Numbers
num1 = 42
num2 = 42
print(f"num1 == num2: {num1 == num2}")

list1 == list2: True
str1 == str2: True
num1 == num2: True

Key Differences in Practice

Let's see all three approaches in a single example ?

class Book:
    def __init__(self, title):
        self.title = title
    
    def __eq__(self, other):
        if isinstance(other, Book):
            return self.title == other.title
        return False

# Create book objects
book1 = Book("Python Guide")
book2 = Book("Python Guide") 
book3 = book1

print("=== Comparison Results ===")
print(f"book1 == book2: {book1 == book2}")  # Uses __eq__
print(f"book1 is book2: {book1 is book2}")  # Different objects
print(f"book1 is book3: {book1 is book3}")  # Same object
print(f"book1 == book3: {book1 == book3}")  # Uses __eq__

=== Comparison Results ===
book1 == book2: True
book1 is book2: False
book1 is book3: True
book1 == book3: True

Conclusion

Use __eq__ to define custom equality logic in your classes. Use is to check if two variables reference the same object. Use == to compare values, which internally calls __eq__ for custom objects.