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What happens if the function of a python thread is completed?

I am using python thread while I found no method to stop it.
Here is how I use the thread:
class MyThread(Thread):
def __init__(self, func, args=()):
Thread.__init__(self)
self.__return_value = None
self.func = func
self.args = args
self.func_name = func.__name__
def run(self):
self.__return_value = self.func(*self.args)
Considering there is no explicit way to stop it, I try to ignore it when it finishes the function to execute.
Will a zombie thread left if I do nothing when it finishes?

No - the thread pack up after itself and shuts down cleanly.
It is how things in Python try to work, after all.
import threading
import time
def worker():
time.sleep(1)
def main():
print (threading.active_count())
t = threading.Thread(target=worker)
t.start()
print(threading.active_count())
time.sleep(2)
print(threading.active_count())
return t
main()
t = main()
t.is_alive()
Running this snippet in ipython (an interactive prompt which uses some threads for its own purposes) will print
4
5
4
False

Stopping eval code dinamically on event fired [duplicate]

What's the proper way to tell a looping thread to stop looping?
I have a fairly simple program that pings a specified host in a separate threading.Thread class. In this class it sleeps 60 seconds, the runs again until the application quits.
I'd like to implement a 'Stop' button in my wx.Frame to ask the looping thread to stop. It doesn't need to end the thread right away, it can just stop looping once it wakes up.
Here is my threading class (note: I haven't implemented looping yet, but it would likely fall under the run method in PingAssets)
class PingAssets(threading.Thread):
def __init__(self, threadNum, asset, window):
threading.Thread.__init__(self)
self.threadNum = threadNum
self.window = window
self.asset = asset
def run(self):
config = controller.getConfig()
fmt = config['timefmt']
start_time = datetime.now().strftime(fmt)
try:
if onlinecheck.check_status(self.asset):
status = "online"
else:
status = "offline"
except socket.gaierror:
status = "an invalid asset tag."
msg =("{}: {} is {}. \n".format(start_time, self.asset, status))
wx.CallAfter(self.window.Logger, msg)
And in my wxPyhton Frame I have this function called from a Start button:
def CheckAsset(self, asset):
self.count += 1
thread = PingAssets(self.count, asset, self)
self.threads.append(thread)
thread.start()

Threaded stoppable function
Instead of subclassing threading.Thread, one can modify the function to allow
stopping by a flag.
We need an object, accessible to running function, to which we set the flag to stop running.
We can use threading.currentThread() object.
import threading
import time
def doit(arg):
t = threading.currentThread()
while getattr(t, "do_run", True):
print ("working on %s" % arg)
time.sleep(1)
print("Stopping as you wish.")
def main():
t = threading.Thread(target=doit, args=("task",))
t.start()
time.sleep(5)
t.do_run = False
if __name__ == "__main__":
main()
The trick is, that the running thread can have attached additional properties. The solution builds
on assumptions:
the thread has a property "do_run" with default value True
driving parent process can assign to started thread the property "do_run" to False.
Running the code, we get following output:
$ python stopthread.py
working on task
working on task
working on task
working on task
working on task
Stopping as you wish.
Pill to kill - using Event
Other alternative is to use threading.Event as function argument. It is by
default False, but external process can "set it" (to True) and function can
learn about it using wait(timeout) function.
We can wait with zero timeout, but we can also use it as the sleeping timer (used below).
def doit(stop_event, arg):
while not stop_event.wait(1):
print ("working on %s" % arg)
print("Stopping as you wish.")
def main():
pill2kill = threading.Event()
t = threading.Thread(target=doit, args=(pill2kill, "task"))
t.start()
time.sleep(5)
pill2kill.set()
t.join()
Edit: I tried this in Python 3.6. stop_event.wait() blocks the event (and so the while loop) until release. It does not return a boolean value. Using stop_event.is_set() works instead.
Stopping multiple threads with one pill
Advantage of pill to kill is better seen, if we have to stop multiple threads
at once, as one pill will work for all.
The doit will not change at all, only the main handles the threads a bit differently.
def main():
pill2kill = threading.Event()
tasks = ["task ONE", "task TWO", "task THREE"]
def thread_gen(pill2kill, tasks):
for task in tasks:
t = threading.Thread(target=doit, args=(pill2kill, task))
yield t
threads = list(thread_gen(pill2kill, tasks))
for thread in threads:
thread.start()
time.sleep(5)
pill2kill.set()
for thread in threads:
thread.join()

