Space Invaders 4

#Space Invaders
import turtle
import os

#Screen
mainscreen = turtle.Screen()
mainscreen.bgcolor("black")
mainscreen.title("Space Invaders")

#Border
border_pen = turtle.Turtle()
border_pen.speed(0)
border_pen.color("white")
border_pen.penup()
border_pen.setposition(-300,-300)
border_pen.pendown()
border_pen.pensize(3)

for side in range(4):
    border_pen.fd(600)
    border_pen.lt(90)
border_pen.hideturtle()

#Create the player turtle
player = turtle.Turtle()
player.color("blue")
player.shape("triangle")
player.penup()
player.speed(0)
player.setposition(0, -250)
player.setheading(90)

playerspeed = 15

#Create the enemy
enemy = turtle.Turtle()
enemy.color("red")
enemy.shape("circle")
enemy.penup()
enemy.speed(0)
enemy.setposition(-140, 250)

enemyspeed = 2

#Player's bullet
bullet = turtle.Turtle()
bullet.color("yellow")
bullet.shape("triangle")
bullet.penup()
bullet.speed(0)
bullet.setheading(90)
bullet.shapesize(0.5, 0.5)
bullet.hideturtle()

bulletspeed = 20

#Define bullet state
#ready - ready to fire
#fire - bullet is firing
bulletstate = "ready"

#Player Movement
def move_left():
    x = player.xcor()
    x-= playerspeed
    if x < -280:
         x = -280
    player.setx(x)

def move_right():
    x = player.xcor()
    x += playerspeed
    if x > 280:
        x = 280
    player.setx(x)

def fire_bullet():
    #Declare bulletsate as a global if it needs changes
    global bulletstate
    if bulletstate == "ready":
        bulletsate = "fire"
        #Move the bullet just above the player
        x = player.xcor()
        y = player.ycor() +10
        bullet.setposition(x, y)
        bullet.showturtle()


#Keyboard bindings
turtle.listen()
turtle.onkey(move_left, "Left")
turtle.onkey(move_right, "Right")
turtle.onkey(fire_bullet, "space")

#Main game loop
while True:

    #Move the enemy
    x = enemy.xcor()
    x += enemyspeed
    enemy.setx(x)

    #Move the enemy back and down
    if enemy.xcor() > 280:
        y = enemy.ycor()
        y -= 40
        enemyspeed *= -1
        enemy.sety(y)

    if enemy.xcor() < -280:
        y = enemy.ycor()
        y-= 40
        enemyspeed *= -1

    #Move the bullet
    if bulletstate == "fire":
        y = bullet.ycor()
        y += bulletspeed
        bullet.sety(y)

    #Check to see if the bullet has gone to the top
    if bullet.ycor() > 275:
        bullet.hideturtle()
        bulletstate = "ready"



delay = raw_input("Press enter to start")