Example

Here is an example that implements all that we learned in this chapter. It creates a window, creates an object to draw to the screen, and then moves that object each iteration.

import pygame

RED = (255, 0, 0)
BLACK = (0, 0, 0)
x = y = 0
width = 100
height = 50

# initializes imported pygame modules
pygame.init()

# creates pygame window that is 500 pixels wide and 400 high
# sets the caption of the window to "My first pygame app!"
window = pygame.display.set_mode((500, 400))
pygame.display.set_caption("My first pygame app!")

# this is where the game loop begins
run = True
while run:
    # change the coordinates
    x, y = x + 1, y + 1

    # draw a black screen over the previous frame
    window.fill(BLACK)

    # draw a new rectangle and update the screen
    pygame.draw.rect(window, RED, (x, y, width, height))
    pygame.display.update()

    for event in pygame.event.get():
        # checks if the close button is pressed
        # if so, exit the game loop
        if event.type == pygame.QUIT:
            run = False

# deactivate pygame modules, opposite of pygame.init()
pygame.quit()

View code on GitHub (useful for line numbers)

Code Breakdown

If you're having trouble understanding the code, here's a full breakdown:

• Line 1 - Imports the pygame module.
• Lines 3-7 - Defines the background color black, the red color that we want our rectangle to be, and our rectangle's 4 attributes (top-left x, top-left y, width, and height).
• Line 10 - Initializes the pygame module so that we can use it.
• Line 14 - Creates the window (just like in the basic window).
• Line 15 - Sets window caption (also just like in the basic window).
• Line 18 - Sets run to True.
• Line 19 - The start of the game loop.
• Line 21 - Change both the x and y coordinates using variable swapping (mentioned in 14.4 Variable Swapping).
• Line 24 - Clear the previous frame by drawing black over it.
• Line 27 - Draw the rectangle onto the screen (just like how we did it in move objects).
• Line 28 - Update the screen. (this must be done if you want to see anything on the screen beside the initial black).
• Line 30-34 - pygame.event.get() returns a list of pygame events. If the close button is pressed, that will add a pygame event of type pygame.QUIT to the list returned by pygame.event.get(). So, if the event's type is pygame.QUIT, then update the game state from running to not running.
• Line 37 - Deactivate the pygame module.