Space Invaders - Python Pygame

import pygame
import base64
import time, math, random, os, sys
import urllib.parse, urllib.request

spritesheet_url = "rwpnG"
spritesheet_filename = "spritesheet.png"

# set up pygame and the display surfaces
SCALE = 4
DW, DH = 320 * SCALE, 200 * SCALE
HDW = int(DW / 2)
FPS = 60

def downloader(url, fn):
    PASTEEE_URL = "http://paste.ee/r/{}"
    USER_AGENT = "SpaceInvaders"
    HEADER = {'user-agent':USER_AGENT}

    req = urllib.request.Request(PASTEEE_URL.format(url), None, HEADER)
    try:
        res = urllib.request.urlopen(req)
    except urllib.error.URLError as e:
        print("Download unsuccessful... {}".format(e.reason))
        return False
    else:
        buffer = res.read().decode('utf-8')
        data = base64.b64decode(buffer)

        fileHandle = open(fn, "wb")
        fileHandle.write(data)
        fileHandle.flush()
        fileHandle.close()
    return True

INVADER_UPDATE_TIME = FPS
INVADER_SPEED = 4 * SCALE

INVADER_COLUMNS = 11
INVADER_ROWS = 5
INVADER_WIDTH = 11 * SCALE
INVADER_HEIGHT = 8 * SCALE
INVADER_MARGIN = 9 * SCALE
INVADER_TOTAL_WIDTH = INVADER_WIDTH + INVADER_MARGIN

INVADER_SPRITE_RECTS = [[0, 0, INVADER_WIDTH, INVADER_HEIGHT], [INVADER_WIDTH, 0, INVADER_WIDTH, INVADER_HEIGHT]]

INVADERS_LEFT_MARGIN = 50 * SCALE
INVADERS_TOP_MARGIN = 20 * SCALE

class invaders:
    class invader:
        def __init__(s, x, y):
            s.x = x
            s.y = y

    def __init__(s):
        global INVADER_SPEED, INVADER_UPDATE_TIME, INVADER_TOTAL_WIDTH, INVADER_COLUMNS, INVADER_ROWS
        global INVADERS_LEFT_MARGIN, INVADERS_TOP_MARGIN

        s.container = []

        for index in range(INVADER_COLUMNS * INVADER_ROWS):
            x = index % INVADER_COLUMNS
            y = int(index / INVADER_COLUMNS)
            s.container += [s.invader(INVADERS_LEFT_MARGIN + x * INVADER_TOTAL_WIDTH, INVADERS_TOP_MARGIN + y * INVADER_TOTAL_WIDTH)]

        s.state = s.moveRight
        s.stateAfterDescent = None

        s.frame = 0
        s.updateTime = INVADER_UPDATE_TIME
        s.updateTick = INVADER_UPDATE_TIME

    def draw(s):
        global INVADER_WIDTH, INVADER_HEIGHT, INVADER_SPRITE_RECTS
        global PRIMARY, SPRITESHEET, SCALE

        for invader in s.container:
            PRIMARY.blit(SPRITESHEET, (invader.x, invader.y), INVADER_SPRITE_RECTS[s.frame])

    def update(s):
        global INVADER_UPDATE_TIME
        global DW

        if s.updateTick:
            s.updateTick -= 1
            return

        s.frame = 1 - s.frame
        s.updateTick = s.updateTime
        s.state()

    def moveRight(s):
        global INVADER_SPEED, INVADER_WIDTH
        global DW

        x = 0
        for invader in s.container:
            invader.x += INVADER_SPEED
            if invader.x > x: x = invader.x
        if x +INVADER_WIDTH >= DW - INVADER_SPEED:
            s.state = s.descend
            s.stateAfterDescent = s.moveLeft

    def moveLeft(s):
        global INVADER_SPEED
        global DW

        x = DW
        for invader in s.container:
            invader.x -= INVADER_SPEED
            if invader.x < x: x = invader.x
        if x <= INVADER_SPEED:
            s.state = s.descend
            s.stateAfterDescent = s.moveRight

    def descend(s):
        global INVADER_HEIGHT
        for invader in s.container:
            invader.y += INVADER_HEIGHT
        s.state = s.stateAfterDescent


# download the spritesheet from paste.ee
if not os.path.exists(spritesheet_filename):
    choice = input("Do you want to download [{}] from paste.ee (y/n)?".format(spritesheet_filename))
    if choice.lower() == "y" or choice.lower() == "yes":
        if downloader(spritesheet_url, spritesheet_filename):
            print("Download Successfull!")
        else:
            sys.exit()
    else:
        print("You can't continue without downloading spritesheet first sorry!")
        sys.exit()

# initilise pygame and create surfaces
pygame.init()
PRIMARY = pygame.display.set_mode((DW, DH))
CLOCK = pygame.time.Clock()

# load the spritesheet and scale
SPRITESHEET = pygame.image.load("spritesheet.png").convert()
SPRITESHEET_WIDTH, SPRITESHEET_HEIGHT = SPRITESHEET.get_size()
SPRITESHEET = pygame.transform.scale(SPRITESHEET, (SPRITESHEET_WIDTH * SCALE, SPRITESHEET_HEIGHT * SCALE))
SPRITESHEET.set_colorkey([0, 0, 0]) # set the backcolor is the spritesheet to transparent

INVADERS = invaders()

while True:
    start = time.time()
    e = pygame.event.get()
    k = pygame.key.get_pressed()

    # quit?
    if k[pygame.K_ESCAPE]: break

    # clear the sprite surface
    PRIMARY.fill([0, 0, 0])

    # draw invader sprites
    INVADERS.draw()

    # update ship, bullets and invaders
    INVADERS.update()

    # update the primary display surface
    pygame.display.update()

    CLOCK.tick(FPS) # keep time