Calling a method in thread from another thread, python

Question

How can I achieve communication between threads?

I have one thread in which I do some stuff, then I need to call a method from an object that lives in the main program thread and this method should be executed in the main process:

class Foo():
    def help(self):
        pass


class MyThread(threading.Thread):

    def __init__(self, connection, parser, queue=DEFAULT_QUEUE_NAME):
        threading.Thread.__init__(self)

    def run(self):
        # do some work
        # here I need to call method help() from Foo()
        # but I need to call it in main process


bar = Foo()

my_work_thread = MyThread()
my_work_thread.run()

Both threads have access to the same memory, so bar in the separate thread is identical to bar in the main thread — Nearoo
– Nearoo, Commented Jul 11, 2018 at 11:51
Hard to tell, what you wanna do. What does help() do? Why you need to call it from the main process? Does help() depend ons some variables available only in the main process? Normally you can share variables between threads, just remember to make them thread-safe (locking) — Frieder
– Frieder, Commented Jul 11, 2018 at 11:52

Andrej Kesely · Accepted Answer · 2018-07-11 12:09:57Z

6

There are many possibilities how to do it, one is using 2 queues:

from time import sleep
import threading, queue

class Foo():
    def help(self):
        print('Running help')
        return 42


class MyThread(threading.Thread):

    def __init__(self, q_main, q_worker):
        self.queue_main = q_main
        self.queue_worker = q_worker
        threading.Thread.__init__(self)

    def run(self):
        while True:
            sleep(1)
            self.queue_main.put('run help')
            item = self.queue_worker.get()      # waits for item from main thread
            print('Received ', item)

queue_to_main, queue_to_worker = queue.Queue(), queue.Queue( )
bar = Foo()

my_work_thread = MyThread(queue_to_main, queue_to_worker)
my_work_thread.start()

while True:
    i = queue_to_main.get()
    if i == "run help":
        rv = Foo().help()
        queue_to_worker.put(rv)

Output:

Running help
Received  42
Running help
Received  42
Running help
Received  42
...etc

answered Jul 11, 2018 at 12:09

Andrej Kesely

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2 Comments

Q-bart Over a year ago

Great thanks! But, the only thing that embarrasses me: a while loop in the main process. Do we can to use something like subscribers? E.g.: when something appends to queue, then run function on_queue_append()?

Andrej Kesely Over a year ago

@Q-bart There aren't any subscribers in Python queue, but you can use get_nowait() just to peek if something is in the queue: docs.python.org/3/library/queue.html#queue.Queue.get

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