This has been asked before on Stack. See the following links:
Is there any way to kill a Thread in Python?
Stopping a thread after a certain amount of time
Basically you just need to set up the thread with a stop function that sets a sentinel value that the thread will check. In your case, you'll have the something in your loop check the sentinel value to see if it's changed and if it has, the loop can break and the thread can die.

I read the other questions on Stack but I was still a little confused on communicating across classes. Here is how I approached it:
I use a list to hold all my threads in the __init__ method of my wxFrame class: self.threads = []
As recommended in How to stop a looping thread in Python? I use a signal in my thread class which is set to True when initializing the threading class.
class PingAssets(threading.Thread):
def __init__(self, threadNum, asset, window):
threading.Thread.__init__(self)
self.threadNum = threadNum
self.window = window
self.asset = asset
self.signal = True
def run(self):
while self.signal:
do_stuff()
sleep()
and I can stop these threads by iterating over my threads:
def OnStop(self, e):
for t in self.threads:
t.signal = False

I had a different approach. I've sub-classed a Thread class and in the constructor I've created an Event object. Then I've written custom join() method, which first sets this event and then calls a parent's version of itself.
Here is my class, I'm using for serial port communication in wxPython app:
import wx, threading, serial, Events, Queue
class PumpThread(threading.Thread):
def __init__ (self, port, queue, parent):
super(PumpThread, self).__init__()
self.port = port
self.queue = queue
self.parent = parent
self.serial = serial.Serial()
self.serial.port = self.port
self.serial.timeout = 0.5
self.serial.baudrate = 9600
self.serial.parity = 'N'
self.stopRequest = threading.Event()
def run (self):
try:
self.serial.open()
except Exception, ex:
print ("[ERROR]\tUnable to open port {}".format(self.port))
print ("[ERROR]\t{}\n\n{}".format(ex.message, ex.traceback))
self.stopRequest.set()
else:
print ("[INFO]\tListening port {}".format(self.port))
self.serial.write("FLOW?\r")
while not self.stopRequest.isSet():
msg = ''
if not self.queue.empty():
try:
command = self.queue.get()
self.serial.write(command)
except Queue.Empty:
continue
while self.serial.inWaiting():
char = self.serial.read(1)
if '\r' in char and len(msg) > 1:
char = ''
#~ print('[DATA]\t{}'.format(msg))
event = Events.PumpDataEvent(Events.SERIALRX, wx.ID_ANY, msg)
wx.PostEvent(self.parent, event)
msg = ''
break
msg += char
self.serial.close()
def join (self, timeout=None):
self.stopRequest.set()
super(PumpThread, self).join(timeout)
def SetPort (self, serial):
self.serial = serial
def Write (self, msg):
if self.serial.is_open:
self.queue.put(msg)
else:
print("[ERROR]\tPort {} is not open!".format(self.port))
def Stop(self):
if self.isAlive():
self.join()
The Queue is used for sending messages to the port and main loop takes responses back. I've used no serial.readline() method, because of different end-line char, and I have found the usage of io classes to be too much fuss.

Depends on what you run in that thread.
If that's your code, then you can implement a stop condition (see other answers).
However, if what you want is to run someone else's code, then you should fork and start a process. Like this:
import multiprocessing
proc = multiprocessing.Process(target=your_proc_function, args=())
proc.start()
now, whenever you want to stop that process, send it a SIGTERM like this:
proc.terminate()
proc.join()
And it's not slow: fractions of a second.
Enjoy :)

My solution is:
import threading, time
def a():
t = threading.currentThread()
while getattr(t, "do_run", True):
print('Do something')
time.sleep(1)
def getThreadByName(name):
threads = threading.enumerate() #Threads list
for thread in threads:
if thread.name == name:
return thread
threading.Thread(target=a, name='228').start() #Init thread
t = getThreadByName('228') #Get thread by name
time.sleep(5)
t.do_run = False #Signal to stop thread
t.join()

