my code

#space invader :0

import turtle
import os
import math
import random

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("white")
player.shape("triangle")
player.penup()
player.speed(0)
player.setposition(0, -250)
player.setheading(90)

playerspeed = 15



number_of_enemies = 5
enemies = []

for i in range(number_of_enemies):
	enemies.append(turtle.Turtle())



for enemy in enemies:

	enemy = turtle.Turtle()
	enemy.color("red")
	enemy.shape("circle")
	enemy.penup()
	enemy.speed(0)
	x = random.randint(-200, 200)
	y = random.randint( 100, 250)
	enemy.setposition(x, y)
	enemyspeed = 1


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

bulletstate = "ready"

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 shoot():

	global bulletstate
	if bulletstate == "ready":
		bulletstate = "fire"
		x = player.xcor()
		y = player.ycor() + 10
		bullet.setposition(x, y)
		bullet.showturtle()

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

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

while(True):

	for enemy in enemies:
		x = enemy.xcor()
		x += enemyspeed
		enemy.setx(x)

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

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

		if isCollision(bullet, enemy):

			bullet.hideturtle()
			bulletstate = "ready"
			bullet.setposition(0, -400)

			enemy.setposition(-200, 250)

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



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

	if bullet.ycor() > 275:
		bullet.hideturtle()
		bulletstate = "ready"




exit = raw_input("press enter to exit.")
#space invader :0

import turtle
import os
import math
import random

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("white")
player.shape("triangle")
player.penup()
player.speed(0)
player.setposition(0, -250)
player.setheading(90)

playerspeed = 15



number_of_enemies = 5
enemies = []

for i in range(number_of_enemies):
	enemies.append(turtle.Turtle())



for enemy in enemies:

	enemy = turtle.Turtle()
	enemy.color("red")
	enemy.shape("circle")
	enemy.penup()
	enemy.speed(0)
	x = random.randint(-200, 200)
	y = random.randint( 100, 250)
	enemy.setposition(x, y)
	enemyspeed = 1


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

bulletstate = "ready"

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 shoot():

	global bulletstate
	if bulletstate == "ready":
		bulletstate = "fire"
		x = player.xcor()
		y = player.ycor() + 10
		bullet.setposition(x, y)
		bullet.showturtle()

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

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

while(True):

	for enemy in enemies:
		x = enemy.xcor()
		x += enemyspeed
		enemy.setx(x)

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

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

		if isCollision(bullet, enemy):

			bullet.hideturtle()
			bulletstate = "ready"
			bullet.setposition(0, -400)

			enemy.setposition(-200, 250)

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



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

	if bullet.ycor() > 275:
		bullet.hideturtle()
		bulletstate = "ready"




exit = raw_input("press enter to exit.")