Scoreboard

Lesson 34 of 40 in Coddy's Make Games with Pygame course.

A ball out on the right is a point for the left player, and the other way round. Split the test in two, so each side knows who scored:

if ball.left > 600:
    left_score += 1
    ball.center = (300, 200)
    vx = -vx
if ball.right < 0:
    right_score += 1
    ball.center = (300, 200)
    vx = -vx

Show both scores in one line at the top, centred: f"{left_score} : {right_score}" reads 2 : 1.

Challenge

Easy

Before the loop, make font = pygame.font.Font(None, 36), left_score = 0 and right_score = 0.
Split the reset in two: a ball past the right edge adds 1 to left_score, a ball past the left edge adds 1 to right_score. Both bring the ball to the middle and flip vx.
Render f"{left_score} : {right_score}" in (240, 240, 240), centred at (300, 30).

Try it yourself

import pygame

pygame.init()
screen = pygame.display.set_mode((600, 400))
pygame.display.set_caption("Pong")
clock = pygame.time.Clock()

ball = pygame.Rect(293, 193, 14, 14)
vx, vy = 5, 4
left = pygame.Rect(20, 160, 12, 80)
right = pygame.Rect(568, 160, 12, 80)

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    keys = pygame.key.get_pressed()
    if keys[pygame.K_w]:
        left.y -= 6
    if keys[pygame.K_s]:
        left.y += 6
    if keys[pygame.K_UP]:
        right.y -= 6
    if keys[pygame.K_DOWN]:
        right.y += 6
    left.clamp_ip(screen.get_rect())
    right.clamp_ip(screen.get_rect())

    ball.x += vx
    ball.y += vy
    if ball.top <= 0 or ball.bottom >= 400:
        vy = -vy
    if ball.colliderect(left):
        vx = abs(vx)
    if ball.colliderect(right):
        vx = -abs(vx)
    if ball.right < 0 or ball.left > 600:
        ball.center = (300, 200)
        vx = -vx

    screen.fill((30, 30, 46))
    pygame.draw.rect(screen, (80, 160, 255), left)
    pygame.draw.rect(screen, (235, 80, 80), right)
    pygame.draw.rect(screen, (255, 196, 0), ball)
    pygame.display.flip()
    clock.tick(60)

pygame.quit()