Bastian Hammer wrote:
Sure, you can use a Semaphore. But it sounds like you are really wanting an exclusive lock. Semaphore can do that for you - actually it's the default behavior. You could try using a regular old Lock. Semaphores are locking counters. You set the counter at initialization to some number (the default is 1). When you enter into a semaphored area of code (using the .acquire() method), the counter attempts to decrement and will do so if it doesn't push it beneath 0. Upon exiting the area of semaphored code (by calling the .release() method on the semaphore), the counter is incremented. An example would look like this:Hi
I´m wondering why there are so few examples with Semaphore. Is it obsolete?
I´ve got a Class Data. It offers 2 Threads methods for updating, editing, .. a private dictionary.
Now I have to make sure, that both threads are synchronal, 1 thread edits something and the other is blocked until the first
thread is ready.
Isn´t it a good idea to do this with a semaphore?
And if I should use a Semaphore here, could anybody give me an example how it should look like?
Everything that I test throws errors :(
Thank you :)
Bye, Bastian
import threading
class locking_writer:
def __init__(self, some_file):
self.sema = threading.Semaphore()
self.f = open(some_file, 'w')
def write(self, content):
self.sema.acquire()
self.f.write(content)
self.sema.release()
and used like this:
In [16]: l = locking_writer('/tmp/foo')In [17]: l.write('test')I haven't tested this with multiple threads, so I'll leave that up to you if you want to use it.
Now, with all that said, the preferred way of synchronizing between threads is to use a Queue (import Queue\nq = Queue.Queue()). If you have a file that more than one thread needs to update, you probably want to create a thread just to update that file and have the threads responsible for getting information to update it with pass that information into a queue. You may have reasons for not wanting to do that, but it's worth looking into and considering.
Jeremy Jones
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