Pygame，win.blit无法正常工作

Question

所以这是我的代码，我想要发生的是当我用箭头键向左移动时，矩形（命名为变量z）变为图像bg.png

if keys[pygame.K_LEFT]:
    x -= vel
    win.blit(z, bg (0,0))

不知道我该怎么做

import pygame
pygame.init()

win = pygame.display.set_mode((500,480))        
pygame.display.set_caption("a")
bg = pygame.image.load('bg.png')
x = 50
y = 50
vel = 5
height = 50
width = 40
def redrawGameWindow():
    win.blit(bg, (0,0))
    z = pygame.draw.rect(win, (255, 0, 0), (x, y, width, height ))
    # use variable as pygame z to change sprites
    pygame.display.update()

run = True
while run:
    pygame.time.delay(100)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
    keys = pygame.key.get_pressed()

    if keys[pygame.K_LEFT]:
        x -= vel
        win.blit(z, bg (0,0))
    if keys[pygame.K_RIGHT]:
        x += vel
    if keys[pygame.K_UP]:
        y -= vel
    if keys[pygame.K_DOWN]:
        y += vel

    redrawGameWindow()