pygame精灵加载动画

import pygame
from pygame.locals import *
class MySprite(pygame.sprite.Sprite):
    def __init__(self, target):
        pygame.sprite.Sprite.__init__(self)
        self.target_surface = target
        self.image = None
        self.master_image = None
        self.rect = None
        self.topleft = 0,0
        self.frame = 0
        self.old_frame = -1
        self.frame_width = 1
        self.frame_height = 1
        self.first_frame = 0
        self.last_frame = 0
        self.columns = 1
        self.last_time = 0
    def load(self, filename, width, height, columns):
        self.master_image = pygame.image.load(filename).convert_alpha()
        self.frame_width = width
        self.frame_height = height
        self.rect = 0,0,width,height
        self.columns = columns
        self.rect = self.master_image.get_rect()
        self.last_frame = (self.rect.width // width) * (self.rect.height // height) - 1
    def update(self, current_time, rate=60):
        if current_time > self.last_time + rate:
            self.frame += 1
        if self.frame > self.last_frame:
            self.frame = self.first_frame
            self.last_time = current_time
        if self.frame != self.old_frame:
            frame_x = (self.frame % self.columns) * self.frame_width
            frame_y = (self.frame // self.columns) * self.frame_height
            rect = ( frame_x, frame_y, self.frame_width, self.frame_height )
            self.image = self.master_image.subsurface(rect)
            self.old_frame = self.frame
            #画面移动
            self.rect.top=self.rect.top+1
            self.rect.left=self.rect.left+1


pygame.init()
screen = pygame.display.set_mode((800,600),0,32)
pygame.display.set_caption("精灵类动画测试")
font = pygame.font.Font(None, 18)
framerate = pygame.time.Clock()

cat = MySprite(screen)
cat.load("sprite2.png", 92, 95, 4)           #精灵图片
#cat.load("sprite3.png", 85, 132, 4) #精灵图片，宽度，高度，列数
group = pygame.sprite.Group()
group.add(cat)
while True:
    framerate.tick(10)
    ticks = pygame.time.get_ticks()
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            exit()
        key = pygame.key.get_pressed()
        if key[pygame.K_ESCAPE]:
            exit()

    screen.fill((255,255,255))
    group.update(ticks)
    group.draw(screen)
    pygame.display.update()