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Engineering LibreTexts

3.4: Game Loops and Game States

  • Page ID
  • import pygame, sys
    from pygame.locals import *
    DISPLAYSURF = pygame.display.set_mode((400, 300))
    pygame.display.set_caption('Hello World!')
    while True: # main game loop
        for event in pygame.event.get():
            if event.type == QUIT:

    Line 7 is a while loop that has a condition of simply the value True. This means that it never exits due to its condition evaluating to False. The only way the program execution will ever exit the loop is if a break statement is executed (which moves execution to the first line after the loop) or sys.exit() (which terminates the program). If a loop like this was inside a function, a return statement will also move execution out of the loop (as well as the function too).

    The games in this book all have these while True loops in them along with a comment calling it the "main game loop". A game loop (also called a main loop) is a loop where the code does three things:

    1. Handles events.
    2. Updates the game state.
    3. Draws the game state to the screen.

    The game state is simply a way of referring to a set of values for all the variables in a game program. In many games, the game state includes the values in the variables that tracks the player’s health and position, the health and position of any enemies, which marks have been made on a board, the score, or whose turn it is. Whenever something happens like the player taking damage (which lowers their health value), or an enemy moves somewhere, or something happens in the game world we say that the game state has changed.

    If you’ve ever played a game that let you save, the "save state" is the game state at the point that you’ve saved it. In most games, pausing the game will prevent the game state from changing.

    Since the game state is usually updated in response to events (such as mouse clicks or keyboard presses) or the passage of time, the game loop is constantly checking and re-checking many times a second for any new events that have happened. Inside the main loop is code that looks at which events have been created (with Pygame, this is done by calling the pygame.event.get() function). The main loop also has code that updates the game state based on which events have been created. This is usually called event handling.

    Figure 5