I find it useful to have a class, derived from threading.Thread, to encapsulate my thread functionality. You simply provide your own main loop in an overridden version of run() in this class. Calling start() arranges for the object’s run() method to be invoked in a separate thread.
Inside the main loop, periodically check whether a threading.Event has been set. Such an event is thread-safe.
Inside this class, you have your own join() method that sets the stop event object before calling the join() method of the base class. It can optionally take a time value to pass to the base class's join() method to ensure your thread is terminated in a short amount of time.
import threading
import time
class MyThread(threading.Thread):
def __init__(self, sleep_time=0.1):
self._stop_event = threading.Event()
self._sleep_time = sleep_time
"""call base class constructor"""
super().__init__()
def run(self):
"""main control loop"""
while not self._stop_event.isSet():
#do work
print("hi")
self._stop_event.wait(self._sleep_time)
def join(self, timeout=None):
"""set stop event and join within a given time period"""
self._stop_event.set()
super().join(timeout)
if __name__ == "__main__":
t = MyThread()
t.start()
time.sleep(5)
t.join(1) #wait 1s max
Having a small sleep inside the main loop before checking the threading.Event is less CPU intensive than looping continuously. You can have a default sleep time (e.g. 0.1s), but you can also pass the value in the constructor.

Sometimes you don't have control over the running target. In those cases you can use signal.pthread_kill to send a stop signal.
from signal import pthread_kill, SIGTSTP
from threading import Thread
from itertools import count
from time import sleep
def target():
for num in count():
print(num)
sleep(1)
thread = Thread(target=target)
thread.start()
sleep(5)
pthread_kill(thread.ident, SIGTSTP)
result
0
1
2
3
4
[14]+ Stopped

Python threads, how do Event and Queue work together?

I was talking with my friend,after looking at example from Beasley's book
class ActorExit(Exception):
pass
class Actor:
def __init__(self):
self._mailbox = Queue()
def send(self, msg):
self._mailbox.put(msg)
def recv(self):
msg = self._mailbox.get()
if msg is ActorExit:
raise ActorExit()
return msg
def close(self):
self.send(ActorExit)
def start(self):
self._terminated = Event()
t = Thread(target=self._bootstrap)
t.daemon = True
t.start()
def _bootstrap(self):
try:
self.run()
except ActorExit:
pass
finally:
self._terminated.set()
def join(self):
self._terminated.wait()
def run(self):
while True:
msg = self.recv()
class PrintActor(Actor):
def run(self):
while True:
msg = self.recv()
print('Got:', msg)
My friend argues that sole purpose of Event is to block the main thread until the other thread performs set operation.
Is that true?
How can we watch thread execution?

