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So, when porting my python language software to iOS (yes, python on iOS), I ran into a problem where my multi-threaded program dependended on a library that was not thread safe.

Without the ability to change the library itself, I had only two options to serialize access to the library:

1. Re-write hundreds of lines of code to "wrap" thread-safe calls with locks around the library
2. Find an easier way.

I chose easier. Enter: Python coroutines.

Serializing non-thread-safe code using Python Coroutines

#!/usr/bin/env python                                                                                                                                                                                                                       
Author: Michael R. Hines                                                                                                                                                                                                                    
This is an example of using Python coroutines to serialize parts                                                                                                                                                                            
of a multi-threaded program without locking.                                                                                                                                                                                                
I ran into this need one day when a library I was using was not                                                                                                                                                                             
thread-safe (without the ability to change the library).                                                                                                                                                                                    
Thus, I needed specific parts of each thread to run serially                                                                                                                                                                                
*in the context* of a different, common thread who only has access                                                                                                                                                                          
to the library.                                                                                                                                                                                                                             
from threading import Thread, current_thread                                                                                                                                                                                                
import Queue
from time import sleep

# The code to be serialized in a different thread context
# (The main thread in this example)
# This is our coroutine.
def serial_function() :
    (msg, input_var, rq) = (yield)
    result = input_var + 10
    rq.put(str(current_thread()) + " " + ": Computed: " + str(result))

    # sleep(5), for example would make the serialization of this
    # code in the main thread much more obvious

# A producer thread with both parallel and serial code requirements
def counter(q) :
    x = 1
    rq = Queue.Queue()

    while True :
        # some parallel code here

        co = serial_function()
        co.next() # prime the generator-based coroutine
        q.put((co, x, rq))

        # Get the result of the computation from the main thread
        result = rq.get()

        print str(current_thread()) + " result: " + result + " from " + str(x)

        x += 1

        # maybe some more parallel code here, if you like

# A consumer which serializes a specific function requested by the producer
def consumer() :
    try :
        while True :
            while not q.empty() :
                (co, x, rq) = q.get()
                try :
                    # Send the input from the thread to the coroutine
                    # The coroutine's function will execuite in the
                    # context of the main thread (or other thread if
                    # you wish.
                    co.send(("sending: " + str(x), x, rq))
                except StopIteration :


    except KeyboardInterrupt, e :

q = Queue.Queue()

num_threads = 2
for idx in range(0, num_threads) :
    a = Thread(target=counter, args=[q])
    a.daemon = True


Michael Hines,
Aug 18, 2014, 8:39 PM