How to fix AttributeError Turtle object has no attribute xcore

Question

# setting up the screen
import turtle
import math


wn = turtle.Screen()
wn.bgcolor('black')
wn.title('S')
# draw a border
bp = turtle.Turtle()
bp.speed(0)
bp.color('white')
bp.penup()
bp.setposition(-300, -300)
bp.pendown()
bp.pensize(3)
bp.hideturtle()
for lll in range(4):
    bp.forward(600)
    bp.left(90)

# mr.TIM
tim = turtle.Turtle()
tim.speed(0)
tim.setposition(0, -250)
tim.setheading(90)
tim.penup()
tim.color('red')
tim.shape('arrow')
# Bad guys
enemy = turtle.Turtle()
enemy.color('red')
enemy.shape('turtle')
enemy.speed(0)
enemy.penup()
enemy.setposition(0, 200)
enemy.setheading(270)
enemy_speed = 2
# Bullet
bullet = turtle.Turtle()
bullet.penup()
bullet.color('gold')
bullet.shape('triangle')
bullet.shapesize(0.5, 0.5)
bullet.speed(0)
bullet.setposition(0, 0)
bullet.setheading(90)
bullet.hideturtle()
bullet_speed = 20
# bullet states
bullet_state = 'ready'
# ready to fire
# fire
# move tim
poi_speed = 15


def move_left():
    x = tim.xcor()
    x -= poi_speed
    tim.setx(x)
    if x < -280:
        x = + 280
        tim.setx(x)


def move_right():
    x = tim.xcor()
    x += poi_speed
    tim.setx(x)
    if x > +280:
        x = - 280
        tim.setx(x)


# show bullet
def fire_bullet():
    global bullet_state
    if bullet_state == 'ready':
        bullet_state = 'fire'
        x = tim.xcor()
        tt = tim.ycor() + 10
        bullet.setposition(x, tt)
        bullet.showturtle()


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


turtle.listen()
turtle.onkey(move_left, 'Left')
turtle.onkey(move_right, 'Right')
turtle.onkey(fire_bullet, 'space')

# move the enemy + game mainloop
while True:
    # Move the enemy left and right
    c = enemy.xcor()
    c += enemy_speed
    enemy.setx(c)
    if enemy.xcor() > 280:
        v = enemy.ycor()
        v -= 40
        enemy.sety(v)
        enemy_speed *= -1
    if enemy.xcor() < -280:
        v = enemy.ycor()
        v -= 40
        enemy.sety(v)
        enemy_speed *= -1
    # move bullet
    if bullet_state == 'fire':
        y = bullet.ycor()
        y += bullet_speed
        bullet.sety(y)
    # stop the bullet and make it ready to fire again
    if bullet.ycor() > 275:
        bullet.hideturtle()
        bullet_state = 'ready'
    # game over for enemy
    if iscollision(bullet, enemy):
        # reset bullet
        bullet_state = 'ready'
        bullet.setposition(0, -400)
        # reset enemy
        enemy.setposition(-200, 250)