6. Class variables

6.1. Class variables common to instances

A class variable is shared by all instances of a class.

In the code below, game_level is a class variable.

In the code below, both game1.game_level and game2.game_level have the same value.

class LevelGame:
    game_level = 1

game1 = LevelGame()
print(game1.game_level)

game2 = LevelGame()
print(game2.game_level)

Tasks

1. Check that the print output is the same for each print statement above.

2. Modify the code so that the class definition has a game_level of 0, then check its value for both instances.

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6.2. Changing Class variables for all instances

A class variable can be altered in the class.

In the code below, LevelGame.game_level = 3, the class variable is changed to 3.

The change in the class variable results in the same change for the instance values of game1.game_level and game2.game_level.

class LevelGame:
    game_level = 1

game1 = LevelGame()
game2 = LevelGame()

LevelGame.game_level = 3
print(game1.game_level)
print(game2.game_level)

Tasks

1. Check that print output is the same for each statement above.

2. Modify the code so that the LevelGame.game_level is set to 5, then check its value for both instances.

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6.3. Changing Class variables in an instance

A class variable can be altered for a particular instance.

game.game_level = 2 changes the value of the variable within the instance.

LevelGame.game_level is not altered.

class LevelGame:
    game_level = 1

game = LevelGame()
game.game_level = 2
print(game.game_level)
print(LevelGame.game_level)

Tasks

1. Check the print output to verify that the instance has a different value to the class.

2. Add code after the instance value is changed so that the LevelGame.game_level is set to 5, then check the value for the instance to see if it is affected.

Tip

1. Use Class variables when the same value is needed in all instances.

2. Avoid changing class variables in instances since it can lead to confusion.

3. Use Instance variables instead of class variables when different values are needed in each instance.

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6.4. Modifying Class variables during instantiation

In the code below, game_number is a class variable.

LevelGame.game_number += 1 is used to increment the game number by 1 each time a new LevelGame is instantiated.

Since game_number is a class variable, it is accessed via LevelGame.game_number within the __init__ function. The class name, LevelGame is used instead of self.

class LevelGame:
    game_number = 0

    def __init__(self, game_level):
        self.game_level = game_level
        LevelGame.game_number += 1

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

game2 = LevelGame(2)
print(game.game_number)
print(game2.game_number)

Tasks

1. Check that print output shows that the class variable is the same for both instances.

2. Add a third instance, game3, then check the class variable value for all three instances.