我对pygame中的子弹运动有问题。

import pygame
from pygame.locals import *

# 初始化Pygame
pygame.init()

# 设置窗口尺寸
screen_width, screen_height = 800, 600
screen = pygame.display.set_mode((screen_width, screen_height))

# 定义子弹的属性
bullet_speed = 5
bullet_width, bullet_height = 10, 20
bullet_color = (255, 0, 0)  # 红色

# 定义子弹的初始位置和状态
bullet_x = screen_width // 2 - bullet_width // 2
bullet_y = screen_height - bullet_height
bullet_state = "ready"  # "ready"表示子弹可以发射，"fire"表示子弹正在飞行

def move_bullet():
    global bullet_y, bullet_state
    
    if bullet_state == "fire":
        bullet_y -= bullet_speed
        if bullet_y < 0:
            bullet_state = "ready"
    
    if bullet_state == "ready":
        bullet_y = screen_height - bullet_height

def draw_bullet():
    pygame.draw.rect(screen, bullet_color, (bullet_x, bullet_y, bullet_width, bullet_height))

# 游戏主循环
running = True
while running:
    screen.fill((0, 0, 0))  # 清空屏幕
    
    for event in pygame.event.get():
        if event.type == QUIT:
            running = False
        
        if event.type == KEYDOWN:
            if event.key == K_SPACE and bullet_state == "ready":
                bullet_state = "fire"
    
    move_bullet()
    draw_bullet()
    
    pygame.display.update()

# 退出游戏
pygame.quit()