77. Custom Hashing for Objects

Here are 10 Python code snippets demonstrating how to implement custom hashing for classes to make them usable in sets and dictionaries:

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1. Basic Custom Hashing

Implementing __hash__ and __eq__ for a simple class.

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

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

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

# Using in set
person1 = Person("Alice", 30)
person2 = Person("Alice", 30)
people = {person1, person2}
print(people)  # Only one entry, because Person objects are hashable and equal

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2. Hashing with Multiple Attributes

Create a custom class with multiple attributes, where __hash__ combines them.

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class Product:
    def __init__(self, id, name):
        self.id = id
        self.name = name

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

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

# Using in a dictionary
product1 = Product(101, "Laptop")
product2 = Product(102, "Phone")
products = {product1: "Electronics", product2: "Electronics"}
print(products)

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3. Custom Hash for Mutable Attributes

Make sure to handle mutable attributes carefully, as they can break hash consistency.

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class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

    def __eq__(self, other):
        return (self.title, self.author) == (other.title, other.author)

    def __hash__(self):
        return hash((self.title, self.author))

# Using in set
book1 = Book("Python 101", "John Doe")
book2 = Book("Python 101", "John Doe")
books = {book1, book2}
print(books)  # Should only store one book as both are equal

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4. Handling Hash Collisions

The __hash__ method must return a unique value for different objects, but sometimes collisions happen.

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class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __eq__(self, other):
        return self.x == other.x and self.y == other.y

    def __hash__(self):
        return hash((self.x, self.y))

# Handling potential collisions
point1 = Point(1, 2)
point2 = Point(2, 3)
points = {point1, point2}
print(points)  # Should differentiate points even if they have similar hash values

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5. Using Only Immutable Attributes for Hashing

Using immutable attributes such as tuples for a hash function to guarantee consistency.

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class Car:
    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year

    def __eq__(self, other):
        return (self.make, self.model, self.year) == (other.make, other.model, other.year)

    def __hash__(self):
        return hash((self.make, self.model, self.year))

car1 = Car("Toyota", "Camry", 2021)
car2 = Car("Toyota", "Camry", 2021)
cars = {car1, car2}
print(cars)  # Only one entry as they are equal and hashable

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6. Custom Hash with Complex Objects

Custom classes that use complex objects, like dictionaries, for hashing.

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class User:
    def __init__(self, username, email):
        self.username = username
        self.email = email

    def __eq__(self, other):
        return self.username == other.username and self.email == other.email

    def __hash__(self):
        return hash((self.username, self.email))

user1 = User("john_doe", "[e-mail]")
user2 = User("john_doe", "[e-mail]")
users = {user1: "Admin", user2: "Admin"}
print(users)  # Should show only one entry

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7. Handling Non-Hashable Attributes

If an attribute is not hashable, it's crucial to exclude it from the __hash__ method.

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class Employee:
    def __init__(self, emp_id, name, dept):
        self.emp_id = emp_id
        self.name = name
        self.dept = dept  # This is a list, which is mutable and non-hashable

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

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

employee1 = Employee(101, "Alice", ["HR"])
employee2 = Employee(102, "Bob", ["Finance"])
employees = {employee1, employee2}
print(employees)  # This works even with non-hashable 'dept' attribute

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8. Custom Hashing with String Representation

Using string representations of objects to generate unique hashes.

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class Movie:
    def __init__(self, title, release_year):
        self.title = title
        self.release_year = release_year

    def __eq__(self, other):
        return self.title == other.title and self.release_year == other.release_year

    def __hash__(self):
        return hash(f"{self.title} {self.release_year}")

movie1 = Movie("Inception", 2010)
movie2 = Movie("Inception", 2010)
movies = {movie1, movie2}
print(movies)  # Only one entry, as they are equal and hashable

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9. Custom Hashing with Frozen Set

Using a frozenset for hashing sets or dictionaries with unordered elements.

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class Group:
    def __init__(self, members):
        self.members = frozenset(members)

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

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

group1 = Group({"Alice", "Bob", "Charlie"})
group2 = Group({"Bob", "Alice", "Charlie"})
groups = {group1, group2}
print(groups)  # Only one entry because frozensets are hashable and unordered

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10. Debugging Hash and Equality

Make sure to implement __eq__ and __hash__ properly to avoid inconsistencies.

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class Song:
    def __init__(self, title, artist):
        self.title = title
        self.artist = artist

    def __eq__(self, other):
        return self.title == other.title and self.artist == other.artist

    def __hash__(self):
        return hash((self.title, self.artist))

song1 = Song("Shape of You", "Ed Sheeran")
song2 = Song("Shape of You", "Ed Sheeran")
song3 = Song("Blinding Lights", "The Weeknd")

songs = {song1, song2, song3}
print(songs)  # Should only have 2 distinct songs (song1 and song2 are equal)

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These examples demonstrate how to implement custom hashing by defining __hash__ and __eq__ methods. For custom objects to work correctly in sets and dictionaries, the hash function must be consistent with equality, and the object attributes used in the hash must be immutable and hashable.