Python多進程同步Lock、Semaphore、Event實例

2019-11-25 18:05:01

字體：大中小

來源：轉載

供稿：網友

同步的方法基本與多線程相同。

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。

復制代碼代碼如下:

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。

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。

復制代碼代碼如下:

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()

上面的實例中使用semaphore限制了最多有2個進程同時執行。

3）Event

Event用來實現進程間同步通信。

復制代碼代碼如下:

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