Why Doesn't My PyGame Code Draw Anything?

Why PyGame Fails to Draw Anything

Your PyGame project aims to create a background with a gray bottom and a black top, using the pygame.draw.rect() function. Despite previous successful experiences with this function, you're encountering inconsistent drawing results. Sometimes, you get a black screen, while other times, a gray rectangle appears partially on the screen.

The confusion arises from the fact that drawing alone does not produce visible output in PyGame. After modifying the associated surface, you need to update the display to make those changes visible.

Display Update

Drawing occurs on a Surface object associated with the PyGame display. However, these changes are not automatically reflected in the display. To make them visible, you must explicitly update the display using pygame.display.update() or pygame.display.flip().

pygame.display.flip() updates the entire display, while pygame.display.update() provides more selective updates for specific areas of the screen. It's typically more optimized for software displays but not supported for hardware accelerated displays.

PyGame Application Loop

A typical PyGame application loop consists of several steps:

1. Event handling: Handle events using pygame.event.pump() or pygame.event.get().
2. State updates: Update game states and object positions based on input events and time (frames).
3. Display clearing: Clear the display or draw a background.
4. Scene drawing: Draw all objects in the scene.
5. Display update: Call pygame.display.update() or pygame.display.flip() to make changes visible.
6. Frame rate limiting: Limit frames per second to avoid excessive CPU usage with pygame.time.Clock.tick.

Code Example

Here is a modified code that includes display updates:

import pygame
from pygame.locals import *
pygame.init()

DISPLAY = pygame.display.set_mode((800,800))
pygame.display.set_caption("thing")
clock = pygame.time.Clock()

run = True
while run:
    # handle events
    for event in pygame.event.get():
        if event.type == QUIT:
            run = False

    # clear display
    DISPLAY.fill(0)

    # draw scene
    pygame.draw.rect(DISPLAY, (200,200,200), pygame.Rect(0,400,800,400))

    # update display
    pygame.display.flip()

    # limit frames per second
    clock.tick(60)

pygame.quit()
exit()

By incorporating the necessary display updates, you should now see the expected background with a gray bottom and a black top.

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