title: Updating object on Python < arcade library >
body: <p>I'm currently coding in some assignment.
  I create a class named Ship and created some methods to make the object class ship be able to move.</p>
  
  <pre><code>class Ship():
  def __init__(self):
      self.center.x = 400
      self.center.y = 300
      self.velocity.dx = .25
      self.velocity.dy = .25
  
  def advance_down(self):
  
      self.center.y = self.center.y - self.velocity.dy
  </code></pre>
  
  <p>This is my code</p>
  
  <pre><code>def check_keys(self):
  """
      This function checks for keys that are being held down.
      You will need to put your own method calls in here.
      """
          if arcade.key.DOWN in self.held_keys:
              self.ship1.advance_down()
  </code></pre>
  
  <p>advance down just changes the position</p>
  
  <pre><code>    def on_key_press(self, key: int, modifiers: int):
      """
      Puts the current key in the set of keys that are being held.
      You will need to add things here to handle firing the bullet.
      """
              if key == arcade.key.DOWN:
                 self.held_keys.add(key)
  </code></pre>
  
  <p>This adds the current key being pressed to the set of self.held_keys.
  I'm calling an update of the positon.</p>
  
  <pre><code> def update(self, delta_time):
      """
      Update each object in the game.
      :param delta_time: tells us how much time has actually elapsed
      ""
  
      self.ship1.advance_down()
  </code></pre>
  
  <p>when I run the code I'm able to display everything just fine but it's not doing anything once I press my keys.
  Any ideas why?</p>