如何在pygame中控制单发子弹

import pygame
import sys

pygame.init()

screen_width = 800
screen_height = 600
screen = pygame.display.set_mode((screen_width, screen_height))
pygame.display.set_caption("控制单发子弹")

white = (255, 255, 255)

class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = pygame.image.load("player.png")
        self.rect = self.image.get_rect()
        self.rect.centerx = screen_width // 2
        self.rect.bottom = screen_height - 10
        self.speed_x = 0

    def update(self):
        self.rect.x += self.speed_x
        if self.rect.left < 0:
            self.rect.left = 0
        if self.rect.right > screen_width:
            self.rect.right = screen_width

    def shoot(self):
        bullet = Bullet(self.rect.centerx, self.rect.top)
        all_sprites.add(bullet)
        bullets.add(bullet)

class Bullet(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        self.image = pygame.Surface((5, 10))
        self.image.fill(white)
        self.rect = self.image.get_rect()
        self.rect.centerx = x
        self.rect.bottom = y
        self.speed_y = -10

    def update(self):
        self.rect.y += self.speed_y
        if self.rect.bottom < 0:
            self.kill()

all_sprites = pygame.sprite.Group()
bullets = pygame.sprite.Group()
player = Player()
all_sprites.add(player)

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_SPACE:
                player.shoot()

    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        player.speed_x = -5
    elif keys[pygame.K_RIGHT]:
        player.speed_x = 5
    else:
        player.speed_x = 0

    all_sprites.update()

    screen.fill((0, 0, 0))
    all_sprites.draw(screen)

    pygame.display.flip()

pygame.quit()
sys.exit()