关于#pygame#修改变量的问题，如何解决？

不知道为什么34行报错；

想让player初始是me1
按下右方向键后player显示为me4

import pygame

speed = 1
me1 = pygame.image.load("./游戏素材/me下.png")
me2 = pygame.image.load("./游戏素材/me上.png")
me3 = pygame.image.load("./游戏素材/me左.png")
me4 = pygame.image.load("./游戏素材/me右.png")
player = me1


def judge():
    if hero.x <= 0:
        hero.x = 0
    if hero.x >= 950:
        hero.x = 950

    if hero.y <= 0:
        hero.y = 0
    if hero.y >= 550:
        hero.y = 550


def _quit():
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()


def handler():

    keys_pressed = pygame.key.get_pressed()
    if keys_pressed[pygame.K_RIGHT]:
        hero.x += speed
        global player = me4
        window.blit(player, hero)
    if keys_pressed[pygame.K_LEFT]:
        hero.x -= speed
    else:
        hero.x += 0
    if keys_pressed[pygame.K_UP]:
        hero.y -= speed
    if keys_pressed[pygame.K_DOWN]:
        hero.y += speed
    else:
        hero.y += 0


pygame.init()

window = pygame.display.set_mode((1000, 618))
background = pygame.image.load("./游戏素材/background.png")

hero = pygame.Rect(20, 20, 40, 40)
clock = pygame.time.Clock()
window.blit(player, (20, 20))

while 1:
    window.blit(background, (0, 0))
    window.blit(player, hero)
    pygame.display.update()

    judge()
    handler()
    _quit()