Python Pygame Pong Game Issues

Question

So basically what I've tried to code here is a game of Pong using the Python programming language and the Pygame module.
My problems are as follows:

1. The game sometimes gets stuck in a loop in which the AI paddle sits there while over and over again it loses
2. The ball jumps up and down sometimes? I have no clue as to why...
3. The ball just goes back and forth? I can't figure out why it isn't following along a random line like I want...
4. And my biggest problem, it seems that when the ball hits the top or bottom, it just disappears and the game just continues as if it was still bouncing around.

My Code

# Pong
import random, pygame, sys
from pygame.locals import *

FPS = 300
WINWID = 640
WINHEI = 480

BROWN = (102, 51, 0)
LIGHTBLUE = (0, 204, 204)
GREEN = (0, 153, 0)
PADDLECOLOR = (255, 255, 255)
BALLCOLOR = PADDLECOLOR
BACKGROUNDCOLOR = (0, 0, 0)

def main():
    global FPSCLOCK, DISPLAYSURF
    SPEED = 1
    DIFF = 1.0 # AI paddle speed handicap
    ball_speed = 1
    AISIGHT = WINWID/6
    FONT = "Times New Roman"
    pygame.init()
    DEFAULTFONT = pygame.font.SysFont(FONT, 16)
    BIGGERFONT = pygame.font.SysFont(FONT, 32)
    FPSCLOCK = pygame.time.Clock()
    DISPLAYSURF = pygame.display.set_mode((WINWID, WINHEI))
    DISPLAYSURF.fill(BACKGROUNDCOLOR)
    BACKGROUND = pygame.Rect(0, 0, WINWID, WINHEI)
    pygame.display.set_caption('Pong')
    paddleUP, paddleDOWN = False, False    
    ballpos = (WINWID/2, WINHEI/2)
    AIpoint, PLAYERpoint = 0, 0
    score = "Player:%2f || Computer:%2f" % (AIpoint, PLAYERpoint)
    paddle = WINHEI/2
    AIpaddle = paddle
    pygame.draw.rect(DISPLAYSURF, BACKGROUNDCOLOR, BACKGROUND)
    pygame.draw.line(DISPLAYSURF, PADDLECOLOR, (9, paddle-20), (9, paddle+20), 3)
    pygame.draw.line(DISPLAYSURF, PADDLECOLOR, (629, AIpaddle-20), (629, AIpaddle+20), 3)
    pygame.draw.circle(DISPLAYSURF, BALLCOLOR, ballpos, 5)
    slope, y_int = ballpath(None, ballpos[0], ballpos[1])
    while True:
        for event in pygame.event.get():
            direction = None
            if event.type == QUIT:
                terminate()
            elif event.type == KEYDOWN:
                if (event.key == K_UP or event.key == K_w):
                    paddleUP = True
                elif (event.key == K_DOWN or event.key == K_s):
                    paddleDOWN = True
            elif event.type == KEYUP:
                if (event.key == K_UP or event.key == K_w):
                    paddleUP = False
                elif (event.key == K_DOWN or event.key == K_s):
                    paddleDOWN = False
        # move player paddle
        if paddleUP and paddle >= 20:
            paddle-=SPEED
        elif paddleDOWN and paddle <= 460:
            paddle+=SPEED
        # move AI paddle
        if ballpos[0] > (AISIGHT):
            if AIpaddle-20 < ballpos[1] and AIpaddle <= 460:
                AIpaddle+=SPEED/DIFF
            elif AIpaddle+20 > ballpos[1] and AIpaddle >= 20:
                AIpaddle-=SPEED/DIFF
        # move ball
        x = ballpos[0] + ball_speed
        y = (slope*x) + y_int
        ballpos = (x, y)
        # if it hits a paddle
        if 9 < ballpos[0] < 11:
            if paddle-20 < ballpos[1] < paddle+20:
                slope, y_int = ballpath(None, ballpos[0], ballpos[1])
                ball_speed*=-1
        elif 629 < ballpos[0] < 631:
            if AIpaddle-20 < ballpos[1] < AIpaddle+20:
                slope, y_int = ballpath(None, ballpos[0], ballpos[1])
                ball_speed*=-1
        # if it hits the top or bottom
        if WINHEI-5 > ballpos[1] or ballpos[1] < 5:
            slope, y_int = ballpath(slope, ballpos[0], ballpos[1])
        # if paddle misses ball
        if ballpos[0] < 5:
            AIpoint+=1
            score = "Player:%2f || Computer:%2f" % (PLAYERpoint, AIpoint)
            ballpos = (WINWID/2, WINHEI/2)
            pygame.draw.rect(DISPLAYSURF, (0, 0, 0, 100), pygame.Rect(0, 0, 640, 480))
            DISPLAYSURF.blit(BIGGERFONT.render("Point: Computer", 1, PADDLECOLOR), (WINWID/3, WINHEI/3))
            paddle, AIpaddle = WINWID/2, WINWID/2
            for i in range(5):
                FPSCLOCK.tick(FPS)
        elif ballpos[0] > 635 and AIpaddle-20 < ballpos[1] < AIpaddle+20:
            PLAYERpoint+=1
            score = "Player:%2f || Computer:%2f" % (PLAYERpoint, AIpoint)
            ballpos = (WINWID/2, WINHEI/2)
            pygame.draw.rect(DISPLAYSURF, (0, 0, 0, 100), pygame.Rect(0, 0, 640, 480))
            DISPLAYSURF.blit(BIGGERFONT.render("Point: Player", 1, PADDLECOLOR), (WINWID/3, WINHEI/3))
            for i in range(5):
                FPSCLOCK.tick(FPS)


        pygame.draw.rect(DISPLAYSURF, BACKGROUNDCOLOR, BACKGROUND)
        pygame.draw.line(DISPLAYSURF, PADDLECOLOR, (9, paddle-20), (9, paddle+20), 3)
        pygame.draw.line(DISPLAYSURF, PADDLECOLOR, (629, AIpaddle-20), (629, AIpaddle+20), 3)
        pygame.draw.circle(DISPLAYSURF, BALLCOLOR, ballpos, 5)
        DISPLAYSURF.blit(DEFAULTFONT.render(score, 1, PADDLECOLOR), (WINWID/3, 20))
        pygame.display.update()
        FPSCLOCK.tick(FPS)


def ballpath(old_slope, ball_x, ball_y):
    if old_slope == None:
        slope = int(random.random() * 2)
    else:
        slope = old_slope * -1
    y_int = ball_y - (slope * ball_x)
    return slope, y_int

def terminate():
    pygame.quit()
    sys.exit()

if __name__ == '__main__':
    main()

Answer 1

I cleaned up your code quite a bit. It should help a bit with how to use the classes. http://pastebin.com/rZF3Gr0D

Things to note:

• prints when score event happens
• paddle movement using keystate vs "on events". It makes that kind of input easier.
• Sprite group's let you 'draw' or check collisions in batches.
• removed global keyword. Game owns most things.
• Using Rect functions and properties all over the place. Such as ball.rect.center = screen.get_rect().center .
• random angle for ball spawns via trig.
• caps framerate. Next you would move onto time-based movement. I left that out for simplicity.
• Color() lets you do many things, including Color("red"), Color("gray40") , etc...