I recently downloaded pygame on my Mac, but for some reason, the window does not pop up with the screen or anything. I don't get an error message, it just runs but doesn't actually pop up a display.

```
import pygame, sys
from pygame.locals import*

pygame.init()
SCREENWIDTH = 800
SCREENHEIGHT = 800
RED = (255,0,0)
screen = pygame.display.set_mode((SCREENWIDTH, SCREENHEIGHT))

pygame.draw.rect(screen, RED, (400, 400, 20, 20),0)
screen.fill(RED)

pygame.display.update()

```