How to get screen to clear?

[metulburr]

here is a more full fledged example

import pygame as pg

pg.init()

def flip_color():
    global bg_white
    bg_white = not bg_white

class Button:
    def __init__(self, rect, command):
        self.color = (255,0,0)
        self.rect = pg.Rect(rect)
        self.image = pg.Surface(self.rect.size)
        self.image.fill(self.color)
        self.command = command
    def render(self, screen):
        screen.blit(self.image, self.rect)
    def get_event(self, event):
        if event.type == pg.MOUSEBUTTONDOWN and event.button == 1:
            if self.rect.collidepoint(pg.mouse.get_pos()):
                self.command()
                
screen = pg.display.set_mode((800,600))
screen_rect = screen.get_rect()
running = True
bg_white = False

btn = Button((10,10,105,25), flip_color)

while running:
    for event in pg.event.get():
        if event.type == pg.QUIT:
            running = False
        btn.get_event(event)
    if bg_white:
        screen.fill((255,255,255))
    else:
        screen.fill((0,0,0))
    btn.render(screen)
    pg.display.update()