Python threads, how do Event and Queue work together?
They don't. You can use Events without queues and queues without Events, there's no dependency on each other. Your example just happens to use both.
My friend argues that sole purpose of Event is to block the main thread until the other thread performs set operation. Is that true?
Calling .wait() on an Event-object will block any calling thread until the internal flag is .set().
If you look at the source for Event, you'll find that Events just consist of a Condition variable with a lock and a boolean flag + methods to handle and communicate (to waiting threads) state changes of that flag.
class Event:
"""Class implementing event objects.
Events manage a flag that can be set to true with the set() method and reset
to false with the clear() method. The wait() method blocks until the flag is
true. The flag is initially false.
"""
def __init__(self):
self._cond = Condition(Lock())
self._flag = False
...
How can we watch thread execution?
A simple method would be to apply some sort of utility function that prints out what you're interested in, for example:
def print_info(info=""):
"""Print calling function's name and thread with optional info-text."""
calling_func = sys._getframe(1).f_code.co_name
thread_name = threading.current_thread().getName()
print(f"<{thread_name}, {calling_func}> {info}", flush=True)
Another possibility would be to use logging like in this answer.
Not sure what Beazly wanted to demonstrate with the code you showed, but it deems a little over-engineered to me for this simple task. Involving Events here on top is unnecessary when you already use a queue. You can initialize thread termination by passing a sentinel-value.
Here's a simplified version of your example with sentinel ('STOP') and some info-prints with print_info from above:
import sys
import time
import threading
from queue import Queue
class Actor(threading.Thread):
def __init__(self):
super().__init__(target=self.run)
self.queue = Queue()
def send(self, msg):
self.queue.put(msg)
print_info(f"sent: {msg}") # DEBUG
def close(self):
print_info() # DEBUG
self.send('STOP')
def run(self):
for msg in iter(self.queue.get, 'STOP'):
pass
class PrintActor(Actor):
def run(self):
for msg in iter(self.queue.get, 'STOP'):
print_info(f"got: {msg}") # DEBUG
if __name__ == '__main__':
pa = PrintActor()
pa.start()
pa.send("Hello")
time.sleep(2)
pa.send("...World!")
time.sleep(2)
pa.close()
pa.join()
Output:
<MainThread, send> sent: Hello
<Thread-1, run> got: Hello
<MainThread, send> sent: ...World!
<Thread-1, run> got: ...World!
<MainThread, close>
<MainThread, send> sent: STOP

Calling GMainLoop.quit() for mainloop running in child process, from parent process

I need to run a gstreamer pipeline to perform video streaming. The GStreamer pipeline requires a GObject.MainLoop object which has a run() method that does not terminate until quit() is called.
For this I create a process (P2) from my main application process (P1), which runs the GObject.MainLoop instance in its main thread. The problem is that loop goes on indefinitly within the process P2 and I'm unable to exit/quit it from the main application process (P1).
Following is the section of code that might help understanding the scenario.
'''
start() spawns a new process P2 that runs Mainloop within its main thread.
stop() is called from P1, but does not quit the Mainloop. This is probably because
processes do not have shared memory
'''
from multiprocessing import Process
import gi
from gi.repository import GObject
class Main:
def __init__(self):
self.process = None
self.loop = GObject.MainLoop()
def worker(self):
self.loop.run()
def start(self):
self.process=Process(target=self.worker, args=())
self.process.start()
def stop(self):
self.loop.quit()
Next, I tried using a multiprocessing Queue for sharing the 'loop' variable between the processes, but am still unable to quit the mainloop.
'''
start() spawns a new process and puts the loop object in a multiprocessing Queue
stop() calls get() from the loop and calls the quit() method, though it still does not quit the mainloop.
'''
from multiprocessing import Process, Queue
import gi
from gi.repository import GObject
class Main:
def __init__(self):
self.p=None
self.loop = GObject.MainLoop()
self.queue = Queue()
def worker(self):
self.queue.put(self.loop)
self.loop.run()
def start(self):
self.p=Process(target=self.worker, args=())
self.p.start()
def stop(self):
# receive loop instance shared by Child Process
loop=self.queue.get()
loop.quit()
How do I call the quit method for the MainLoop object which is only accessible within the child Process P2?

Ok firstly we need to be using threads not processes. Processes will be in a different address space.
What is the difference between a process and a thread?
Try passing the main loop object to a separate thread that does the actual work. This will make your main method in to nothing but a basic GLib event processing loop, but that is fine and the normal behavior in many GLib applciations.
Lastly, we need to handle the race condition of the child process finishing its work before the main loop activates. We do this with the while not loop.is_running() snippet.
from threading import Thread
import gi
from gi.repository import GObject
def worker(loop):
while not loop.is_running():
print("waiting for loop to run")
print("working")
loop.quit()
print("quitting")
class Main:
def __init__(self):
self.thread = None
self.loop = GObject.MainLoop()
def start(self):
self.thread=Thread(target=worker, args=(self.loop,))
self.thread.start()
self.loop.run()
def main():
GObject.threads_init()
m = Main()
m.start()
if __name__ =='__main__' : main()

