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.

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

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

game = LevelGame(1)


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

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

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

game1 = LevelGame(1)

game2 = LevelGame(2)


  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.

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

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

    def make_game(cls, game_level, 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)


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