Move an object every few seconds in Pygame

You can use an Event for this together with pygame.time.set_timer():

pygame.time.set_timer()
repeatedly create an event on the event queue
set_timer(eventid, milliseconds) -> None

Set an event type to appear on the event queue every given number of milliseconds

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Here’s a simple, complete example. Note how the enemies move every 1000ms sideways, every 3500ms downwards, and you can shoot every 450ms (all using events).

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import pygame

# you'll be able to shoot every 450ms
RELOAD_SPEED = 450

# the foes move every 1000ms sideways and every 3500ms down
MOVE_SIDE = 1000
MOVE_DOWN = 3500

screen = pygame.display.set_mode((300, 200))
clock = pygame.time.Clock()

pygame.display.set_caption("Micro Invader")

# create a bunch of events 
move_side_event = pygame.USEREVENT + 1
move_down_event = pygame.USEREVENT + 2
reloaded_event  = pygame.USEREVENT + 3

move_left, reloaded = True, True

invaders, colors, shots = [], [] ,[]
for x in range(15, 300, 15):
    for y in range(10, 100, 15):
        invaders.append(pygame.Rect(x, y, 7, 7))
        colors.append(((x * 0.7) % 256, (y * 2.4) % 256))

# set timer for the movement events
pygame.time.set_timer(move_side_event, MOVE_SIDE)
pygame.time.set_timer(move_down_event, MOVE_DOWN)

player = pygame.Rect(150, 180, 10, 7)

while True:
    clock.tick(40)
    if pygame.event.get(pygame.QUIT): break
    for e in pygame.event.get():
        if e.type == move_side_event:
            for invader in invaders:
                invader.move_ip((-10 if move_left else 10, 0))
            move_left = not move_left
        elif e.type == move_down_event:
            for invader in invaders:
                invader.move_ip(0, 10)
        elif e.type == reloaded_event:
            # when the reload timer runs out, reset it
            reloaded = True
            pygame.time.set_timer(reloaded_event, 0)

    for shot in shots[:]:
        shot.move_ip((0, -4))
        if not screen.get_rect().contains(shot):
            shots.remove(shot)
        else:
            hit = False
            for invader in invaders[:]:
                if invader.colliderect(shot):
                    hit = True
                    i = invaders.index(invader)
                    del colors[i]
                    del invaders[i]
            if hit:
                shots.remove(shot)

    pressed = pygame.key.get_pressed()
    if pressed[pygame.K_LEFT]: player.move_ip((-4, 0))
    if pressed[pygame.K_RIGHT]: player.move_ip((4, 0))

    if pressed[pygame.K_SPACE]: 
        if reloaded:
            shots.append(player.copy())
            reloaded = False
            # when shooting, create a timeout of RELOAD_SPEED
            pygame.time.set_timer(reloaded_event, RELOAD_SPEED)

    player.clamp_ip(screen.get_rect())

    screen.fill((0, 0, 0))

    for invader, (a, b) in zip(invaders, colors): 
        pygame.draw.rect(screen, (150, a, b), invader)

    for shot in shots: 
        pygame.draw.rect(screen, (255, 180, 0), shot)

    pygame.draw.rect(screen, (180, 180, 180), player)    
    pygame.display.flip()