Dealing with a spritesheet of 16 images in a 4x4 matrix

sprite_sheet.py

import sys
import pygame

class Character(pygame.sprite.Sprite):
    def __init__(self, *group):
        super(Character, self).__init__(*group)
        self.sheet = pygame.image.load('char.png')
        self.at = 0
        self.elapsed = 0
        self.rect = pygame.rect.Rect(50, 100, 50, 50)

    @property
    def image(self):
        return self.image_at(self.at)

    def image_at(self, slot):
        i, j = slot % 5, slot / 5
        SW, SH = 40, 49
        SX, SY = (2 + SW) * i, (2 + SH) * j
        rect = pygame.Rect(SX, SY, SW, SH)
        image = pygame.Surface(rect.size).convert()
        image.blit(self.sheet, (0,0), rect)
        return image

    def update(self, millis):
        self.elapsed += millis
        if self.elapsed > 50:
            self.elapsed = 0
            if self.at == 24:
                self.at = 0
            else:
                self.at += 1


class SeqCharacter(pygame.sprite.Sprite):
    def __init__(self, *group):
        super(SeqCharacter, self).__init__(*group)
        self.sheet = [pygame.image.load('%s.png' % i) for i in range(1,11)]
        self.at = 0
        self.elapsed = 0
        self.rect = pygame.rect.Rect(0, 0, 50, 50)

    @property
    def image(self):
        return self.sheet[self.at]

    def update(self, millis):
        self.elapsed += millis
        if self.elapsed > 50:
            self.elapsed = 0
            if self.at == 9:
                self.at = 0
            else:
                self.at += 1


pygame.init()

SIZE = W, H = 640, 400
FPS = 60
BG = pygame.color.Color('white')
is_running = True

screen = pygame.display.set_mode(SIZE)
clock = pygame.time.Clock()

if len(sys.argv) > 1 and sys.argv[1] == 'seq':
    play = SeqCharacter()
else:
    play = Character()
group = pygame.sprite.Group()
group.add(play)

while is_running:
    millis = clock.tick(FPS)

    ev = pygame.event.poll()
    if ev.type == pygame.KEYDOWN:
        key = ev.dict['key']
        if key == 27:
            is_running = False

    screen.fill(BG)
    group.update(millis)
    group.draw(screen)
    pygame.display.flip()