pygame教程添加第二个精灵不起作用

我一直在使用本教程中的这段代码https://opensource.com/article/18/5/pygame-enemy，但是我不能仅仅通过使用添加更多敌人

enemy   = Enemy(40,100,'spr.png')# spawn enemy
enemy_list = pygame.sprite.Group()   # create enemy group
enemy_list.add(enemy)                # add enemy to group

原始图像来自此处https://opengameart.org/sites/default/files/opp2_sprites.zip，但为了便于说明，我在imgur中进行了分隔：

内容文件夹图像：

yeti.png： https ：//imgur.com/GNNcU6z

stage.png： https ：//imgur.com/YyiEJ0q

以及我想放置的第二个敌人（精神）的图像

spr.png： https ://imgur.com/1fYXa7Y

我尝试使用类似问题的解决方案，如何在pygame中添加敌人？，但只有错误或没有结果。

我的代码：

import pygame
import sys
import os
'''
Objects
'''

class Enemy(pygame.sprite.Sprite):
    '''
    Spawn an enemy
    '''

    def __init__(self, x, y, img):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.image.load(os.path.join('images', img))
        #self.image.convert_alpha()
        #self.image.set_colorkey(ALPHA)
        self.rect = self.image.get_rect()
        self.rect.x = x
        self.rect.y = y
        self.counter = 0

    def move(self):
        '''
        enemy movement
        '''
        distance = 20
        speed = 8

        if self.counter >= 0 and self.counter <= distance:
            self.rect.x += speed
        elif self.counter >= distance and self.counter <= distance * 2:
            self.rect.x -= speed
        else:
            self.counter = 0

        self.counter += 1


class Level():
    def bad(lvl,eloc):
        if lvl == 1:
            enemy = Enemy(eloc[0],eloc[1],'yeti.png') # spawn enemy
            enemy_list = pygame.sprite.Group() # create enemy group
            enemy_list.add(enemy)              # add enemy to group
        if lvl == 2:
            print("Level " + str(lvl) )

        return enemy_list

class Player(pygame.sprite.Sprite):
    '''
    Spawn a player
    '''

    def __init__(self):
        pygame.sprite.Sprite.__init__(self)
        self.movex = 0
        self.movey = 0
        self.frame = 0
        self.health = 10
        self.frame = 0
        self.images = []
        for i in range(1, 5):
            img = pygame.image.load(
                os.path.join('images', 'hero' + str(i) + '.png')).convert()
            img.convert_alpha()
            img.set_colorkey(ALPHA)
            self.images.append(img)
            self.image = self.images[0]
            self.rect = self.image.get_rect()

    def control(self, x, y):
        '''
        control player movement
        '''
        self.movex += x
        self.movey += y

    def update(self):
        '''
        Update sprite position
        '''

        self.rect.x = self.rect.x + self.movex
        self.rect.y = self.rect.y + self.movey

        # moving left
        if self.movex < 0:
            self.frame += 1
            if self.frame > 3 * ani:
                self.frame = 0
            self.image = self.images[self.frame // ani]
        # collisions
        enemy_hit_list = pygame.sprite.spritecollide(self, enemy_list, False)
        for enemy in enemy_hit_list:
            self.health -= 1
            print(self.health)

        # moving right
        if self.movex > 0:
            self.frame += 1
            if self.frame > 3 * ani:
                self.frame = 0
            self.image = self.images[(self.frame // ani)]


'''
Setup
'''

worldx = 560
worldy = 520

fps = 40  # frame rate
ani = 4  # animation cycles
clock = pygame.time.Clock()
pygame.init()
main = True

BLUE = (25, 25, 200)
BLACK = (23, 23, 23)
WHITE = (254, 254, 254)
ALPHA = (0, 255, 0)

world = pygame.display.set_mode([worldx, worldy])
backdrop = pygame.image.load(os.path.join('images', 'stage.png')).convert()
backdropbox = world.get_rect()
player = Player()  # spawn player
player.rect.x = 0
player.rect.y = 0
player_list = pygame.sprite.Group()
player_list.add(player)
steps = 10  # how fast to move
eloc = []
eloc = [200,20]
enemy_list = Level.bad( 1, eloc )
'''
Main loop
'''
while main == True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
            main = False

        if event.type == pygame.KEYDOWN:
            if event.key == pygame.K_LEFT or event.key == ord('a'):
                player.control(-steps, 0)
            if event.key == pygame.K_RIGHT or event.key == ord('d'):
                player.control(steps, 0)
            if event.key == pygame.K_UP or event.key == ord('w'):
                print('jump')

        if event.type == pygame.KEYUP:
            if event.key == pygame.K_LEFT or event.key == ord('a'):
                player.control(steps, 0)
            if event.key == pygame.K_RIGHT or event.key == ord('d'):
                player.control(-steps, 0)
            if event.key == ord('q'):
                pygame.quit()
                sys.exit()
                main = False

#    world.fill(BLACK)
    world.blit(backdrop, backdropbox)
    player.update()
    player_list.draw(world)  #refresh player position
    enemy_list.draw(world)
    for e in enemy_list:
        e.move()
    pygame.display.flip()
    clock.tick(fps)