Condition variable to pause a thread after finishing a particular task (in Python)

Question

I am implementing a GUI for a program that builds and modifies a big graph for a bus-routing problem.

I want to have a Play-button, that should start the simulations (create a new thread simThread that executes a simulate()-method, which is pretty much an infinite loop calling a method modifyGraph() over and over again).

I also want a Pause-button, that when pressed causes the simThread to wait, but not UNTIL it has finished its current call to modifyGraph()). This is because during modifyGraph(), the graph is often in a state that shouldn't be plotted. When I press Play again, simThread should ideally continue where it was.

I am relatively new to threading in general and in Python in particular, but I have a feeling Condition Variables are the way to go. Anybody knows how to go about this?

Answer 1

If your simThread looks something like this:

class simThread(threading.Thread):
    def run(self):
        while True:
            modifyGraph()

You can introduce an event to unpause like this:

class simThread(threading.Thread):
    def __init__(self):
        self.unpaused = threading.Event()
        self.unpaused.set()
        threading.Thread.__init__(self)

    def run(self):
        while True:
            modifyGraph()
            unpaused.wait()

You would use this functionality like this:

thread = simThread()
thread.start()
thread.unpaused.clear()
[...]
thread.unpaused.set()

If you think an event called unpaused is a bit awkward, you can also have an additional boolean variable pause that you initially set to False . In the thread, you would check if it is True and then wait for an unpause event. So something like this:

class simThread(threading.Thread):
    def __init__(self):
        self.pause = False
        self.unpause = threading.Event()
        threading.Thread.__init__(self)

    def run(self):
        while True:
            modifyGraph()
            if self.pause:
                unpause.wait()

    def pause(self):
        self.unpause.clear()
        self.pause = True

    def unpause(self):
        self.pause = False
        self.unpause.set()

thread = simThread()
thread.start()
thread.pause()
thread.unpause()

The reason why you should use an event instead of just having another while loop spinning on self.pause is to avoid wasting CPU cycles.