Equality & Comparison in Python: Essential OOP Guide

In Python, comparing objects can be trickier than it seems. By default, objects are compared based on their memory address, not the values they hold. This can lead to unexpected results when comparing custom objects. Thankfully, Python’s magic methods provide a way to define custom equality and comparison behaviors, unlocking powerful features like object sorting and flexible comparisons.

1. The Default Behavior: Identity vs. Equality

Understanding the difference between identity and equality is crucial:

• Identity (is): Two variables refer to the same object in memory.
• Equality (==): Two objects have the same value, even if they aren’t the same object in memory.

b1 = Book("Pride and Prejudice", "Jane Austen", 9.99)
b2 = Book("Pride and Prejudice", "Jane Austen", 9.99)

print(b1 is b2)  # False (different objects in memory)
print(b1 == b2)  # False (by default, object equality isn't implemented)

2. The __eq__ Magic Method: Defining Equality

To customize how equality is checked, you can override the __eq__ magic method:

class Book:
    # ...
    def __eq__(self, other):
        if not isinstance(other, Book):
            raise ValueError("Can't compare book to a non-book")
        return (self.title == other.title and
                self.author == other.author and
                self.price == other.price)

Now, comparing two Book objects will check if their attributes match.

3. Comparison Magic Methods: Beyond Equality

Python provides magic methods for other comparisons as well:

• __lt__ (Less than): Implement the < operator.
• __le__ (Less than or equal to): Implement the <= operator.
• __gt__ (Greater than): Implement the > operator.
• __ge__ (Greater than or equal to): Implement the >= operator.

class Book:
    # ...
    def __ge__(self, other): 
        if not isinstance(other, Book):
            raise ValueError("Can't compare book to a non-book")
        return self.price >= other.price

4. Sorting Objects: The Power of Comparison

By implementing comparison magic methods, you can now sort lists of objects:

books = [b1, b3, b2, b4]
books.sort()  # Sorts by price ascending (because of __lt__)

5. Key Takeaways: Beyond the Basics

• Default Behavior: Python compares objects by identity unless you override the __eq__ method.
• Customizable Comparisons: You have full control over how your objects are compared.
• Sorting: Implementing comparison methods enables easy sorting of object lists.
• Other Magic Methods: Explore the full range of comparison magic methods for more advanced scenarios.

Frequently Asked Questions (FAQ)