Stop a thread on deletion

Hello all,

I'm using threading for generating video content. The trouble is how to
kill the thread, because there are multiple (simultaneous) owners of a
thread. Ideally, a flag would be set when the reference count of the
thread becomes zero, causing the run() loop to quit. Example:

import threading
import time
import gc

class myThread(threading.Thread):
def __init__(self):
self.passedOut = threading.Event()
threading.Thread.__init__(self)
def __del__(self):
self.passedOut.set()
def run(self):
i = 0
while not self.passedOut.isSet():
i += 1
print "Hi %d" % i
time.sleep(0.25)
a = myThread()
a.start()
time.sleep(2.5)
a = None
time.sleep(2.5)

Unfortunately, this doesn't work. When I remove the while-loop, __del__
is called, actually. Appearantly there is still some reference to the
thread while it is running.

I tried gc.get_referrers(self), but it seems to need some parsing. I'm
not sure how to implement that and I'm not sure whether it will work
always or not.

On 8/8/07, Sjoerd Op 't Land <sj****@intercue.nlwrote:

>Hello all,

I'm using threading for generating video content. The trouble is how to
kill the thread, because there are multiple (simultaneous) owners of a
thread. Ideally, a flag would be set when the reference count of the
thread becomes zero, causing the run() loop to quit. Example:

import threading
import time
import gc

class myThread(threading.Thread):
def __init__(self):
self.passedOut = threading.Event()
threading.Thread.__init__(self)
def __del__(self):
self.passedOut.set()
def run(self):
i = 0
while not self.passedOut.isSet():
i += 1
print "Hi %d" % i
time.sleep(0.25)
a = myThread()
a.start()
time.sleep(2.5)
a = None
time.sleep(2.5)

Unfortunately, this doesn't work. When I remove the while-loop, __del__
is called, actually. Appearantly there is still some reference to the
thread while it is running.

I tried gc.get_referrers(self), but it seems to need some parsing. I'm
not sure how to implement that and I'm not sure whether it will work
always or not.

gc.get_referrers returns a list of object instances that hold a
reference to the object. The important one in this case is, of course,
the thread itself. The thread holds a reference to the run method
which (of course) requires a reference to the object. In other words,
a running thread cannot be refcounted to zero. You are going to need a
better method of handling your resources.

Perhaps instead of holding a reference to the thread, they could hold
a reference to a proxy object:

import threading
import time
import gc
import pprint

class myThread(threading.Thread):
def __init__(self):
self.passedOut = threading.Event()
threading.Thread.__init__(self)
def run(self):
i = 0
while not self.passedOut.isSet():
i += 1
print "Hi %d" % i
time.sleep(1)
print "stopped"
class ThreadProxy(object):
def __init__(self, proxy_for):
self.proxy_for = proxy_for
def __del__(self):
self.proxy_for.passedOut.set()
def start(self):
self.proxy_for.start()