Galaxy Invaders

import turtle
from turtle import *

sc = Screen()
sc.setup(500, 620)

can_shoot = True
alien_speed = 5;
delay(0)
t = Turtle()
t.up()
t.shapesize(2)
t.tilt(90)
t.speed(0)
t.goto(0, -280)
bullets = []
enemies = []

def create_aliens():
    for j in range(3):
       for i in range(8):
           bad = Turtle()
           bad.speed(0)
           bad.up()
           bad.goto(-250 + i * 40, 250 -40*j)
           bad.color("green")
           bad.shape("square")
           enemies.append(bad)

create_aliens()
counter = 0


def move():
   global alien_speed, counter

   counter += 1
   if counter > 20:
       counter = 0
   if counter %2 == 0:
       for i in bullets:
           i.fd(15)  # forward = fd
           if i.ycor() > 300:
               i.hideturtle()
               bullets.remove(i)
           # proverka za sblusuk
           for y in enemies:
               if (y.xcor() - 10) < i.xcor() < (y.xcor() + 10) and (y.ycor() - 13) < i.ycor() < (y.ycor() + 13):
                   y.hideturtle()
                   enemies.remove(y)
                   i.hideturtle()
                   bullets.remove(i)
                   # boom
                   if len(enemies)<1:
                       if alien_speed > 1:
                           alien_speed -= 1
                       create_aliens()


   # aliens movement
   if counter %alien_speed == 0:
        for bad in enemies:
           if bad.xcor() > 250 or bad.xcor() < -250:
               for badx in enemies:
                   badx.sety(badx.ycor() - 20)
                   badx.left(180)
               break
        for bad in enemies:
           bad.fd(5)


   ontimer(move, 5)


def move_left():
   t.bk(10)


def move_right():
   t.fd(10)

def allow_shoot():
    global can_shoot
    can_shoot = True

def shoot():
    global can_shoot
    if can_shoot:
       s = Turtle()
       s.shape("circle")
       s.speed(0)
       s.shapesize(0.5)
       s.up()
       s.left(90)
       s.goto(t.xcor(), t.ycor())
       bullets.append(s)
       can_shoot = False
       ontimer(allow_shoot, 100)


sc.onkeypress(move_left, "Left")
sc.onkeypress(move_right, "Right")
sc.onkey (shoot, "space")
sc.listen()

move()

done()