Issue with updating display in Pygame

Question

I am developing a program that will allow user to create and edit a GUI made in Pygame. The program itself will have a nice GUI and for that I am making a module to simplify its creation. While everything else for now works as I expect, I am hitting on that one problem that I simply can't solve, neither understand:

When I start the program a black window shows up. The problem is that the window should be white and containing a black rectangle with an image in it. I tried clicking in it and moving it around, but it didn't show anything.

First I thought that I didn't update the window, but I checked the script and found out that I did. Then I started trying different things with the window and found out that only the part of the window that exits my actual screen (monitor) gets updated and it gets updated instantly the moment I drag it out and back into screen.

Can someone please explain me what is happening, why is it happening and how I can fix it?

EDIT

I found out that the window also updates correctly if I minimize it and then maximize it. So the problem is not so critical but it is very annoying.

Here is the code:

import pygame
import time

def initialize():
    global display
    global screen
    pygame.init()
    display = pygame.display
    screen = None

class Screen():
    def __init__(self, size, color=[255, 255, 255]):
        global screen
        self.size = size
        self.color = color
        self.screen = display.set_mode(self.size)
        self.fill(self.color)
        self.update()
        screen = self
    def update(self, rectangle=None):
        display.update(rectangle)
    def fill(self, color, rectangle=None):
        self.screen.fill(color, rectangle)
    def draw(self, sprite):
        self.screen.blit(sprite.image, sprite.rectangle[0])

class Engine():
    def __init__(self):
        self.events = []
        self.running = False
    def update_events(self):
        self.events += pygame.event.get()
    def get_events(self):
        events = self.events
        self.events = []
        return events
    def start(self, function=None, args=[], kwargs={}, frequency=21, function_first=False):
        self.running = True
        while self.running:
            if function_first and function:
                function(*args, **kwargs)
            self.update_events()
            if not function:
                for event in self.events:
                    if event.type == pygame.QUIT:
                        self.running = False
                        pygame.quit()
                        break
            if not function_first and function:
                function(*args, **kwargs)
            if self.running:
                tick = 1.0 / frequency
                time.sleep(tick)
    def stop(self):
        self.running = False

class Sprite():
    def __init__(self, rectangle, color=[0, 0, 0]):
        self.rectangle = rectangle
        self.color = color
        self.image = None
    def update(self):
        global screen
        screen.fill(self.color, self.rectangle)
        screen.draw(self)
    def load_image(self, image):
        self.image = pygame.image.load(image).convert()

if __name__ == "__main__":
    initialize()
    s = Screen([500, 500])
    e = Engine()
    sprite = Sprite([[10, 10], [200, 200]])
    sprite.load_image("test.png")
    sprite.update()
    s.update()
    e.start()

Here are the screenshots:

When I start the program:

When I pull the window out of my screen:

When I pull it back in:

Answer 1

You have some errors in your code.

1. You don't update the screen in your main loop.

2. You should always call pygame.event.get in every iteration of your main loop.

3. Don't use time.sleep, use pygame.time.clock.tick