title: Detecting collision in Python turtle game
body: <p>I am trying to make a Python game where the red turtle chases the blue turtle. When the red turtle catches the blue turtle, I want it to say 'COLLISION' on the screen but it is not working. When it collides, nothing happens and it gives me an error 'Turtle' object is not callable'. </p>
  
  <pre><code>from turtle import Turtle, Screen
  
  playGround = Screen()
  
  playGround.screensize(250, 250)
  playGround.title("Turtle Keys")
  
  run = Turtle("turtle")
  run.speed("fastest")
  run.color("blue")
  run.penup()
  run.setposition(250, 250)
  
  follow = Turtle("turtle")
  follow.speed("fastest")
  follow.color("red")
  follow.penup()
  follow.setposition(-250, -250)
  
  def k1():
      run.forward(45)
  
  def k2():
      run.left(45)
  
  def k3():
      run.right(45)
  
  def k4():
      run.backward(45)
  
  def quitThis():
      playGround.bye()
  
  def follow_runner():
      follow.setheading(follow.towards(run))
      follow.forward(8)
      playGround.ontimer(follow_runner, 10)
  
  playGround.onkey(k1, "Up")  # the up arrow key
  playGround.onkey(k2, "Left")  # the left arrow key
  playGround.onkey(k3, "Right")  # you get it!
  playGround.onkey(k4, "Down")
  
  playGround.listen()
  
  follow_runner()
  
  def is_collided_with(self, run):
      return self.rect.colliderect(run.rect)
  
  runner = run(10, 10, 'my_run')
  follower = follow(20, 10)
  if follow.is_collided_with(run):
      print 'collision!'
  
   playGround.mainloop()
  </code></pre>