我试着用PyGame做一个简单的迷宫游戏作为初学者的项目，但是当我试着使用 event.type 时，它说，

 Traceback (most recent call last):
  File "simple_maze.py", line 22, in <module>
    if event.type == pygame.KEYDOWN:
NameError: name 'event' is not defined 

作为参考，这是我当前的代码。

import pygame
screendim = (500, 500)
blu = (0, 0, 255)
red = (255, 0, 0)
sc = (0, 0)
pr = 50
pygame.init()
screen = pygame.display.set_mode(screendim)
pygame.draw.circle(screen, blu, (250, 10), 10)
pygame.draw.rect(screen, red, pygame.Rect(0, 25, 450, 15))