Pong Game in PyGame

CODE:

import pygame

BLACK = (0,0,0)

WHITE = (255,255,255)

RED = (255,0,0)

GREEN = (0,255,0)

BLUE = (0,0,255)

pygame.init()

#Initializing the display window

size = (800,600)

screen = pygame.display.set_mode(size)

pygame.display.set_caption("pong")

#Starting coordinates of the paddle

rect_x = 400

rect_y = 580

#initial speed of the paddle

rect_change_x = 0

rect_change_y = 0

#initial position of the ball

ball_x = 50

ball_y = 50

#speed of the ball

ball_change_x = 5

ball_change_y = 5

score = 0

#draws the paddle. Also restricts its movement between the edges

#of the window.

def drawrect(screen,x,y):

    if x <= 0:

        x = 0

    if x >= 699:

        x = 699    

    pygame.draw.rect(screen,RED,[x,y,100,20])

#game's main loop    

done = False

clock=pygame.time.Clock()

while not done:

    for event in pygame.event.get():

        if event.type == pygame.QUIT:

            done = True

        elif event.type == pygame.KEYDOWN:

            if event.key == pygame.K_LEFT:

                rect_change_x = -6

            elif event.key == pygame.K_RIGHT:

                rect_change_x = 6

            #elif event.key == pygame.K_UP:

                #rect_change_y = -6

            #elif event.key == pygame.K_DOWN:

                #rect_change_y = 6'''            

        elif event.type == pygame.KEYUP:

            if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:

                rect_change_x = 0

            elif event.key == pygame.K_UP or event.key == pygame.K_DOWN:

                rect_change_y = 0            

    screen.fill(BLACK)

    rect_x += rect_change_x

    rect_y += rect_change_y

    ball_x += ball_change_x

    ball_y += ball_change_y

    #this handles the movement of the ball.

    if ball_x<0:

        ball_x=0

        ball_change_x = ball_change_x * -1

    elif ball_x>785:

        ball_x=785

        ball_change_x = ball_change_x * -1

    elif ball_y<0:

        ball_y=0

        ball_change_y = ball_change_y * -1

    elif ball_x>rect_x and ball_x600:

        ball_change_y = ball_change_y * -1

        score = 0                        

    pygame.draw.rect(screen,WHITE,[ball_x,ball_y,15,15])

    #drawball(screen,ball_x,ball_y)

    drawrect(screen,rect_x,rect_y)
    
    #score board

    font= pygame.font.SysFont('Calibri', 15, False, False)

    text = font.render("Score = " + str(score), True, WHITE)

    screen.blit(text,[600,100])    

    pygame.display.flip()         

    clock.tick(60)

pygame.quit()