Python Secret Double Underscore Tricks

Dunder Methods Explained

If you’ve used Python classes, you know __init__ - the method that creates objects. But Python has many more dunder (double underscore) methods that control how objects behave. Let’s explore the most useful ones.

Building the Class: Dunder by Dunder

__init__: The Foundation

Sets up your object’s initial state

class Coffee:
    def __init__(self, size, price):
        self.size = size
        self.price = price

# Usage
morning_coffee = Coffee("Large", 4.99)
print(f"Object - {morning_coffee}") # this will print the object
print(f"Coffee size: {morning_coffee.size}")  # Output: Coffee size: Large

__str__: The Human-Friendly Display

Controls what print() shows

class Coffee:
    def __init__(self, size, price):
        self.size = size
        self.price = price
    
    def __str__(self):
        return f"Coffee size: {self.size} and price: {self.price}"

# Usage
morning_coffee = Coffee("Large", 4.99)
print(f"Object - {morning_coffee}") # this will print the object
# this will disappeared once added __str__ method
print(f"Coffee size: {morning_coffee.size}")  # Output: Coffee size: Large
print(f"__str__ - {morning_coffee}")  # Coffee size: Large and price: 4.99

__repr__: The Debugger’s Truth

Shows how to recreate the objec

class Coffee:
    def __init__(self, size, price):
        self.size = size
        self.price = price
    
    def __str__(self):
        return f"{self.size} coffee (${self.price})"

    def __repr__(self):
        return f"Coffee(size='{self.size}', price={self.price})"

# Usage
morning_coffee = Coffee("Large", 4.99)
print(f"Object - {morning_coffee}") # this will print the object
# this will disappeared once added __str__ method
print(f"Coffee size: {morning_coffee.size}")  # Output: Coffee size: Large
print(f"__str__ - {morning_coffee}")  # Coffee size: Large and price: 4.99
print(f"Debugging: {repr(morning_coffee)}") # Debugging: Coffee(size='Large', price=4.99)

__eq__: The Equality Judge

Defines what == means for your object

class Coffee:
    def __init__(self, size, price):
        self.size = size
        self.price = price
    
    def __str__(self):
        return f"{self.size} coffee (${self.price})"

    def __repr__(self):
        return f"Coffee(size='{self.size}', price={self.price})"

    def __eq__(self, other):
        return (self.size == other.size) and (self.price == other.price)

# Usage
morning_coffee = Coffee("Large", 4.99)
print(f"Object - {morning_coffee}") # this will print the object
# this will disappeared once added __str__ method
print(f"Coffee size: {morning_coffee.size}")  # Output: Coffee size: Large
print(f"__str__ - {morning_coffee}")  # Coffee size: Large and price: 4.99
print(f"Debugging: {repr(morning_coffee)}") # Debugging: Coffee(size='Large', price=4.99)

large_coffee = Coffee("Large", 4.99)
print(morning_coffee == large_coffee) # True
small_coffee = Coffee("Small", 3.99)
print(morning_coffee == small_coffee) # False

__len__: The Size Inspector

Makes objects work with len()

class Coffee:
    def __init__(self, size, price):
        self.size = size
        self.price = price
    
    def __str__(self):
        return f"{self.size} coffee (${self.price})"

    def __repr__(self):
        return f"Coffee(size='{self.size}', price={self.price})"

    def __eq__(self, other):
        return (self.size == other.size) and (self.price == other.price)

    def __len__(self):
        return len(self.size)  # Returns character count of size (e.g., "Large" → 5)

# Usage
morning_coffee = Coffee("Large", 4.99)
print(f"Object - {morning_coffee}") # this will print the object
# this will disappeared once added __str__ method
print(f"Coffee size: {morning_coffee.size}")  # Output: Coffee size: Large
print(f"__str__ - {morning_coffee}")  # Coffee size: Large and price: 4.99
print(f"Debugging: {repr(morning_coffee)}") # Debugging: Coffee(size='Large', price=4.99)

large_coffee = Coffee("Large", 4.99)
print(morning_coffee == large_coffee) # True
med_coffee = Coffee("Medium", 3.99)
print(morning_coffee == med_coffee) # False

print(len(morning_coffee)) # 5
print(len(med_coffee)) # 6

Recap

We built a Coffee class step by step using special Python methods:

• __init__ - Sets up the object when created
• __str__ - Controls what prints when you display the object
• __repr__ - Shows how to recreate the object (helpful for debugging)
• __eq__ - Defines how to compare two objects
• __len__ - Lets you check the object’s size with len()

These special methods (called “dunder” methods because of their double underscores) make your objects work naturally with Python’s built-in functions. Each one adds useful behavior while keeping everything working together smoothly.

Thank you!

Thank you for your time and for reading this!