由于采用了具有“NameError”麻烦

我是新来的类，这是我在做三分之一的尝试。我碰到一个NameError，我真的不知道如何解决。看看我的程序，看看您是否可以提供帮助。

import random
import math
import pygame
import pickle 

# initialise pygame
pygame.init()

# player class 
class player(object):
    def __init__(self, playerimage, playerX, playerY = 700, playerX_change = 0):
        self.playerimage = pygame.image.load("Main Player.png") 
        self.playerX = 365
        self.playerY = 700 
        self.playerX_change = 0 

# laser class 
# ready - bullet not on screen
# fire - bullet is shown on screen and is moving

class laser(object):
    def __init__(self, laserimage, laserX, laserY, laserY_change):
        self.laserimage = pygame.image.load("laser.png")
        self.laserX = 0
        self.laserY = 700 
        self.laserY_change = 10
        self.laser_state = "ready"

# alien player / random movement = random.randint()
class alien(object):
    def __init__(self, alienimage, alienX, alienY, alienX_change, alienY_change, amount_aliens):
        self.alienimage = pygame.image.load("alien.png")
        self.alienX = []
        self.alienY = []
        self.alienX_change = []
        self.alienY_change = []
        self.amount_aliens = 10

    for i in range(ufo.amount_aliens):
        ufo.alienimage.append(pygame.image.load('alien.png'))
        ufo.alienX.append(random.randint(0, 735))
        ufo.alienY.append(random.randint(50, 200))
        ufo.alienX_change.append(3)
        ufo.alienY_change.append(7)

score = 0


# define player
def main_player(x, y):
    screen.blit(male.playerimage, (x, y))


# define laster
def fire_laser(x, y):
    lasr.laser_state = "fire"
    screen.blit(lasr.laserimage, (x + 16, y + 10))


# define alien
def alien(x, y, i):
    screen.blit(ufo.alienimage[i], (x, y))


# collision detection
def hascollision(alienX, alienY, laserX, laserY):
    distance = math.sqrt((math.pow(alienX - laserX, 2)) + (math.pow(alienY - laserY, 2)))
    if distance < 27:
        return True
    else:
        return False

#frames per second 
clock = pygame.time.Clock()

# background
background = pygame.image.load('stars.png')

# display and screen title/icon
(width, height) = (800, 800)
screen = pygame.display.set_mode((width, height))
flip = pygame.display.flip()
pygame.display.set_caption("space fighters")
pygame.event.get()
icon = pygame.image.load('logo.png')
pygame.display.set_icon(icon)

from sys import exit

ufo = alien()
lasr = laser(0, 700, 32, 32)
male = player(365, 700,)
# loop of functions
executed = True
while executed:
    screen.fill((63, 62, 63))
    # image background
    screen.blit(background, (0, 0))
    for event in pygame.event.get():
        # if key pressed, check which input, right or left?
        if event.type == pygame.KEYDOWN:
            print("key pressed")
            if event.key == pygame.K_a:
                male.playerX_change = -6
            if event.key == pygame.K_s:
                male.playerX_change = 6
        if event.type == pygame.KEYUP:
            if event.key == pygame.K_a or event.key == pygame.K_s:
                male.playerX_change = 0
            if event.key == pygame.K_SPACE:
                if lasr.laser_state is "ready":
                    lasr.laserX = male.playerX
                fire_laser(lasr.laserX, lasr.laserY)
    #frames per second is 60fps
    clock.tick(60)
    # bounrary algorithm, prevents player moving out/enemy.
    male.playerX += male.playerX_change

    if male.playerX <= 0:
        male.playerX = 0
    elif male.playerX >= 736:
        male.playerX = 736

    # boundry algorithm, make sure alien doesn't go out of bountry
    for i in range(ufo.amount_aliens):
        ufo.alienX[i] += ufo.alienX_change[i]
        if ufo.alienX[i] <= 0:
            ufo.alienX_change[i] = 4
            ufo.alienY[i] += ufo.alienY_change[i]
        elif ufo.alienX[i] >= 736:
            ufo.alienX_change[i] = -4
            ufo.alienY[i] += ufo.alienY_change[i]

    # collision
        collision = hascollision(ufo.alienX[i], ufo.alienY[i], lasr.laserX, lasr.laserY)
        if collision:
            lasr.laserY = 650
            lasr.laser_state = "ready"
            score += 5
            print(score)
            alienX[i] = random.randint(0, 735)
            alienY[i] = random.randint(50, 200)

        alien(ufo.alienX[i], ufo.alienY[i], i)

    # movement of laser shot
    if lasr.laserY <= 0:
        lasr.laserY = 650
        lasr.laser_state = "ready"

    if lasr.laser_state is "fire":
        fire_laser(lasr.laserX, lasr.laserY)
        lasr.laserY -= lasr.laserY_change

    # updates screen to show screen
    main_player(male.playerX, male.playerY)
    pygame.display.update()
pygame.quit()

这是错误的输出由Visual Studio码（它是在第39行）中给出
在范围（ufo.amount_aliens）对于i：NameError：名称“UFO”没有被定义