Space Invaders with Pygame

class Bullet:
  def __init__(self,x,y):
    self.x = x
    self.y = y
    self.width = 5
    self.height = 10
    self.last = pygame.time.get_ticks()
    
  def draw(self):    
    screen.blit(bulletImg,(self.x,self.y))

  def checkCollide(self, otherthing):
    if self.x < otherthing.x + otherthing.width and self.x + self.width > otherthing.x and self.y < otherthing.y + otherthing.height and self.y+self.height > otherthing.y:
      print ("collide")
      return True
    return False
      

class Gun:
  def __init__(self,x,y):
    self.x = x
    self.y = y
  def draw(self):   
    screen.blit(gunImg,(self.x,self.y))


class Alien:
  def __init__(self,x,y):
    self.x = x
    self.y = y
    self.dir = "left"
    self.width=30
    self.height=30
  
  def draw(self):
    #pygame.draw.rect(screen,WHITE,[self.x, self.y, 30,30],5)
    screen.blit(alienImg,(self.x,self.y))
  
  def move(self,speed):
    #self.x = self.x + speed
    if self.x > 500 - 30:
      self.dir="left"
      self.y = self.y + 30
    if self.x < 0:
      self.dir="right"
      self.y = self.y + 30

    if self.dir == "right":
      self.x = self.x + speed
    if self.dir == "left":
      self.x = self.x - speed
      


import pygame
pygame.init()
BLACK   = (0,0,0)
WHITE   = (255,255,255)
GREEN   = (0,255,0)
RED     = (255,0,0)
BLUE    = (0,0,255)

size    =[500,500]
screen  = pygame.display.set_mode(size)
alienImg = pygame.image.load("alien2.png")
gunImg = pygame.image.load("gun.png")
bulletImg = pygame.image.load("bullet.png")
pygame.display.set_caption("space invaderz")

clock=pygame.time.Clock()
last = pygame.time.get_ticks()
done=False
#instantiate the aliens
aliens = []
for i in range(18):
  aliens.append(Alien(i*60,20))

#instantiate gun
spacegun = Gun(235,470)

#empty array for bullets
bullets = []

#----------------- MAIN GAME LOOP START----------------------------
while done==False:

#----------------- CHECK FOR EVENTS START--------------------------
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            done=True

        if event.type == pygame.KEYDOWN: 
            if event.key == pygame.K_RIGHT:
              print("You pressed right")
              spacegun.x = spacegun.x + 10
            if event.key == pygame.K_LEFT:
              print("You pressed left")
              spacegun.x = spacegun.x - 10
            if event.key == pygame.K_SPACE:
              print("You pressed space")
              
    
    

#----------------- CHECK FOR EVENTS END----------------------------

#----------------- GAME LOGIC START--------------------------------
#code to spew bullets constantly - every 250 ms
    now = pygame.time.get_ticks()
    if now - last >= 250:
      last = now
      bullets.append(Bullet(spacegun.x + 13,spacegun.y))
#----------------- GAME LOGIC END ---------------------------------

#----------------- DRAWING START-----------------------------------
    screen.fill(BLACK) # starts off with a BLACK screen
    
    
    for i in range(len(aliens)):
      aliens[i].move(5)
      aliens[i].draw()

    spacegun.draw()
    # loop through the bullets
    for bul in bullets:
      bul.y = bul.y - 10
      # loop through the aliens
      for al in aliens:
        # check if bullet has hit the alien
        if bul.checkCollide(al):
          print("collide!!!")
          aliens.remove(al)
      bul.draw()

    pygame.display.flip() # have to flip the display to show result

#----------------- DRAWING END-------------------------------------

    # limit to 20 frames per second
    clock.tick(20)
#----------------- MAIN GAME LOOP END ----------------------------
pygame.quit()