I'm baffled with a situation that involves:
1) an instance of some class that defines __del__,
2) a thread which is created, started and referenced by that instance,
and
3) a weakref proxy to the instance that is passed to the thread
instead of 'self', to prevent a cyclic reference.
This probably sounds like gibberish so here's a simplified example:
==========================================
import time
import weakref
import threading
num_main = num_other = 0
main_thread = threading.currentThread()
class Mystery(object):
def __init__(self):
proxy = weakref.proxy(self)
self._thread = threading.Thread(target=target, args=(proxy,))
self._thread.start()
def __del__(self):
global num_main, num_other
if threading.currentThread() is main_thread:
num_main += 1
else:
num_other += 1
def sleep(self, t):
time.sleep(t)
def target(proxy):
try: proxy.sleep(0.01)
except weakref.ReferenceError: pass
if __name__ == '__main__':
for i in xrange(1000):
Mystery()
time.sleep(0.1)
print '%d __del__ from main thread' % num_main
print '%d __del__ from other threads' % num_other
==========================================
When I run it, I get around 950 __del__ from the main thread and the
rest from non-main threads. I discovered this accidentally when I
noticed some ignored AssertionErrors caused by a __del__ that was
doing "self._thread.join()", assuming that the current thread is not
self._thread, but as it turns out that's not always the case.
So what is happening here for these ~50 minority cases ? Is __del__
invoked through the proxy ?
George 6 2840
On Jun 10, 8:15 pm, George Sakkis <george.sak...@gmail.comwrote:
I'm baffled with a situation that involves:
1) an instance of some class that defines __del__,
2) a thread which is created, started and referenced by that instance,
and
3) a weakref proxy to the instance that is passed to the thread
instead of 'self', to prevent a cyclic reference.
This probably sounds like gibberish so here's a simplified example:
==========================================
import time
import weakref
import threading
num_main = num_other = 0
main_thread = threading.currentThread()
class Mystery(object):
def __init__(self):
proxy = weakref.proxy(self)
self._thread = threading.Thread(target=target, args=(proxy,))
self._thread.start()
def __del__(self):
global num_main, num_other
if threading.currentThread() is main_thread:
num_main += 1
else:
num_other += 1
def sleep(self, t):
time.sleep(t)
def target(proxy):
try: proxy.sleep(0.01)
except weakref.ReferenceError: pass
if __name__ == '__main__':
for i in xrange(1000):
Mystery()
time.sleep(0.1)
print '%d __del__ from main thread' % num_main
print '%d __del__ from other threads' % num_other
==========================================
When I run it, I get around 950 __del__ from the main thread and the
rest from non-main threads. I discovered this accidentally when I
noticed some ignored AssertionErrors caused by a __del__ that was
doing "self._thread.join()", assuming that the current thread is not
self._thread, but as it turns out that's not always the case.
So what is happening here for these ~50 minority cases ? Is __del__
invoked through the proxy ?
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
If the child thread gets the GIL before __init__ returns it will enter
Mystery.sleep, then the main thread will return from Mystery.__init__
and release its strong reference, followed by the child thread
returning from Mystery.sleep, releasing its strong reference, and (as
it just released the last strong reference) calling Mystery.__del__.
If the main thread returns from __init__ before the child thread gets
the GIL, it will release the only strong reference to the Mystery
instance, causing it to clear the weakref proxy and call __del__
before the child thread ever gets a chance. If you added counters to
the target function you should see them match the counters of the
__del__ function.
Incidentally, += 1 isn't atomic in Python. It is possible for updates
to be missed.
On Jun 11, 1:40 am, Rhamphoryncus <rha...@gmail.comwrote:
On Jun 10, 8:15 pm, George Sakkis <george.sak...@gmail.comwrote:
I'm baffled with a situation that involves:
1) an instance of some class that defines __del__,
2) a thread which is created, started and referenced by that instance,
and
3) a weakref proxy to the instance that is passed to the thread
instead of 'self', to prevent a cyclic reference.
This probably sounds like gibberish so here's a simplified example:
==========================================
import time
import weakref
import threading
num_main = num_other = 0
main_thread = threading.currentThread()
class Mystery(object):
def __init__(self):
proxy = weakref.proxy(self)
self._thread = threading.Thread(target=target, args=(proxy,))
self._thread.start()
def __del__(self):
global num_main, num_other
if threading.currentThread() is main_thread:
num_main += 1
else:
num_other += 1
def sleep(self, t):
time.sleep(t)
def target(proxy):
try: proxy.sleep(0.01)
except weakref.ReferenceError: pass
if __name__ == '__main__':
for i in xrange(1000):
Mystery()
time.sleep(0.1)
print '%d __del__ from main thread' % num_main
print '%d __del__ from other threads' % num_other
==========================================
When I run it, I get around 950 __del__ from the main thread and the
rest from non-main threads. I discovered this accidentally when I
noticed some ignored AssertionErrors caused by a __del__ that was
doing "self._thread.join()", assuming that the current thread is not
self._thread, but as it turns out that's not always the case.
So what is happening here for these ~50 minority cases ? Is __del__
invoked through the proxy ?
