How I can fix pygame background not working?

I want to add background to my app in python pygame but its not working help? heres the code

import pygame
import sys
import os

# initial pygame
pygame.init()

# frame :
screen = pygame.display.set_mode((800, 600))

# Background 
background = pygame.image.load('background.png')

# the title
pygame.display.set_caption("Untitled")

# Loop
running = True
while True:
    screen.blit(background, (0, 0))
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            exit()

also i haven't finish the whole app yet