如何在pygame中的函数定义中分配sprite层？

import pygame
import sys

pygame.init()
screen = pygame.display.set_mode((800, 600))

all_sprites = pygame.sprite.Group()

class MySprite(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.Surface((50, 50))
        self.image.fill((255, 0, 0))
        self.rect = self.image.get_rect()
        self.rect.center = (400, 300)

    def update(self):
        self.rect.x += 1

def create_sprite():
    sprite = MySprite()
    all_sprites.add(sprite)

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()

    all_sprites.update()
    screen.fill((0, 0, 0))
    all_sprites.draw(screen)
    pygame.display.flip()