I extended multiprocessing.Process module in my class Main and overridden its run() method to actually run the GObject.Mainloop instance inside another thread (T1) instead of its main thread. And then implemented a wait-notify mechanism which will make the main thread of Process (P2) to go under wait-notify loop and used multiprocessing.Queue to forward messages to the main thread of P2 and P2 will be notified at the same time. For eg, stop() method, which will send the quit message to P2 for which a handler is defined in the overridden run() method.
This module can be extended to parse any number of messages to the Child Process provided their handlers are to be defined also.
Following is the code snippet which I used.
from multiprocessing import Process, Condition, Queue
from threading import Thread
import gi
from gi.repository import GObject
loop=GObject.MainLoop()
def worker():
loop.run()
class Main(Process):
def __init__(self, target=None, args=()):
self.target=target
self.args=tuple(args)
print self.args
self.message_queue = Queue()
self.cond = Condition()
self.thread = None
self.loop = GObject.MainLoop()
Process.__init__(self)
def run(self):
if self.target:
self.thread = Thread(target=self.target, args=())
print "running target method"
self.thread.start()
while True:
with self.cond:
self.cond.wait()
msg = self.message_queue.get()
if msg == 'quit':
print loop.is_running()
loop.quit()
print loop.is_running()
break
else:
print 'message received', msg
def send_message(self, msg):
self.message_queue.put(msg)
with self.cond:
self.cond.notify_all()
def stop(self):
self.send_message("quit")
self.join()
def func1(self):
self.send_message("msg 1") # handler is defined in the overridden run method
# few others functions which will send unique messages to the process, and their handlers
# are defined in the overridden run method above
This method is working fine for my scenerio but suggestions are welcomed if there is a better way to do the same.

In Python, how do I know when a process is finished?

From within a Python GUI (PyGTK) I start a process (using multiprocessing). The process takes a long time (~20 minutes) to finish. When the process is finished I would like to clean it up (extract the results and join the process). How do I know when the process has finished?
My colleague suggested a busy loop within the parent process that checks if the child process has finished. Surely there is a better way.
In Unix, when a process is forked, a signal handler is called from within the parent process when the child process has finished. But I cannot see anything like that in Python. Am I missing something?
How is it that the end of a child process can be observed from within the parent process? (Of course, I do not want to call Process.join() as it would freeze up the GUI interface.)
This question is not limited to multi-processing: I have exactly the same problem with multi-threading.

I think as a part of making python multi-platform, simple things like SIGCHLD must be done yourself. Agreed, this is a little more work when all you want to do is know when the child is done, but it really isn't THAT painful. Consider the following that uses a child process to do the work, two multiprocessing.Event instances, and a thread to check if the child process is done:
import threading
from multiprocessing import Process, Event
from time import sleep
def childsPlay(event):
print "Child started"
for i in range(3):
print "Child is playing..."
sleep(1)
print "Child done"
event.set()
def checkChild(event, killEvent):
event.wait()
print "Child checked, and is done playing"
if raw_input("Do again? y/n:") == "y":
event.clear()
t = threading.Thread(target=checkChild, args=(event, killEvent))
t.start()
p = Process(target=childsPlay, args=(event,))
p.start()
else:
cleanChild()
killEvent.set()
def cleanChild():
print "Cleaning up the child..."
if __name__ == '__main__':
event = Event()
killEvent = Event()
# process to do work
p = Process(target=childsPlay, args=(event,))
p.start()
# thread to check on child process
t = threading.Thread(target=checkChild, args=(event, killEvent))
t.start()
try:
while not killEvent.is_set():
print "GUI running..."
sleep(1)
except KeyboardInterrupt:
print "Quitting..."
exit(0)
finally:
print "Main done"
EDIT
Joining to all processes and threads created is a good practice because it will help indicate when zombie (never-finishing) processes/threads are being created. I've altered the above code making a ChildChecker class that inherits from threading.Thread. It's sole purpose is to start a job in a separate process, wait for that process to finish, and then notify the GUI when everything is complete. Joining on the ChildChecker will also join the process it is "checking". Now, if the process doesn't join after 5 seconds, the thread will force terminate the process. Enter "y" creates starts a child process running "endlessChildsPlay" that must demonstrate force termination.
import threading
from multiprocessing import Process, Event
from time import sleep
def childsPlay(event):
print "Child started"
for i in range(3):
print "Child is playing..."
sleep(1)
print "Child done"
event.set()
def endlessChildsPlay(event):
print "Endless child started"
while True:
print "Endless child is playing..."
sleep(1)
event.set()
print "Endless child done"
class ChildChecker(threading.Thread):
def __init__(self, killEvent):
super(ChildChecker, self).__init__()
self.killEvent = killEvent
self.event = Event()
self.process = Process(target=childsPlay, args=(self.event,))
def run(self):
self.process.start()
while not self.killEvent.is_set():
self.event.wait()
print "Child checked, and is done playing"
if raw_input("Do again? y/n:") == "y":
self.event.clear()
self.process = Process(target=endlessChildsPlay, args=(self.event,))
self.process.start()
else:
self.cleanChild()
self.killEvent.set()
def join(self):
print "Joining child process"
# Timeout on 5 seconds
self.process.join(5)
if self.process.is_alive():
print "Child did not join! Killing.."
self.process.terminate()
print "Joining ChildChecker thread"
super(ChildChecker, self).join()
def cleanChild(self):
print "Cleaning up the child..."
if __name__ == '__main__':
killEvent = Event()
# thread to check on child process
t = ChildChecker(killEvent)
t.start()
try:
while not killEvent.is_set():
print "GUI running..."
sleep(1)
except KeyboardInterrupt:
print "Quitting..."
exit(0)
finally:
t.join()
print "Main done"

