__repr__, __len__, __iter__, __contains__ & __add__

Objects that feel built-in

Lesson 11 of 20 · 12 min · Module 3: Objects that behave

Python’s own types work with len(), for loops, in, and +. Your classes can too, by defining special methods whose names start and end with double underscores.

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By the end, you’ll be able to…

Implement common special methods
Make objects print helpfully
Return new objects from operators

The idea

__repr__ controls how an object prints in the console. __len__ powers len(obj), __iter__ powers for loops, __contains__ powers in, and __add__ powers +. Return NotImplemented from __add__ for types you do not support.

class Basket:
    def __init__(self, items):
        self.items = list(items)
    def __len__(self):
        return len(self.items)
    def __iter__(self):
        return iter(self.items)

print(len(Basket(["apple", "pear"])))

01Special methods connect to syntax

len(p) calls p.__len__(), for song in p calls p.__iter__(), and "x" in p calls p.__contains__("x"). Python looks these up on your class automatically.

02A good repr helps debugging

When a list of playlists prints as [Playlist('Road trip', 3 songs)] instead of [<__main__.Playlist object at 0x…>], bugs become far easier to spot.

class P:
    def __repr__(self):
        return "P(42)"

print([P()])

Output

[P(42)]

03Operators should not mutate

a + b on built-in types returns a new object and leaves a and b alone. Follow the same rule, so users of your class are never surprised.

Your turn

Write a class Playlist(name, songs=None). Support len(), iteration over songs in order, case-insensitive in, and repr() as Playlist('Road trip', 3 songs) (use 1 song for one). a + b returns a new playlist named "A + B" with the songs of a, then the songs of b that are not already in it (case-insensitive).

What the checks look for

Supports len() and iteration in order
in ignores letter case
repr shows the name and song count
+ merges without duplicates and without changing the originals
Copies the songs list it receives

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Take this with you

Special methods make your objects work naturally with Python’s syntax.

Go a little further

Add __eq__ so two playlists with the same name and songs compare equal.