Python 3.2+threadingterm

threading.Barrier

synchronization

A barrier: a fixed number of threads wait for each other in wait() and then all move on together — a shared starting line before the next phase of work.

Syntax

barrier = threading.Barrier(parties, action=None, timeout=None)
index = barrier.wait(timeout=None)

The barrier opens only when exactly parties threads have entered wait(): if one crashed or fewer were started, the rest hang forever, so pass a timeout. A timeout breaks the whole barrier: every waiting and every later wait() raises BrokenBarrierError until you call reset(). wait() hands each thread a distinct number from 0 to parties-1, so exactly one thread can do the shared step (the action parameter does the same). Unlike Event, a barrier is reusable: after a pass it is ready for the next round by itself.

Examples

import threading
barrier = threading.Barrier(3)
lock = threading.Lock()
log, numbers = [], []
def worker(name):
    with lock:
        log.append('ready')
    n = barrier.wait()
    with lock:
        log.append('go')
        numbers.append(n)
threads = [threading.Thread(target=worker, args=(c,)) for c in 'abc']
for t in threads:
    t.start()
for t in threads:
    t.join()
print(log[:3], sorted(numbers))  # → ['ready', 'ready', 'ready'] [0, 1, 2]

import threading
barrier = threading.Barrier(2, timeout=0.1)
try:
    barrier.wait()  # второй участник так и не пришёл
except threading.BrokenBarrierError:
    print('broken:', barrier.broken)  # → broken: True

See also

Common errors

Open in the full glossarydocs.python.orgReport an inaccuracy