Named semaphores in Python?

Question

Hi,

I have a script in python which uses a resource which can not be used by more than a certain amount of concurrent scripts running.

Classically, this would be solved by a named semaphores but I can not find those in the documentation of the multiprocessing module or threading .

Am I missing something or are named semaphores not implemented / exposed by Python? and more importantly, if the answer is no, what is the best way to emulate one?

Thanks, Boaz

PS. For reasons which are not so relevant to this question, I can not aggregate the task to a continuously running process/daemon or work with spawned processes - both of which, it seems, would have worked with the python API.

Answer 1

You can emulate them by using the filesystem instead of a kernel path (named semaphores are implemented this way on some platforms anyhow). You'll have to implement sem_[open|wait|post|unlink] yourself, but it ought to be relatively trivial to do so. Your synchronization overhead might be significant (depending on how often you have to fiddle with the semaphore in your app), so you might want to initialize a ramdisk when you launch your process in which to store named semaphores.

Alternatively if you're not comfortable rolling your own, you could probably wrap boost::interprocess::named_semaphore (docs here) in a simple extension module.

Answer 2

I suggest a third party extension like these, ideally the posix_ipc one -- see in particular the sempahore section in the docs.

These modules are mostly about exposing the "system V IPC" (including semaphores) in a unixy way, but at least one of them (posix_ipc specifically) is claimed to work with Cygwin on Windows (I haven't verified that claim). There are some documented limitations on FreeBSD 7.2 and Mac OSX 10.5, so take care if those platforms are important to you.