8. Class methods

A class method isn’t bound to any specific instance. It’s bound to the class only.

The class methods can access and modify the class state.

To define a class method:

First place the @classmethod decorator above the method definition.

The @classmethod decorator will change an instance method to a class method.

Second, rename the self parameter to cls.

The cls means class. Since class is a keyword, it can’t be used as a parameter.

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8.1. @classmethod and cls parameter

Class methods use the cls parameter as the first parameter for passing the class.

In the code below, game_number is a class variable.

The set_game_number function takes cls as the first parameter, and has game_number as a second parameter.

The class variable, cls.game_number, is set to the value of game_number.

The function is preceded by the decorator, @classmethod, which is required to make the function work as a class method, so the function acts on the class rather than an instance of the class.

The class method is called on the class using LevelGame.set_game_number(1) which sets the class variable, game_number, to 1.

The class method, set_game_number, can also be called on an instance, game, such as, game.set_game_number(1).

While this works like calling on the class, it makes more sense to call it on the class itself.

class LevelGame:
    game_number = 0

    def __init__(self, game_level):
        self.game_level = game_level

    @classmethod
    def set_game_number(cls, game_number):
        cls.game_number = game_number

game = LevelGame(1)
LevelGame.set_game_number(1)
print(game.game_number)

Tasks

1. Modify the code so that the game_number is set to 5, then check its value for the instance, game.

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8.2. Using a Class method in the __init__ function

In the code below, the class method increase_lives is called by the __init__ function.

It increases the class variable, lives, by 1.

class LevelGame:
    lives = 3

    def __init__(self, game_level):
        self.game_level = game_level
        self.increase_lives()

    @classmethod
    def increase_lives(cls):
        cls.lives += 1

game1 = LevelGame(1)
print(game1.lives)

game2 = LevelGame(2)
print(game2.lives)

Tasks

1. Modify the code so that the lives starts at 1 for the first time that LevelGame is called, then increase_lives increases lives by 2 each time it is called.

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8.3. Class methods as alternative constructors

In the code below, the class method make_game provides an alternate constructor to that of just calling the class to create a new instance.

game1 = LevelGame(game_level=1) results in game_level = 1 and game_lives = 5

game2 = LevelGame.make_game(game_level=1, game_lives=3) results in game_level = 1 and game_lives = 3

The last line of code, return cls(game_level), calls the __init__ function to create the new instance.

class LevelGame:
    game_lives = 5

    def __init__(self, game_level):
        self.game_level = game_level

    @classmethod
    def set_game_lives(cls, game_lives):
        cls.game_lives = game_lives

    @classmethod
    def make_game(cls, game_level, game_lives):
        cls.set_game_lives(game_lives)
        return cls(game_level)


game1 = LevelGame(game_level=1)
print(game1.game_level, game1.game_lives)

game2 = LevelGame.make_game(game_level=1, game_lives=3)
print(game2.game_level, game2.game_lives)

Tasks

1. Write code to create a game at level 10 with 2 lives.