Why the draw rectangle doesn't show in the scr

Question

I've tried several times running code to display something in the screen but it never shows up anything, it is always a blank screen as you can see:

I want to draw a rectangle, but why is it not displaying?

import pygame, sys
from pygame.locals import *

def main():
    pygame.init()

    DISPLAY=pygame.display.set_mode((500,400),0,32)

    WHITE=(255,255,255)
    blue=(0,0,255)

    DISPLAY.fill(WHITE)

    pygame.draw.rect(DISPLAY,blue,(200,150,100,50))

    while True:
        for event in pygame.event.get():
            if event.type==QUIT:
                pygame.quit()
                sys.exit()
        pygame.display.update()

main()

I'm running Python 3.7.1 and pygame 1.9.4 on macos