I have a Python script that handles aynchronous callbacks from .NET Remoting. These callbacks execute in a dummy (worker) thread. From inside my callback handler, I need to call a function I’ve defined in my script, but I need the function to execute in the main thread.

The Main thread is a remote client that sends commands to a server. Some of these commands result in asynchronous callbacks.

Basically, I need the equivalent of .NET’s Invoke method. Is this possible?

You want to use the Queue (now queue from python 3) class to set up a queue that your dummy threads populate with functions and that your main thread consumes.

import Queue

#somewhere accessible to both:
callback_queue = Queue.Queue()

def from_dummy_thread(func_to_call_from_main_thread):
    callback_queue.put(func_to_call_from_main_thread)

def from_main_thread_blocking():
    callback = callback_queue.get() #blocks until an item is available
    callback()

def from_main_thread_nonblocking():
    while True:
        try:
            callback = callback_queue.get(False) #doesn't block
        except Queue.Empty: #raised when queue is empty
            break
        callback()

Demo:

import threading
import time

def print_num(dummyid, n):
    print "From %s: %d" % (dummyid, n)
def dummy_run(dummyid):
    for i in xrange(5):
        from_dummy_thread(lambda: print_num(dummyid, i))
        time.sleep(0.5)
    
threading.Thread(target=dummy_run, args=("a",)).start()
threading.Thread(target=dummy_run, args=("b",)).start()

while True:
    from_main_thread_blocking()

Prints:

From a: 0
From b: 0
From a: 1
From b: 1
From b: 2
From a: 2
From b: 3
From a: 3
From b: 4
From a: 4

and then blocks forever