python - Why is my pygame application loop not working properly?

Question

This question already has answers here:

Answer 1

Your basic misunderstanding, is that you try to draw the background at the position of an object, then you move the object and blit it finally on its new position. That all is not necessary.
In common the entire scene is drawn in each frame in the main application loop. It is sufficient to draw the background to the entire window and blit each object on top of it. Note, you do not see the changes of the window surface immediately. The changes become visible, when the display is updated by pygame.display.update() or pygame.display.flip():

The main application loop has to:

• handle the events by either pygame.event.pump() or pygame.event.get().
• update the game states and positions of objects dependent on the input events and time (respectively frames)
• clear the entire display or draw the background
• draw the entire scene (blit all the objects)
• update the display by either pygame.display.update() or pygame.display.flip()

e.g.:

while 1:

    # handle events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()

    # update objects (depends on input events and frames)
    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        objects[0].move_left()    
    if keys[pygame.K_RIGHT]:
        objects[0].move_right()
    if keys[pygame.K_UP]:
        objects[0].move_up()
    if keys[pygame.K_DOWN]:
        objects[0].move_down()

    for num in range(num_objects - 1):
        objects[num + 1].rand_move()

    # draw background
    screen.blit(background, (0, 0))

    # draw scene
    for o in objects:
        screen.blit(o.image, o.pos)

    # update dispaly
    pygame.display.update()
    pygame.time.delay(100)

---

Minimal example: repl.it/@Rabbid76/PyGame-MinimalApplicationLoop