learn-python/contrib/advanced-python/magic-methods.md

Magic Methods

Magic methods, also known as dunder (double underscore) methods, are special methods in Python that start and end with double underscores (__). These methods allow you to define the behavior of objects for built-in operations and functions, enabling you to customize how your objects interact with the language's syntax and built-in features. Magic methods make your custom classes integrate seamlessly with Python’s built-in data types and operations.

Commonly Used Magic Methods

1. Initialization and Representation
   • __init__(self, ...): Called when an instance of the class is created. Used for initializing the object's attributes.
   • __repr__(self): Returns a string representation of the object, useful for debugging and logging.
   • __str__(self): Returns a human-readable string representation of the object.

Example :

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

    def __repr__(self):
        return f"Person({self.name}, {self.age})"

    def __str__(self):
        return f"{self.name}, {self.age} years old"

p = Person("Alice", 30)
print(repr(p))
print(str(p))  

Output :

Person("Alice",30)
Alice, 30 years old

2. Arithmetic Operations
   • __add__(self, other): Defines behavior for the + operator.
   • __sub__(self, other): Defines behavior for the - operator.
   • __mul__(self, other): Defines behavior for the * operator.
   • __truediv__(self, other): Defines behavior for the / operator.

Example :

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

    def __add__(self, other):
        return Vector(self.x + other.x, self.y + other.y)

    def __repr__(self):
        return f"Vector({self.x}, {self.y})"

v1 = Vector(2, 3)
v2 = Vector(1, 1)
v3 = v1 + v2
print(v3)  

Output :

Vector(3, 4)

3. Comparison Operations
   • __eq__(self, other): Defines behavior for the == operator.
   • __lt__(self, other): Defines behavior for the < operator.
   • __le__(self, other): Defines behavior for the <= operator.

Example :

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

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

    def __lt__(self, other):
        return self.age < other.age

p1 = Person("Alice", 30)
p2 = Person("Bob", 25)
print(p1 == p2) 
print(p1 < p2)  

Output :

False
False

5. Container and Sequence Methods

   • __len__(self): Defines behavior for the len() function.
   • __getitem__(self, key): Defines behavior for indexing (self[key]).
   • __setitem__(self, key, value): Defines behavior for item assignment (self[key] = value).
   • __delitem__(self, key): Defines behavior for item deletion (del self[key]).

Example :

class CustomList:
    def __init__(self, *args):
        self.items = list(args)

    def __len__(self):
        return len(self.items)

    def __getitem__(self, index):
        return self.items[index]

    def __setitem__(self, index, value):
        self.items[index] = value

    def __delitem__(self, index):
        del self.items[index]

    def __repr__(self):
        return f"CustomList({self.items})"

cl = CustomList(1, 2, 3)
print(len(cl))
print(cl[1])    
cl[1] = 5
print(cl)
del cl[1]
print(cl)      

Output :

3
2
CustomList([1, 5, 3])
CustomList([1, 3])

Magic methods provide powerful ways to customize the behavior of your objects and make them work seamlessly with Python's syntax and built-in functions. Use them judiciously to enhance the functionality and readability of your classes.