Container dunder methods in Python

Teaching a class to behave like a built-in container.

class Playlist:
    def __init__(self, *songs):
        self.songs = list(songs)

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

    def __getitem__(self, i):
        return self.songs[i]

    def __contains__(self, title):
        return title in self.songs


mix = Playlist("Africa", "Take On Me", "Hey Ya")
count = len(mix)
opener = mix[0]
has_it = "Hey Ya" in mix

How it works

1. __len__ powers len() on the object.
2. __getitem__ enables indexing with [].
3. __contains__ answers the in operator.

Keywords and builtins used here

• Playlist
• class
• def
• len
• list
• return
• self

The run, in numbers

Lines

18

Characters to type

327

Tokens

97

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 55 seconds.

Type this snippet

Step 3 of 5 in Protocols & context managers, step 17 of 53 in Pythonic Python.

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