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Engineering LibreTexts

5.14: Checking for a Specific Event, and Posting Events to Pygame’s Event Queue

  • Page ID
  • def checkForQuit():
        for event in pygame.event.get(QUIT): # get all the QUIT events
            terminate() # terminate if any QUIT events are present
        for event in pygame.event.get(KEYUP): # get all the KEYUP events
            if event.key == K_ESCAPE:
                terminate() # terminate if the KEYUP event was for the Esc key
   # put the other KEYUP event objects back

    The checkForQuit() function will check for QUIT events (or if the user has pressed the Esc key) and then call the terminate() function. But this is a bit tricky and requires some explanation.

    Pygame internally has its own list data structure that it creates and appends Event objects to as they are made. This data structure is called the event queue. When the pygame.event.get() function is called with no parameters, the entire list is returned. However, you can pass a constant like QUIT to pygame.event.get() so that it will only return the QUIT events (if any) that are in the internal event queue. The rest of the events will stay in the event queue for the next time pygame.event.get() is called.

    You should note that Pygame’s event queue only stores up to 127 Event objects. If your program does not call pygame.event.get() frequently enough and the queue fills up, then any new events that happen won’t be added to the event queue.

    Line 2 [123] pulls out a list of QUIT events from Pygame’s event queue and returns them. If there are any QUIT events in the event queue, the program terminates.

    Line 4 [125] pulls out all the KEYUP events from the event queue and checks if any of them are for the Esc key. If one of the events is, then the program terminates. However, there could be KEYUP events for keys other than the Esc key. In this case, we need to put the KEYUP event back into Pygame’s event queue. We can do this with the function, which adds the Event object passed to it to the end of the Pygame event queue. This way, when line 70 calls pygame.event.get() the non-Esc key KEYUP events will still be there. Otherwise calls to checkForQuit() would "consume" all of the KEYUP events and those events would never be handled.

    The function is also handy if you ever want your program to add Event objects to the Pygame event queue.