Python Game Programming Tutorial: Space Invaders 2

import turtle
import os

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

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()

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

playerspeed = 15

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)

    wn.listen()
    wn.onkey(move_left, "Left")
    wn.onkey(move_right, "Right")




delay = input("Press enter to finsh.")