为什么在程序中循环时没有定义屏幕？

Question

当我试图在鼠标坐标处形成一个圆圈时，我使用以下代码：

def main():
    screen.clear()
    mx, my = pygame.mouse.get_pos()
    screen.draw.circle((mx, my), 30, (255, 255, 255))

它只起过一次作用，但当我将它改为如下时：

def main():
    screen.clear()
    mx, my = pygame.mouse.get_pos()
    screen.draw.circle((mx, my), 30, (255, 255, 255))
    screen.clear()
while (1):
    main()

它会引发此错误：

Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3/dist-packages/pgzero/__main__.py", line 3, in <module>
    main()
  File "/usr/lib/python3/dist-packages/pgzero/runner.py", line 92, in main
    exec(code, mod.__dict__)
  File "virus-simulation!.py", line 6, in <module>
    main()
  File "virus-simulation!.py", line 2, in main
    screen.clear()
NameError: name 'screen' is not defined

有什么原因吗？

Answer 1

我不知道你的屏幕是在哪里定义的，所以我不能确定是什么导致了这个问题。下面是一些基于示例的工作代码。它在鼠标位置画一个圆圈。

import pygame

pygame.init()

class CDraw():
   def __init__(self, sc): 
       self.scrn = sc
   def circle(self, p, r, c):
       pygame.draw.circle(self.scrn, c, p, r)
       
class CScreen():        
   def __init__(self):
       self.scrn = pygame.display.set_mode((400,400))
       self.draw = CDraw(self.scrn)
   def clear(self):
       self.scrn.fill((0,200,0))

screen = CScreen()

def main():
    screen.clear()
    mx, my = pygame.mouse.get_pos()
    screen.draw.circle((mx, my), 30, (255, 255, 255))
    #screen.clear()  # will remove everything
    for event in pygame.event.get():   # needed for os events
        if event.type == pygame.QUIT:
            exit()
    pygame.display.update()  # needed to do screen update
    
while (1):
    main()