11. Class Composition

See: https://www.geeksforgeeks.org/inheritance-and-composition-in-python/

See: https://medium.com/swlh/the-best-way-to-understand-composition-in-python-5-case-studies-and-solution-4b23a6a2cc38

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11.1. Composition

Composition models a has-a-relationship.

Use composition to create components that can be reused by multiple classes.

A composite class can be assigned to an instance variable.

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11.2. Board Asteroid

In the code below, the composite class, Asteroid, is accessed via the self.asteroid1 and self.asteroid2 attribute of the Board class.

import random

class Asteroid:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def move(self):
        self.y = self.y + 1
        if self.y == 5:
            self.y = 0
            self.x = random.randint(0, 4)


class Board:
    def __init__(self):
        self.score = 0
        self.asteroid1 = Asteroid(random.randint(0, 4), 0)
        self.asteroid2 = Asteroid(random.randint(0, 4), 1)

    def get_board(self):
        self.board = []
        self.board.append((self.asteroid1.x, self.asteroid1.y))
        self.board.append((self.asteroid2.x, self.asteroid2.y))
        return self.board


game_board = Board()
print(game_board.get_board())

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11.3. Board Asteroid Ship

In the code below, the composite class, Ship has been added so that it is accessed via the self.ship attribute of the Board class.

import random

class Asteroid:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def move(self):
        self.y = self.y + 1
        if self.y == 5:
            self.y = 0
            self.x = random.randint(0, 4)

class Ship:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def move(self):
        if button_a.was_pressed() and self.x != 0:
            self.x = self.x - 1
        elif button_b.was_pressed() and self.x != 4:
            self.x = self.x + 1

class Board:
    def __init__(self):
        self.score = 0
        self.ship = Ship(2, 4)
        self.asteroid1 = Asteroid(random.randint(0, 4), 0)
        self.asteroid2 = Asteroid(random.randint(0, 4), 1)

    def get_board(self):
        self.board = []
        self.board.append((self.ship.x, self.ship.y))
        self.board.append((self.asteroid1.x, self.asteroid1.y))
        self.board.append((self.asteroid2.x, self.asteroid2.y))
        return self.board


game_board = Board()
print(game_board.get_board())