Object Composition

You already know that objects can hold data as attributes. Those attributes can be numbers, strings, or booleans. They can also be other objects.

When one object holds other objects as attributes, that’s called composition. It lets you build complex things out of simpler, focused pieces.

A Playlist Made of Songs

Say you’re building a playlist. Without composition, every song is a separate set of variables:

playlist_name = "My Favorites"
 
song1_title = "Bohemian Rhapsody"
song1_artist = "Queen"
song1_duration = 354
 
song2_title = "Thriller"
song2_artist = "Michael Jackson"
song2_duration = 357
 
song3_title = "Billie Jean"
song3_artist = "Michael Jackson"
song3_duration = 294

Every new song means three new variables. There’s no way to pass “a song” anywhere — it’s always three separate pieces.

With composition, you create a Song class to bundle that data, and a Playlist class to hold a collection of songs:

class Song:
    def __init__(self, title, artist, duration):
        self.title = title
        self.artist = artist
        self.duration = duration
 
class Playlist:
    def __init__(self, name):
        self.name = name
        self.songs = []
 
    def add(self, song):
        self.songs.append(song)

playlist = Playlist("My Favorites")
playlist.add(Song("Bohemian Rhapsody", "Queen", 354))
playlist.add(Song("Thriller", "Michael Jackson", 357))
playlist.add(Song("Billie Jean", "Michael Jackson", 294))

Each song travels as one unit. The playlist holds a list of them and doesn’t need to know anything about the songs themselves.

Why This Is Useful

Composition lets you group related data into one object and pass it around as a single unit. When something needs to change, you change it in one place.

Adding a 50th song means one new line — not three new variables.

Has-a vs Is-a

Composition describes a has-a relationship. A playlist has songs. A game has players. A team has members.

This is different from inheritance, where a Circle is a Shape. You’ll see how those two ideas work together in the next section.

In the Generative Art Project

Canvas and Shape follow the same pattern. A Canvas holds a list of shape objects, and each shape carries its own data. When it’s time to render, Canvas loops through its shapes and hands each one off to be drawn.

Try it out