Python in a Nutshell

Name

wait

Synopsis

                           c.wait(timeout=None)

wait releases L, then suspends the calling thread until some other thread calls notify or notifyAll on c. The calling thread must hold L before it calls c .wait( ). timeout is covered earlier in Section 14.4.2.1. After a thread wakes up, either by notification or timeout, the thread becomes ready when it acquires L again. When wait returns, the calling thread always holds L again.

In typical use, a Condition object c regulates access to some global state s that is shared between threads. When a thread needs to wait for s to change, the thread loops as follows:

                           c.acquire( )
while not is_ok_state(s):
    c.wait( )
do_some_work_using_state(s)
c.release( )

Meanwhile, each thread that modifies s calls notify (or notifyAll, if it needs to wake up all waiting threads, not just one) each time s changes:

                           c.acquire( )
do_something_that_modifies_state(s)
c.notify( )    # or, c.notifyAll( )
c.release( )

As you see, you always need to acquire and release c around each use of c’s methods, which makes using Condition somewhat error-prone.