Python 2 Threading Primitives, Locks and Events
Recently I’ve had to deal with some inter-process communication in Python 2.7, in which I had several threads sharing data. I figured doing a brain-dump of some of this data would be useful for me in the future as well as anyone else wandering by.
In this specific instance, I’m retrieving frames from a camera of some sort, pulling them in over a websocket, then passing them on to an OpenCV processor. Nothing crazy, but the websocket server couldn’t be blocked by the OpenCV processor, so threads were introduced. Really just two.
The primary things we care about here are thread
events and thread
locks.. I wrote this when I was dealing with Python 2, but I don’t
see why you couldn’t do this stuff in 3+. In general, use
asyncio in favor of threads and locks and processes and
whatever. Easier to think about.
Heres something similar to what I ended up writing:
import thread, threading class Server: def __init__(self, lock, event): self.lock = lock self.event = event # Assume this server is doing a bunch of server-y things, and this # method is the callback triggered when new data comes in: def msg_received(self, client, server, msg): with self.frame_lock: self.frame = msg self.event.set() def get_frame(self): return self.frame class Processor: def __init__(self, lock, event): self.lock = lock self.event = event def process(self, server): while self.event.wait(): with self.lock: frame = server.get_frame() self.event.clear() # Do fancy processing here ... def main(): frame_lock = thread.allocate_lock() frame_event = threading.Event() # Server receives frames from upstream: server = Server(frame_lock, frame_event, ip='localhost', port=8999) # Processor does fancy computation on frames: processor = Processor(frame_lock, frame_event) t = Threading.Thread(target=processor.process, args=(server,)) t.start() server.run_forever() if __name__ == '__main__': main()
This is more or less what I have. I haven’t tested the above code, but the point is you have a Server of some sort operating independently of a processor of some other sort. Later on I might write another post detailing how you might do this with a UNIX pipe instead of this Locking/Event style, but we can talk about that when the time comes. Or doesn’t. Whatever.
The fancy (but still basic) thing here is the event, and the lock.
You can use the event to avoid spinlocking (eg.
while not frame: ...), and instead only wake up your
expensive processing Thread when it has something to do. Remember,
sleep() is a sign that you’re being lazy!
One thread will call
event.set() when it is done
operating on the shared resource, then back off. The next thread will
then wake up and wait to acquire the lock, so it can operate on the
shared resource, doing it’s thing, and then calling
event.clear(). This ends up working really nicely because
each thread only works when it can, no one is stepping on anyone elses
toes and it all just works out.