Dunder Methods in Python

🤔 What Are Dunder Methods?

Dunder methods are special functions that have double underscores before and after their names, like:

__init__, __str__, __repr__, __eq__, __len__ ...

“Dunder” means “double underscore”. These methods are not magic — they are Python’s way of saying:

“If you want your object to work with print(), ==, len(), for, etc., then define this method.”

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🔄 The Core Idea

When you use normal Python code, like print(obj) or obj == other, Python checks: “Does this object have a special method that tells me how to do this?”

It automatically looks for a dunder method to use — for example:

• print(obj) → Python tries obj.__str__()
• obj == other → Python tries obj.__eq__(other)
• len(obj) → Python tries obj.__len__()

These methods connect your object to Python’s built-in behavior.

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🧱 Let’s Build a Simple Class

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

This creates a Dog object with a name and age.

Now let’s add dunder methods one at a time.

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1. __str__: Friendly Display

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

What it does: Used when you do print(dog) — gives a nice, human-readable output.

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2. __repr__: Debugging Output

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

What it does: Used in debugging, lists, shell — shows how the object is built.

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3. __eq__: Compare by Value

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

What it does: Lets dog1 == dog2 work based on their values, not memory location.

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4. __len__: Custom Length

    def __len__(self):
        return self.age

What it does: Lets you use len(dog) — maybe to represent dog’s age.

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5. __call__: Act Like a Function

    def __call__(self):
        print(f"{self.name} says woof!")

What it does: Lets you write dog() like calling a function.

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✅ Full Class Together

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

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

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

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

    def __len__(self):
        return self.age

    def __call__(self):
        print(f"{self.name} says woof!")

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🧠 Summary Table

Python Code	What It Does	Dunder Method Behind It
Dog("Buddy", 5)	Creates object	__init__
print(dog)	Friendly string	__str__
dog == other	Compare values	__eq__
len(dog)	Custom length	__len__
dog()	Makes object callable	__call__
print([dog])	Shows object in debug/log form	__repr__

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🪄 Final Advice for Beginners

• Start with just __init__ and __str__. Then add more as you need them.
• Think: “What does this object need to do?” and then see if there’s a dunder method for it.
• Don’t memorize them — just understand the pattern: Python uses your methods automatically if you name them correctly.