How to Implement __hash__ in Python: Simple Guide

To implement __hash__ in Python, define the method inside your class to return an integer hash value representing the object. This allows instances to be used as keys in dictionaries or stored in sets. Make sure that objects that compare equal have the same hash value.

Syntax

The __hash__ method is defined inside a class and must return an integer. This integer is the hash value of the object.

• def __hash__(self): — method header
• Return an integer representing the object’s hash

python

class MyClass:
    def __hash__(self):
        # return an integer hash value
        return 42

Example

This example shows a class with __hash__ implemented based on its attributes. It allows objects to be used as dictionary keys.

python

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

    def __eq__(self, other):
        if not isinstance(other, Person):
            return False
        return self.name == other.name and self.age == other.age

    def __hash__(self):
        return hash((self.name, self.age))

# Create two identical persons
p1 = Person('Alice', 30)
p2 = Person('Alice', 30)

# Use as dictionary keys
people = {p1: 'Engineer'}

print(people[p2])  # Output: Engineer

Output

Engineer

Common Pitfalls

Common mistakes when implementing __hash__ include:

• Not defining __eq__ consistently with __hash__. Objects that compare equal must have the same hash.
• Using mutable attributes in the hash calculation, which can cause errors if the object changes after hashing.
• Returning non-integer values from __hash__.

python

class BadPerson:
    def __init__(self, name):
        self.name = name

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

    def __hash__(self):
        # Wrong: returns a string instead of int
        return self.name

# Correct way
class GoodPerson:
    def __init__(self, name):
        self.name = name

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

    def __hash__(self):
        return hash(self.name)

Quick Reference

• Define __hash__(self) to return an int.
• Ensure __eq__ and __hash__ are consistent.
• Use only immutable attributes in hash calculation.
• Use Python’s built-in hash() function on tuples of attributes.

Key Takeaways

Implement __hash__ to return an integer hash value for your object.

Ensure __hash__ and __eq__ are consistent: equal objects must have the same hash.

Use only immutable attributes to compute the hash to avoid errors.

Use Python’s built-in hash() on tuples of attributes for simplicity.

Objects with __hash__ can be used as dictionary keys and in sets.