同步的方法基本與多線程相同。

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1) Lock

當多個進程需要訪問共享資源的時候，Lock可以用來避免訪問的沖突。

import?multiprocessing
import?sys

def?worker_with(lock,?f):
????with?lock:
????????fs?=?open(f,"a+")
????????fs.write('Lock?acquired?via?with\n')
????????fs.close()
????????
def?worker_no_with(lock,?f):
????lock.acquire()
????try:
????????fs?=?open(f,"a+")
????????fs.write('Lock?acquired?directly\n')
????????fs.close()
????finally:
????????lock.release()

if?__name__?==?"__main__":

????f?=?"file.txt"
??
????lock?=?multiprocessing.Lock()
????w?=?multiprocessing.Process(target=worker_with,?args=(lock,?f))
????nw?=?multiprocessing.Process(target=worker_no_with,?args=(lock,?f))

????w.start()
????nw.start()

????w.join()
????nw.join()

? ???

在上面的例子中，如果兩個進程沒有使用lock來同步,則他們對同一個文件的寫操作可能會出現混亂。

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2）Semaphore

Semaphore用來控制對共享資源的訪問數量，例如池的最大連接數。

import?multiprocessing
import?time?

def?worker(s,i):
????s.acquire()
????print(multiprocessing.current_process().name?+?"?acquire")
????time.sleep(i)
????print(multiprocessing.current_process().name?+?"?release")
????s.release()

if?__name__?==?"__main__":
??
????s?=?multiprocessing.Semaphore(2)
????for?i?in?range(5):
????????p?=?multiprocessing.Process(target=worker,?args=(s,i*2))
? ? ? ? p.start()

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上面的實例中使用semaphore限制了最多有2個進程同時執行。

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3）Event

Event用來實現進程間同步通信。

import?multiprocessing
import?time

def?wait_for_event(e):
????"""Wait?for?the?event?to?be?set?before?doing?anything"""
????print?('wait_for_event:?starting')
????e.wait()
????print?('wait_for_event:?e.is_set()->'?+?str(e.is_set()))

def?wait_for_event_timeout(e,?t):
????"""Wait?t?seconds?and?then?timeout"""
????print?('wait_for_event_timeout:?starting')
????e.wait(t)
????print?('wait_for_event_timeout:?e.is_set()->'?+?str(e.is_set()))


if?__name__?==?'__main__':
????e?=?multiprocessing.Event()
????w1?=?multiprocessing.Process(name='block',?
?????????????????????????????????target=wait_for_event,
?????????????????????????????????args=(e,))
????w1.start()

????w2?=?multiprocessing.Process(name='non-block',?
?????????????????????????????????target=wait_for_event_timeout,?
?????????????????????????????????args=(e,?2))
????w2.start()

????time.sleep(3)
????e.set()
????print?('main:?event?is?set')
????
#the?output?is:
#wait_for_event_timeout:?starting
#wait_for_event:?starting
#wait_for_event_timeout:?e.is_set()->False
#main:?event?is?set
#wait_for_event:?e.is_set()->True

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參考：http://www.doughellmann.com/PyMOTW/multiprocessing/communication.html?

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完！

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