我一直在尝试让敌人每20秒生成一次，以解决敌人在玩家正下方时卡在平台下面的问题。我已经留下了我打算使用的方法，但实施起来并不顺利。

原计划是使用Python内置的timer.tick来计时每20秒，然而我意识到这只是基于帧率的，如你所见，我在顶部使用了start = time.time()来启动计时，然后在20秒后使用end = time.time()来结束计时。

import pygame    #导入Pygame
import time    #导入计时器，以便使用tick函数使游戏达到60fps
start= time.time()#为每个函数编写这个
end= time.time()
import math    #导入数学模块
import sys    #导入系统模块
import random
from random import *
from time import *    #导入time模块的所有内容
from pygame import *    #导入所有Pygame文件
from pygame.math import *
from pygame.mixer import *
print(start)
win_height = 750    #窗口高度为750像素
win_width = 1050    #窗口宽度为1050像素
half_win_width = int(win_width / 2)    #用于居中摄像头
half_win_height = int(win_height / 2)
white=(255, 255, 255)
black=(0, 0, 0)
gray=(50, 50, 50)
red=(255, 0, 0)
green=(0, 255, 0)
blue=(0, 0, 255)
yellow=(255, 255, 0)
display = (win_width, win_height)    #创建500*500像素的窗口
depth = 32    #防止无限递归
flags = 0    #给Les的信息：我不是很清楚这是做什么用的，但我看到很多地方都在使用，所以我认为这是重要的
camera_slack = 30    #玩家移动多少像素后摄像头会随之移动
pygame.init()
mixer.init()
pygame.mixer.music.load('Caravan Palace - Lone Digger [Clip officiel].mp3')    #在我的游戏文件夹中播放音乐
#pygame.mixer.music.load('Toby Fox - Megalovania [Electro Swing Remix].mp3')    #在我的游戏文件夹中播放音乐
pygame.mixer.music.play(-1)    #无限循环播放音乐
myfont = pygame.font.SysFont('Comic Sans MS', 30)
def main_menu():
    pygame.init()
    screen = pygame.display.set_mode(display, flags, depth)
    pygame.display.set_caption("超级恶魔城人")
    timer = pygame.time.Clock()
def main():
    #主游戏函数
    global cameraX, cameraY
    pygame.init()
    screen = pygame.display.set_mode(display, flags, depth)
    pygame.display.set_caption("超级恶魔城人")
    timer = pygame.time.Clock()
    move_cameraX = 0
    move_cameraY = 0
    up = down = left = right = running = False
    background = pygame.Surface((32,32))    #背景在屏幕上占用空间
    background.convert()
    background.fill(pygame.Color("#000000"))    #背景为黑色
    entities = pygame.sprite.Group()
    player = Player_class(32, 32*15)    #玩家大小为32*32像素
    platforms = []
    x = y = 0
    blank_level = [
        "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "P                               P",
        "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP"]
    def print_level(level):
        for row in level:
            print(row)
    new_level = blank_level    #随机生成关卡
    #首先处理平台
    #随机选择平台数量（6 - 12）
    #对于这些平台，选择没有平台的行（即只有两个P的行），然后选择一个起始单元格和长度（3-7）->确保在墙边截断
    #随机选择一个空单元格放置E
    for z in range (0,15):
        plat_len = (randint(1, 7))
        plat_start = (randint(3, 29))
        row = (randint(3, 19))
        new_level[row] = new_level[row][0:plat_start]+("P"*plat_len)+new_level[row][plat_start+plat_len:]
    #随机选择一个空单元格放置E
    new_level[5] = new_level[5][0:4]+"E"+new_level[5][5:]
    for row in blank_level:
        for col in row:
            if col == "P":
                p = Platform(x, y)    #使P成为固体对象
                platforms.append(p)
                entities.add(p)
            if col == "E":
                e = Exit_block(x, y)
                platforms.append(e)
                entities.add(e)
            x += 32
        y += 32
        x = 0
    entities.add(player)
    enemy = Enemy(60, 200, player)    #生成敌人
    enemy_list = pygame.sprite.Group()    #创建敌人组
    enemy_list.add(enemy)    #将敌人添加到组中
    while 1:
        timer.tick(60)    #使游戏以60帧每秒运行
        for e in pygame.event.get():    #将event缩写为e
            if e.type == QUIT:
                return
            if e.type == KEYDOWN and e.key == K_ESCAPE:
                return
            if e.type == KEYDOWN and e.key == K_UP:
                up = True
                move_cameraY = -10
            if e.type == KEYDOWN and e.key == K_DOWN:
                down = True
                move_cameraY = 10
            if e.type == KEYDOWN and e.key == K_LEFT:
                left = True
                move_cameraX = -10
            if e.type == KEYDOWN and e.key == K_RIGHT:
                right = True
                move_cameraX = 10
            if e.type == KEYDOWN and e.key == K_SPACE:
                running = True
            if e.type == KEYUP and e.key == K_UP:
                up = False
                move_cameraY = 0
            if e.type == KEYUP and e.key == K_DOWN:
                down = False
                move_cameraY = 0
            if e.type == KEYUP and e.key == K_RIGHT:
                right = False
                move_cameraX = 0
            if e.type == KEYUP and e.key == K_LEFT:
                left = False
                move_cameraX = 0
            if e.type == KEYUP and e.key == K_RIGHT:
                right = False
                # 更新游戏。
        for e in enemy_list:
            e.update(platforms)
        player.update(up, down, left, right, running, platforms)
        # 绘制所有内容。
        for y in range(32):    #绘制背景
