Magic & Dunder Methods (__str__, __repr__)

In Python, methods starting and ending with double underscores are called Dunder Methods (short for Double Underscore) or Magic Methods. You already know the most famous one: __init__().

Dunder methods allow custom objects to interact seamlessly with built-in Python syntax like print(), len(), +, or ==.

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1. Why Do We Need __str__?

If you print a custom object without defining how it should be displayed, Python outputs an unreadable memory address:

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

my_book = Book("Python Mastery", 350)
print(my_book)  # Output: <__main__.Book object at 0x000001F8C9E4B1C0> ❌ Ugly!

To fix this, define __str__() to return a human-readable string representation:

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

    # Defines what happens when print() is called on the object
    def __str__(self):
        return f"📘 '{self.title}' ({self.pages} pages)"

my_book = Book("Python Mastery", 350)
print(my_book)  # Output: 📘 'Python Mastery' (350 pages) ✅ Clean!

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2. Commonly Used Dunder Methods Table

Here are the most essential dunder methods you will use in advanced software development:

Dunder Method	Triggered By	Purpose	Example Use Case
__init__(self, ...)	Object(...)	Constructor	Initializing attributes on creation.
__str__(self)	print(obj) or str(obj)	User-friendly text	Displaying clean info to end-users.
__repr__(self)	repr(obj) or terminal	Developer debug representation	Accurate string representation for debugging logs.
__len__(self)	len(obj)	Object length	Returning the size of a custom playlist or collection.
__eq__(self, other)	obj1 == obj2	Equality comparison	Checking if two custom user objects have matching IDs.

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3. Practical Code Example: Custom Playlist

class Playlist:
    def __init__(self, name):
        self.name = name
        self.songs = []

    def add_song(self, song_title):
        self.songs.append(song_title)

    # Magic method for print()
    def __str__(self):
        return f"🎵 Playlist: {self.name} ({len(self.songs)} tracks)"

    # Magic method for len()
    def __len__(self):
        return len(self.songs)

my_music = Playlist("Telugu Super Hits")
my_music.add_song("Naatu Naatu")
my_music.add_song("Samajavaragamana")

print(my_music)      # Output: 🎵 Playlist: Telugu Super Hits (2 tracks)
print(len(my_music)) # Output: 2