Traffic lights

import pygame
import time

DW = DH = 400 # DisplayWidth, DisplayHeight
HDW = int(DW / 2) # HalfDisplayWidth
HDH = int(DH / 2) # HalfDisplayHeight
pygame.init()
PD = pygame.display.set_mode((DW, DH)) # PD = Primary Display

# colors
RED = [255, 0, 0]
AMBER = [255,191,0]
GREEN = [0, 255, 0]

colors = [RED, AMBER, GREEN]
colorIndex = 0
timeStamp = -1

# press escape to get out!
while True:
    e = pygame.event.get()
    if pygame.key.get_pressed()[pygame.K_ESCAPE]: break

    now = time.time()
    if now - timeStamp >= 0.5:
        pygame.draw.circle(PD, colors[colorIndex], (100, 100), 100)
        pygame.display.update()
        colorIndex += 1
        if colorIndex == len(colors): colorIndex = 0
        timeStamp = now
pygame.quit()