在pygame中无法获取'function‘来左右移动船体

import pygame

# 初始化pygame
pygame.init()

# 设置屏幕宽高
screen_width = 800
screen_height = 600
screen = pygame.display.set_mode((screen_width, screen_height))
pygame.display.set_caption("Move Ship")

# 定义船体初始位置和速度
ship_x = screen_width // 2
ship_y = screen_height - 50
ship_speed = 5

# 游戏主循环
running = True
while running:
    screen.fill((255, 255, 255))  # 填充背景色

    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT:
                ship_x -= ship_speed
            elif event.key == pygame.K_RIGHT:
                ship_x += ship_speed

    # 边界检测
    if ship_x < 0:
        ship_x = 0
    elif ship_x > screen_width - 50:  # 假设船体宽度为50
        ship_x = screen_width - 50

    # 绘制船体
    pygame.draw.rect(screen, (0, 0, 255), (ship_x, ship_y, 50, 50))

    # 更新屏幕
    pygame.display.flip()

# 退出游戏
pygame.quit()