Why isn't the ball bouncing back?

Question

I'm making a simple pong game, but when the ball drops from the ceiling, it doesn't bounce back and forth. It just goes off the screen. I can't figure out why this is happening! I am only concerned with the ball bouncing off the top and bottom of the screen and I want it to bounce back and forth in a straight path. Any help is appreciated!

EDIT: I've found the problem! Thanks for all the help!

Here's my base code:

import math
import random
import sys, pygame
from pygame.locals import *

import ball
import colors
import paddle

# draw the scene
def draw(screen, ball1, paddle1) :
   screen.fill((128, 128, 128))
   ball1.draw_ball(screen)
   paddle1.draw_paddle(screen)

#function to start up the main drawing
def main():

   pygame.init()
   width = 600
   height = 600
   screen = pygame.display.set_mode((width, height))

   ball1 = ball.Ball(300, 1, 40, colors.YELLOW, 0, 5)
   paddle1 = paddle.Paddle(250, 575, colors.GREEN, 100, 20)

   while 1:
      for event in pygame.event.get():
         if event.type == QUIT: sys.exit()
         elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_RIGHT:
               paddle1.update_paddle('right', 20)
            if event.key == pygame.K_LEFT:
               paddle1.update_paddle('left', 20)

      ball1.test_collide_top_ball(600)
      ball1.test_collide_bottom_ball(0)
      ball1.update_ball()
      draw(screen, ball1, paddle1)
      pygame.display.flip()

if __name__ == '__main__':
   main()

And here is my code for the ball class/methods:

import pygame

class Ball:
   def __init__(self, x, y, radius, color, dx, dy):
      self.x = x
      self.y = y
      self.radius = radius
      self.color = color
      self.dx = dx
      self.dy = dy

   def draw_ball(self, screen):
      pygame.draw.ellipse(screen, self.color,
         pygame.Rect(self.x, self.y, self.radius, self.radius))

   def update_ball(self):
      self.x += self.dx
      self.y += self.dy

   def test_collide_top_ball(self, top_height):
      if (self.y >= top_height):
         self.dy *= -1

   def test_collide_bottom_ball(self, coll_height):
      if (self.y >= coll_height):
         self.dy *= -1

You always use the same hard-coded `dx` and `dy`, and never *use* the value returned from e.g. `test_collide_bottom_ball` — jonrsharpe, Apr 06 '14 at 21:51

score 1 · Accepted Answer · answered Apr 06 '14 at 21:53

Your test collide functions return the value of velocity. You never use it anywhere.

You call the update ball with dx=0 dy=5.

Instead of returning the a value after collision, it would be better to hold the dx and dy in the object. So it would become:

class Ball:
   def __init__(self, x, y, radius, color):
      self.x = x
      self.y = y
      self.radius = radius
      self.color = color
      self.dx = 0
      self.dy = 5

   def draw_ball(self, screen):
      pygame.draw.ellipse(screen, self.color,
         pygame.Rect(self.x, self.y, self.radius, self.radius))

   def update_ball(self):
      self.x += self.dx
      self.y += self.dy

   def test_collide_top_ball(self, top_height):
      if (self.y >= top_height):
         self.dy *= -1

   def test_collide_bottom_ball(self, coll_height):
      if (self.y >= coll_height):
         self.dy *= -1

Thank you so much! So I've revised my code (I'll edit my post), but now the ball seems to be bouncing in the wrong direction. Any suggestions? Sorry, I'm new to python. — , Apr 06 '14 at 22:08

Why isn't the ball bouncing back?

1 Answers1

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