This answer is really simple! (It just took me days to work it out.)
Combined with PyGTK's idle_add(), you can create an AutoJoiningThread. The total code is borderline trivial:
class AutoJoiningThread(threading.Thread):
def run(self):
threading.Thread.run(self)
gobject.idle_add(self.join)
If you want to do more than just join (such as collecting results) then you can extend the above class to emit signals on completion, as is done in the following example:
import threading
import time
import sys
import gobject
gobject.threads_init()
class Child:
def __init__(self):
self.result = None
def play(self, count):
print "Child starting to play."
for i in range(count):
print "Child playing."
time.sleep(1)
print "Child finished playing."
self.result = 42
def get_result(self, obj):
print "The result was "+str(self.result)
class AutoJoiningThread(threading.Thread, gobject.GObject):
__gsignals__ = {
'finished': (gobject.SIGNAL_RUN_LAST,
gobject.TYPE_NONE,
())
}
def __init__(self, *args, **kwargs):
threading.Thread.__init__(self, *args, **kwargs)
gobject.GObject.__init__(self)
def run(self):
threading.Thread.run(self)
gobject.idle_add(self.join)
gobject.idle_add(self.emit, 'finished')
def join(self):
threading.Thread.join(self)
print "Called Thread.join()"
if __name__ == '__main__':
print "Creating child"
child = Child()
print "Creating thread"
thread = AutoJoiningThread(target=child.play,
args=(3,))
thread.connect('finished', child.get_result)
print "Starting thread"
thread.start()
print "Running mainloop (Ctrl+C to exit)"
mainloop = gobject.MainLoop()
try:
mainloop.run()
except KeyboardInterrupt:
print "Received KeyboardInterrupt. Quiting."
sys.exit()
print "God knows how we got here. Quiting."
sys.exit()
The output of the above example will depend on the order the threads are executed, but it will be similar to:
Creating child
Creating thread
Starting thread
Child starting to play.
Child playing.
Running mainloop (Ctrl+C to exit)
Child playing.
Child playing.
Child finished playing.
Called Thread.join()
The result was 42
^CReceived KeyboardInterrupt. Quiting.
It's not possible to create an AutoJoiningProcess in the same way (because we cannot call idle_add() across two different processes), however we can use an AutoJoiningThread to get what we want:
class AutoJoiningProcess(multiprocessing.Process):
def start(self):
thread = AutoJoiningThread(target=self.start_process)
thread.start() # automatically joins
def start_process(self):
multiprocessing.Process.start(self)
self.join()
To demonstrate AutoJoiningProcess here is another example:
import threading
import multiprocessing
import time
import sys
import gobject
gobject.threads_init()
class Child:
def __init__(self):
self.result = multiprocessing.Manager().list()
def play(self, count):
print "Child starting to play."
for i in range(count):
print "Child playing."
time.sleep(1)
print "Child finished playing."
self.result.append(42)
def get_result(self, obj):
print "The result was "+str(self.result)
class AutoJoiningThread(threading.Thread, gobject.GObject):
__gsignals__ = {
'finished': (gobject.SIGNAL_RUN_LAST,
gobject.TYPE_NONE,
())
}
def __init__(self, *args, **kwargs):
threading.Thread.__init__(self, *args, **kwargs)
gobject.GObject.__init__(self)
def run(self):
threading.Thread.run(self)
gobject.idle_add(self.join)
gobject.idle_add(self.emit, 'finished')
def join(self):
threading.Thread.join(self)