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
Ah, that was the missing part; I thought that anything accessed
through a proxy didn't create a strong reference. The good thing is
that it seems you can get a proxy to a bounded method and then call it
without creating a strong reference to 'self':
num_main = num_other = 0
main_thread = threading.currentThread()
class MysterySolved(object):
def __init__(self):
sleep = weakref.proxy(self.sleep)
self._thread = threading.Thread(target=target, args=(sleep,))
self._thread.start()
def __del__(self):
global num_main, num_other
if threading.currentThread() is main_thread:
num_main += 1
else:
num_other += 1
def sleep(self, t):
time.sleep(t)
def target(sleep):
try: sleep(0.01)
except weakref.ReferenceError: pass
if __name__ == '__main__':
for i in xrange(1000):
MysterySolved()
time.sleep(.1)
print '%d __del__ from main thread' % num_main
print '%d __del__ from other threads' % num_other
==========================================
Output:
1000 __del__ from main thread
0 __del__ from other threads
Thanks a lot, I learned something new :)
George
On Jun 11, 10:43 am, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 1:40 am, Rhamphoryncus <rha...@gmail.comwrote:
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
Ah, that was the missing part; I thought that anything accessed
through a proxy didn't create a strong reference. The good thing is
that it seems you can get a proxy to a bounded method and then call it
without creating a strong reference to 'self':
That's not right. Of course a bound method has a strong reference to
self, otherwise you'd never be able to call it. There must be
something else going on here. Try using sys.setcheckinterval(1) to
make threads switch more often.
>
num_main = num_other = 0
main_thread = threading.currentThread()
class MysterySolved(object):
def __init__(self):
sleep = weakref.proxy(self.sleep)
self._thread = threading.Thread(target=target, args=(sleep,))
self._thread.start()
def __del__(self):
global num_main, num_other
if threading.currentThread() is main_thread:
num_main += 1
else:
num_other += 1
def sleep(self, t):
time.sleep(t)
def target(sleep):
try: sleep(0.01)
except weakref.ReferenceError: pass
if __name__ == '__main__':
for i in xrange(1000):
MysterySolved()
time.sleep(.1)
print '%d __del__ from main thread' % num_main
print '%d __del__ from other threads' % num_other
==========================================
Output:
1000 __del__ from main thread
0 __del__ from other threads
Thanks a lot, I learned something new :)
George
On Jun 11, 2:01 pm, Rhamphoryncus <rha...@gmail.comwrote:
On Jun 11, 10:43 am, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 1:40 am, Rhamphoryncus <rha...@gmail.comwrote:
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
Ah, that was the missing part; I thought that anything accessed
through a proxy didn't create a strong reference. The good thing is
that it seems you can get a proxy to a bounded method and then call it
without creating a strong reference to 'self':
That's not right. Of course a bound method has a strong reference to
self, otherwise you'd never be able to call it. There must be
something else going on here. Try using sys.setcheckinterval(1) to
make threads switch more often.
I tried that and it still works; all objects die at the main thread.
Any other idea to break it ?
George
On Jun 11, 2:15 pm, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 2:01 pm, Rhamphoryncus <rha...@gmail.comwrote:
On Jun 11, 10:43 am, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 1:40 am, Rhamphoryncus <rha...@gmail.comwrote:
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
Ah, that was the missing part; I thought that anything accessed
through a proxy didn't create a strong reference. The good thing is
that it seems you can get a proxy to a bounded method and then call it
without creating a strong reference to 'self':
That's not right. Of course a bound method has a strong reference to
self, otherwise you'd never be able to call it. There must be
something else going on here. Try using sys.setcheckinterval(1) to
make threads switch more often.
I tried that and it still works; all objects die at the main thread.
Any other idea to break it ?
Nothing comes to mind.
However, none of this is guaranteed anyway, so you can't rely on it.
__del__ might be called by any thread at any point (even when you're
holding a lock, updating a datastructure.)
On Jun 11, 8:37*pm, Rhamphoryncus <rha...@gmail.comwrote:
On Jun 11, 2:15 pm, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 2:01 pm, Rhamphoryncus <rha...@gmail.comwrote:
On Jun 11, 10:43 am, George Sakkis <george.sak...@gmail.comwrote:
On Jun 11, 1:40 am, Rhamphoryncus <rha...@gmail.comwrote:
The trick here is that calling proxy.sleep(0.01) first gets a strong
reference to the Mystery instance, then holds that strong reference
until it returns.
Ah, that was the missing part; I thought that anything accessed
through a proxy didn't create a strong reference. The good thing is
that it seems you can get a proxy to a bounded method and then call it
without creating a strong reference to 'self':
That's not right. *Of course a bound method has a strong reference to
self, otherwise you'd never be able to call it. *There must be
something else going on here. *Try using sys.setcheckinterval(1) to
make threads switch more often.
I tried that and it still works; all objects die at the main thread.
Any other idea to break it ?
Nothing comes to mind.
However, none of this is guaranteed anyway, so you can't rely on it.
__del__ might be called by any thread at any point (even when you're
holding a lock, updating a datastructure.)
Ok, you scared me enough to cut the link to the thread altogether,
avoiding the cyclic reference and the weakref proxy. The original
intent for keeping a reference to the thread was to be able to join()
it later before some cleanup takes place. I have since found a less
spooky way to synchronize them but regardless, I'd be interested for
an explanation of what's really going on in the posted snippets.
George This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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