            for x in range(32):
                screen.blit(background, (x * 32, y * 32))
        entities.draw(screen)
        enemy_list.draw(screen)
        pygame.display.flip()  # 每帧只需要一次flip或update调用。
class Entity(pygame.sprite.Sprite):
    #使玩家成为精灵
    def __init__(self):
        pygame.sprite.Sprite.__init__(self)    #设置精灵初始化
class Player_class(Entity):
    #定义玩家类
    def __init__(self, x, y):    #x是玩家的x坐标，y是玩家的y坐标
        Entity.__init__(self)    #玩家是一个实体
        self.xvel = 0    #玩家左右移动的速度
        self.yvel = 0    #玩家上下移动的速度
        self.onGround = False    #假设玩家在空中
        self.image = pygame.Surface((32,32))    #玩家大小为32*32像素
        self.image.fill(pygame.Color("#0000FF"))    #使玩家变蓝
        self.rect = pygame.Rect(x, y, 32, 32)
        self.x = x
        self.y = y
    def update(self, up, down, left, right, running, platforms):
        if up:
            if self.onGround:
                self.yvel -= 10    #只有在地面上时才跳跃
        if down:
            pass
        if running:
            self.xvel = 12
        if left:
            self.xvel = -8
        if right:
            self.xvel = 8
        if not self.onGround:
            self.yvel += 0.3    #只有在空中时才受到重力加速
            if self.yvel > 100: self.yvel = 100    #终端速度=100
        if not(left or right):
            self.xvel = 0
        self.rect.left += self.xvel    #下落或跳跃
        self.collide(self.xvel, 0, platforms)    #在x轴上创建碰撞
        self.rect.top += self.yvel    #在y轴上创建碰撞
        self.onGround = False;    #假设玩家在空中
        # 进行y轴碰撞检测
        self.collide(0, self.yvel, platforms)
    def collide(self, xvel, yvel, platforms):
        for p in platforms:
            if pygame.sprite.collide_rect(self, p):
                if isinstance(p, Exit_block):
                    pygame.quit()
                    sys.exit()
                if xvel > 0:
                    self.rect.right = p.rect.left
                if xvel < 0:
                    self.rect.left = p.rect.right
                if yvel > 0:
                    self.rect.bottom = p.rect.top
                    self.onGround = True
                    self.yvel = 0
                if yvel < 0:
                    self.rect.top = p.rect.bottom
class Platform(Entity):
    def __init__(self, x, y):
        Entity.__init__(self)
        self.image = pygame.Surface((32, 32))
        self.image.fill(pygame.Color("#FFFFFF"))
        self.rect = pygame.Rect(x, y, 32, 32)
class Exit_block(Platform):
    def __init__(self, x, y):
        Platform.__init__(self, x, y)
        self.image.fill(pygame.Color("#00FF00"))
#出口块是绿色的
class Enemy(Entity):
    def __init__(self, x, y, player):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.Surface((32, 32))
        self.xvel = 0
        self.yvel = 0
        self.image.fill(pygame.Color("#FF0000"))    #敌人是红色的
        self.rect = pygame.Rect(250, 50, 32, 32)
        #
        #
        #
        #
        """        if pygame.time.Clock == 20:
            self.rect = pygame.Rect(250, 50, 32, 32)
        """
        self.player = player
    def collide(self, xvel, yvel, platforms):
    # 检查敌人是否与玩家碰撞。
        if self.rect.colliderect(self.player.rect):
            pygame.quit()
            sys.exit()
        for p in platforms:
            if pygame.sprite.collide_rect(self, p):
                if xvel > 0:
                    self.rect.right = p.rect.left
                if xvel < 0:
                    self.rect.left = p.rect.right
                if yvel > 0:
                    self.rect.bottom = p.rect.top
                if yvel < 0:
                    self.rect.top = p.rect.bottom
    def move(self, xvel, platforms, speed=4):
   # 追逐移动
        for p in platforms:    #如果敌人的顶部触碰到平台的底部，将敌人向右移动
            if pygame.sprite.collide_rect(self, p):
                if xvel == 15:
                    self.rect.top = p.rect.bottom
        if self.rect.x > self.player.rect.x:    # 沿x方向移动
             self.xvel = -speed
        elif self.rect.x < self.player.rect.x:
            self.xvel = speed
        if self.rect.y < self.player.rect.y:    # 沿y方向移动
            self.yvel = speed
        elif self.rect.y > self.player.rect.y:
            self.yvel = -speed
    def update(self, platforms):
        self.move(self, platforms)  # 设置速度。
        self.rect.left += self.xvel
        self.collide(self.xvel, 0, platforms)    #在x轴上创建碰撞
        self.rect.top += self.yvel    #在y轴上创建碰撞
        self.collide(0, self.yvel, platforms)
if __name__ == "__main__":
    main()
    pygame.quit()

回答：

在while循环之前定义start，然后从开始时间中减去当前时间（now）以查看是否已经过了所需的时间，然后将start设置为now。

start = pygame.time.get_ticks()
while 1:
    # ... 省略的代码。
    now = pygame.time.get_ticks()
    # 当20000毫秒过去后。
    if now - start > 20000:
        start = now
        enemy = Enemy(60, 200, player)
        enemy_list.add(enemy)