print "Called Thread.join()"
class AutoJoiningProcess(multiprocessing.Process, gobject.GObject):
__gsignals__ = {
'finished': (gobject.SIGNAL_RUN_LAST,
gobject.TYPE_NONE,
())
}
def __init__(self, *args, **kwargs):
multiprocessing.Process.__init__(self, *args, **kwargs)
gobject.GObject.__init__(self)
def start(self):
thread = AutoJoiningThread(target=self.start_process)
thread.start()
def start_process(self):
multiprocessing.Process.start(self)
self.join()
gobject.idle_add(self.emit, 'finished')
def join(self):
multiprocessing.Process.join(self)
print "Called Process.join()"
if __name__ == '__main__':
print "Creating child"
child = Child()
print "Creating thread"
process = AutoJoiningProcess(target=child.play,
args=(3,))
process.connect('finished',child.get_result)
print "Starting thread"
process.start()
print "Running mainloop (Ctrl+C to exit)"
mainloop = gobject.MainLoop()
try:
mainloop.run()
except KeyboardInterrupt:
print "Received KeyboardInterrupt. Quiting."
sys.exit()
print "God knows how we got here. Quiting."
sys.exit()
The resulting output will be very similar to the example above, except this time we have both the process joining and it's attendant thread joining too:
Creating child
Creating thread
Starting thread
Running mainloop (Ctrl+C to exit)
Child starting to play.
Child playing.
Child playing.
Child playing.
Child finished playing.
Called Process.join()
The result was [42]
Called Thread.join()
^CReceived KeyboardInterrupt. Quiting.
Unfortunately:
This solution is dependent on gobject, due to the use of idle_add(). gobject is used by PyGTK.
This is not a true parent/child relationship. If one of these threads is started by another thread, then it will nonetheless be joined by the thread running the mainloop, not the parent thread. This problem holds true for AutoJoiningProcess too, except there I imagine an exception would be thrown.
Thus to use this approach, it would be best to only create threads/process from within the mainloop/GUI.

You can use a queue to communicate with child processes. You can stick intermediate results on it, or messages indicating that milestones have been hit (for progress bars) or just a message indicating that the process is ready to be joined. Polling it with empty is easy and fast.
If you really only want to know if it's done, you can watch the exitcode of your process or poll is_alive().

In my efforts to try to find an answer to my own question, I stumbled across PyGTK's idle_add() function. This gives me the following possibility:
Create a new child process that communicates via a Queue.
Create a listener thread that listens to the Queue, when the child process sends the listener a message saying that it is finished, the listener calls idle_add() that sets up a callback.
During the next time around the main loop the parent process will call the callback.
The callback can extract results, join the child process and join the listener-thread.
This seems an overly complex way to re-create Unix's call-callback-when-child-process-is-done.
This must be an uber-common problem with GUIs in Python. Surely there is a standard pattern to solve this problem?

have a look at the subprocess module:
http://docs.python.org/library/subprocess.html
import subprocess
let pipe = subprocess.Popen("ls -l", stdout=subprocess.PIPE)
allText = pipe.stdout.read()
pipe.wait()
retVal = pipe.returncode