Screen only updates when I check for user input pygame

Question

I'm using pygame and updating to the screen every loop of the main loop. What I don't understand is nothing will update until I add a for loop looking for events, then suddenly all the updating does occur. Why is this?

   def run(self):

        two_pm = get_stand_up_timestamp()

        pygame.init()
        font = pygame.font.Font(None, 72)
        screen = pygame.display.set_mode(self._dimensions)



        before_two = True
        while before_two:

            # Blit the time to the window.
            # Update Screen.
            current_time = datetime.datetime.now()
            text = font.render(f'{current_time.hour} : {current_time.minute} : {current_time.second}', True, (0, 0, 0))
            blit_center = (
                self._dimensions[0] // 2 - (text.get_width() // 2),
                self._dimensions[1] // 2 - (text.get_height() // 2)
            )
            screen.fill((255, 255, 255))
            screen.blit(text, blit_center)
            pygame.display.flip()

            # Get events.
            for event in pygame.event.get():

                if event.type == pygame.QUIT:
                    sys.exit()
                elif event.type == pygame.KEYDOWN:
                    if event.key == pygame.K_q:
                        sys.exit()

Hard to guess without seeing actual code. Would you mind showing it? — Alderven, Oct 25 '19 at 14:30
uploaded, works fine as that but if I just remove the loop checking for the key q or exiting then it doesn't update. — Alesi Rowland, Oct 25 '19 at 14:37

score 1 · Answer 1 · answered Oct 25 '19 at 15:30

When you call pygame.event.get() (or pump()), pygame processes all events that your window manager send to the window managed by pygame.

You don't see these events as they are not returned by get(), but pygame handles them internally. These events could be WM_PAINT on Windows or Expose on Linux (IIRC pygame uses Xlib), or other events (I guess you could look them up in pygame's source code).

E.g. if you run pygame on Windows, Pygame has to call Windows' GetMessage function, otherwise:

If a top-level window stops responding to messages for more than several seconds, the system considers the window to be not responding and replaces it with a ghost window that has the same z-order, location, size, and visual attributes. This allows the user to move it, resize it, or even close the application. However, these are the only actions available because the application is actually not responding.

So the typical behaviour if you don't let pygame process the events is that it will basically run, but the mouse cursor will change to the busy cursor and you can't move the window before it will eventually freeze.

If you run pygame on other systems, e.g. Linux, you only see a black screen. I don't know the internals of the message loop when pygame runs on Linux, but it's similiar to the Windows message loop: you have to process the events in the queue to have pygame call Xlib's XNextEvent function (IIRC) to give the window manager a chance to draw the window.

See e.g. Message loop in Microsoft Windows and/or Xlib for more information on that topic.

score 0 · Answer 2 · answered Oct 25 '19 at 15:30

No idea why it doesn't work on your end, however when I run

 def run():
    width = 500
    height = 500
    pygame.init()
    font = pygame.font.Font(None, 72)
    screen = pygame.display.set_mode((width, height))

    before_two = True
    while before_two:

        # Blit the time to the window.
        # Update Screen.
        current_time = datetime.datetime.now()
        text = font.render(f'{current_time.hour} : {current_time.minute} :      {current_time.second}', True, (0, 0, 0))
        blit_center = (
            width // 2 - (text.get_width() // 2),
            height // 2 - (text.get_height() // 2)
        )
        screen.fill((255, 255, 255))
        screen.blit(text, blit_center)
        pygame.display.flip()




run()

Everything works fine update wise. The clock ticks every second so it may be something with your version of python or pygame. Try updating them both. Alternately it could be a problem with how you get pass pygame the dimensions of the window with the run(self) and self._dimensions. Trying using static dimensions like I did above and see if that works on your end. Sadly without more code to see how you call run() its difficult to fully debug whats wrong.

I may work for you but not OP because you're using different operating systems / window managers. — sloth, Oct 25 '19 at 15:32

Screen only updates when I check for user input pygame

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