25. Python's __magic__ Methods

Here Python code snippets that showcase the use of Python's dunder methods (a.k.a. magic methods) like __init__, __str__, __repr__, and others, to customize class behavior.

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1. __init__: Constructor Method

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

p = Person("Alice", 30)
print(p.name)  # Alice
print(p.age)   # 30

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2. __str__: String Representation for Humans

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

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

p = Person("Alice", 30)
print(p)  # Alice is 30 years old.

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3. __repr__: String Representation for Debugging

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

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

p = Person("Alice", 30)
print(repr(p))  # Person(name=Alice, age=30)

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4. __add__: Overloading the + Operator

class Number:
    def __init__(self, value):
        self.value = value

    def __add__(self, other):
        return Number(self.value + other.value)

    def __repr__(self):
        return f"Number({self.value})"

n1 = Number(5)
n2 = Number(10)
print(n1 + n2)  # Number(15)

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5. __getitem__: Index Access for Custom Objects

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

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

cl = CustomList([10, 20, 30])
print(cl[1])  # 20

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6. __len__: Custom Length Calculation

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

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

cl = CustomList([1, 2, 3, 4])
print(len(cl))  # 4

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7. __eq__: Overloading == for Equality Check

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

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

p1 = Person("Alice", 30)
p2 = Person("Alice", 30)
p3 = Person("Bob", 25)
print(p1 == p2)  # True
print(p1 == p3)  # False

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8. __call__: Making Objects Callable

class Greeter:
    def __init__(self, greeting):
        self.greeting = greeting

    def __call__(self, name):
        return f"{self.greeting}, {name}!"

greet = Greeter("Hello")
print(greet("Alice"))  # Hello, Alice!

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9. __del__: Destructor Method

class Resource:
    def __init__(self, name):
        self.name = name
        print(f"Resource {self.name} created.")

    def __del__(self):
        print(f"Resource {self.name} destroyed.")

res = Resource("FileHandler")
del res  # Resource FileHandler destroyed.

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10. __contains__: Custom Behavior for in

class CustomContainer:
    def __init__(self, items):
        self.items = items

    def __contains__(self, item):
        return item in self.items

container = CustomContainer([1, 2, 3])
print(2 in container)  # True
print(5 in container)  # False

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These examples demonstrate how dunder methods let you override and customize the behavior of built-in Python operations, allowing you to create intuitive, powerful, and expressive classes.