code: def update(self, dt):
          
          self.group.update(dt)
  
          # check if the sprite's feet are colliding with wall
          # sprite must have a rect called feet, and move_back method,
          # otherwise this will fail
          for sprite in self.group.sprites():
              if sprite.feet.collidelist(self.walls) > -1:
                  sprite.move_back(dt)
docs: Tasks that occur over time should be handled here