multiprocessing-utils 0.4

Multiprocessing utils (shared locks and thread locals)

Multiprocessing utilities

Shared locks

“Shared” version of the standard Lock() and RLock() classes found in the multiprocessing/threading modules.

Shared locks can be acquired in two modes, shared and exclusive.

Any number of processes/threads can acquire the lock in shared mode.

Only one process/thread can acquire the lock in exclusive mode.

A process/thread attempting to exclusively acquire the lock will block until the lock has been released by all other threads/processes.

A process/thread attempting to shared acquire the lock will only block while there is an exclusive lock.

This is a little like database locks, which can be acquired for shared reading, or exclusive writing.

lock = multiprocessing_utils.SharedLock()

def exclusive_worker():
    with lock.exclusive():
        # this code will only run when no other
        # process/thread in a lock context

def shared_worker():
    with lock:
        # this code will run so long as no
        # thread/process holds an exclusive lock

multiprocess-safe threading.local()

A process (and thread) safe version of threading.local()

l = multiprocessing_utils.local()
l.x = 1

def f():
    try:
        print(l.x)
    except Attribute:
        print("x not set")
f()                                        # prints "1"
threading.Thread(target=f).start()         # prints "x not set"
multiprocessing.Process(target=f).start()  # prints "x not set"

Difference to standard threading.local()

A standard threading.local() instance created before forking (via os.fork() or multiprocessing.Process()) will be “local” as expected and the new process will have access to any data set before the fork.

Using a standard threading.local() in the example above would yield:

f()                                        # prints "1"
threading.Thread(target=f).start()         # prints "x not set"
multiprocessing.Process(target=f).start()  # prints "1" :(