Space Invaders code

import turtle
import os
import time
import math

#Set up the screen
wn = turtle.Screen()
wn.bgcolor("black")
wn.title("Space Invaders")

#Draw border
border_pen = turtle.Turtle()
border_pen.hideturtle()
border_pen.speed(0)
border_pen.color("white")
border_pen.penup()
border_pen.setposition(-300,-300)
border_pen.pendown()
border_pen.pensize(3)
#Loop for drawing
for side in range(4):
    #Forward 600 steps
    border_pen.fd(600)
    #Turn 90 degrees
    border_pen.lt(90)


#Create the player
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(-200, 250)

enemyspeed = 20

#Create the weapon
bullet = turtle.Turtle()
bullet.color("yellow")
bullet.shape("triangle")
bullet.penup()
bullet.speed(0)
bullet.setheading (90)
bullet.shapesize(0.5, 0.5)



bulletspeed = 20



#Define bullet states
#ready - ready to fire
#fire - bullet if firing
bulletstate = "ready"


#Move the player left and right
def move_left():
    #Get the player's x coords
    x = player.xcor()
    x-= playerspeed
    #Setting left boundary
    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 bulletstate as a global if it needs changed
    global bulletstate
    if bulletstate == "ready":
        bulletstate = "fire"



def isCollision(t1, t2):
    distance = math.sqrt(math.pow(t1.xcor()-t2.xcor(),2)+math.pow(t1.ycor()-t2.ycor(),2))
    if distance < 15:
        return True
    else:
        return False


#Create keyboard bindings
turtle.listen()
#When pressed left, call function move left
turtle.onkey(move_left, "Left")
turtle.onkey(move_right, "Right")
turtle.onkey(fire_bullet, "space")

#Main game loop
while True:
    if bulletstate == "ready":
        bullet.hideturtle()
        #Move the bullet to above player
        x = player.xcor()
        y = player.ycor()
        bullet.setposition(x, y+10)

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

    #Move the enemy back and down
    if enemy.xcor() > 280:
        y = enemy.ycor()
        #Bring enemy closer to the player
        y -= 50
        enemy.sety(y)
        #When the enemy is touching the right wall, change enemyspeed to
        #enemyspeed * -1, (2*-1 is -2) so that it will go the other way.
        enemyspeed *= -1

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

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

    #Check bullet if touching top
    if bullet.ycor() > 275:
        bullet.hideturtle()
        #Move the bullet to above player
        x = player.xcor()
        y = player.ycor()
        bullet.setposition(x, y+10)
        bulletstate = "ready"

    #Check for contact with enemy
    if isCollision(bullet, enemy):
        #Reset the bullet
        bullet.hideturtle()
        bulletstate = "ready"
        x = player.xcor()
        y = player.ycor()
        bullet.setposition (x, y+10)
        #Reset the enemy
        enemy.setposition (-200, 250)

    #If enemy touches player
    if isCollision(player, enemy):
        player.hideturtle()
        enemy.hideturtle()
        print("Game over :(")
        break