import pygame
import sys

pygame.init()
screen = pygame.display.set_mode((400, 300))

surface = pygame.Surface((50, 50))
surface.fill((0,255,0))

while True:
    event = pygame.event.get()
    for event in event:
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
    screen.blit(surface, (175, 125))
    pygame.display.update()