When I am running the code of pygame it is showing black screen.

import pygame,sys
import random
screen = pygame.display.set_mode( (800,600) )

rectangle = pygame.Rect(400, 300, 60, 40)

clock = pygame.time.Clock()
while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()

# Process Player Input
up = pygame.key.get_pressed() [pygame.K_UP]
down = pygame.key.het_pressed() [pygame.K_DOWN]
left = pygame.key.get_pressed() [pygame.K_LEFT]
right = pygame.key.get_pressed() [pygame.K_RIGHT]

#Updating game logic
if up:
    rectangle.y += -3
if down:
    rectangle.y += 3
if left:
    rectangle.x += -3
if right:
    rectangle.x += 3

if rectangle.x < 0:
    rectangle.x = 0
if rectangle.y < 0:
    rectangle.y = 0
if rectangle.x > screen.get_width() - rectangle.width:
    rectangle.x = screen.get_width() - rectangle.width
if rectangle.y > screen.get_height() - rectangle.height:
    rectangle.y = screen.get_height() - rectangle.height

#Rendering
screen.fill( (200,200,255) )
pygame.draw.rect(screen, (255,255,255), rectangle)
pygame.display.flip()

#Stimulate Some Delay
randDelay = random.randrange(randDelay)

clock.tick(60)