4. Instance variables

4.1. Variables unique to the instance

Instance variables are variables, created in a class, that are unique to the instance.
The __init__() function assigns values to instance variables when the object is created.
e.g. self.game_level = 1
self.game_level is an instance variable. It is referenced using the self keyword.

class LevelGame:
    def __init__(self):
        self.game_level = 1

game = LevelGame()
print(game.game_level)

4.2. Instantiating a class instance with arguments

Parameters can be used in the __init__ definition so that arguments can be passed when the object is instantiated.
e.g. the level parameter has been used in __init__(self, level).
When game is instantiated using game = LevelGame(1), a value of 1 is passed in as the argument, so that level = 1.

class LevelGame:
    def __init__(self, level):
        self.game_level = level

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

4.3. Customary variable names

Is it customary to use the same name for the parameters as for the instance variables.
e.g. self.game_level = game_level
In the code above, a different variable name, level, has been used instead of game_level to help see what is happening with the variables.
The code below follows the custom.

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

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

Tasks

Run the code above and check the print output is 1.
Modify the code to use game = LevelGame(2) and check the print output.

4.4. Default paramaters

A default value for a pareter can be set.
e.g. the level parameter can be set to a value of 1 by default in __init__(self, level=1).
When game is instantiated using game = LevelGame(), level = 1 by default.

class LevelGame:
    def __init__(self, level=1):
        self.game_level = level

game = LevelGame()
print(game.game_level)

4.5. Changing Instance variables in an instance

An instance variable can be altered for a particular instance.

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

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

game = LevelGame(1)
game.game_level = 2
print(game.game_level)

Tasks

Check the print output to verify that the instance has a different value to that set when the instance was created.
Add code after the instance value is changed so that the game.game_level is set to 3, then check the value for the instance to see if it is affected.

Tip

Direct access to instance variables outside of the class code can be prevented by using double underscores.
self.game_level could be changed to self.__game_level so that game.__game_level would be blocked.
However, game._LevelGame__game_level still enables access to the attribute.
Setters (a method to set self.__game_level) and getters (a method to return self.__game_level) are methods used along with this approach, known as encapsulation.

class LevelGame:
    def __init__(self, game_level):
        self.__game_level = game_level

    def set_game_level(self, game_level):
        self.__game_level = game_level

    def get_game_level(self):
        return self.__game_level

game = LevelGame(3)
print(game._LevelGame__game_level)
print(game.get_game_level())