Python condition variable timeout

Question

I have thread1 which is waiting on a condition from thread2. But it could be that thread2 is never signalling the condition variable. So I have added a timeout to the wait call in thread 1, like this:

cv.acquire()
cv.wait(1.0)
cv.release()

How can I know if the condition variable was signaled or a timeout occurred? wait does not seem to return any value. The python documentation on Condition Objects gives no clues about this.

Answer 1

You are not supposed to care; the typical case is that your waiting thread checks some shared state until that state matches some condition.

The documentation example is thus:

cv.acquire()
while not an_item_is_available():
    cv.wait()
get_an_available_item()
cv.release()

and the documentation also states:

[…] threads that are interested in a particular change of state call wait() repeatedly until they see the desired state

If you do have a pressing need to distinguish between a timeout and a signal, you'll need to use Event objects instead; the .wait(timeout) call on an Event object returns None if the flag wasn't set (which only happens when the timeout was reached).

Answer 2

Martijn's answer was true for older versions of Python, however in 3.2, this changed. The method wait() now returns False on timeout.

Interestingly, there is now a wait_for() method as well. This takes a predicate method, and waits for it to evaluate to True. Upon timing out, it returns False.