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Python線程同步的實(shí)現(xiàn)代碼

更新時(shí)間：2018年10月03日 09:05:38 作者：Harvard_Fly

本文介紹了threading 模塊提供的線程同步原語包括：Lock、RLock、Condition、Event、Semaphore等對(duì)象。對(duì)大家的學(xué)習(xí)具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友可以參考下

本文介紹Python中的線程同步對(duì)象，主要涉及 thread 和 threading 模塊。

threading 模塊提供的線程同步原語包括：Lock、RLock、Condition、Event、Semaphore等對(duì)象。

線程執(zhí)行

join與setDaemon

子線程在主線程運(yùn)行結(jié)束后，會(huì)繼續(xù)執(zhí)行完，如果給子線程設(shè)置為守護(hù)線程(setDaemon=True)，主線程運(yùn)行結(jié)束子線程即結(jié)束；

如果join()線程，那么主線程會(huì)等待子線程執(zhí)行完再執(zhí)行。

import threading
import time


def get_thread_a():
 print("get thread A started")
 time.sleep(3)
 print("get thread A end")


def get_thread_b():
 print("get thread B started")
 time.sleep(5)
 print("get thread B end")


if __name__ == "__main__":
 thread_a = threading.Thread(target=get_thread_a)
 thread_b = threading.Thread(target=get_thread_b)
 start_time = time.time()
 thread_b.setDaemon(True)
 thread_a.start()
 thread_b.start()
 thread_a.join()
 
 end_time = time.time()
 print("execution time: {}".format(end_time - start_time))

thread_a是join，首先子線程thread_a執(zhí)行，thread_b是守護(hù)線程，當(dāng)主線程執(zhí)行完后，thread_b不會(huì)再執(zhí)行執(zhí)行結(jié)果如下：

get thread A started
get thread B started
get thread A end
execution time: 3.003199815750122

線程同步

當(dāng)線程間共享全局變量，多個(gè)線程對(duì)該變量執(zhí)行不同的操作時(shí)，該變量最終的結(jié)果可能是不確定的(每次線程執(zhí)行后的結(jié)果不同)，如：對(duì)count變量執(zhí)行加減操作，count的值是不確定的，要想count的值是一個(gè)確定的需對(duì)線程執(zhí)行的代碼段加鎖。

python對(duì)線程加鎖主要有Lock和Rlock模塊

Lock：

from threading import Lock
lock = Lock()
lock.acquire()
lock.release()

Lock有acquire()和release()方法，這兩個(gè)方法必須是成對(duì)出現(xiàn)的，acquire()后面必須release()后才能再acquire()，否則會(huì)造成死鎖

Rlock：

鑒于Lock可能會(huì)造成死鎖的情況，RLock(可重入鎖)對(duì)Lock進(jìn)行了改進(jìn)，RLock可以在同一個(gè)線程里面連續(xù)調(diào)用多次acquire()，但必須再執(zhí)行相同次數(shù)的release()

from threading import RLock
lock = RLock()
lock.acquire()
lock.acquire()
lock.release()
lock.release()

condition(條件變量)，線程在執(zhí)行時(shí)，當(dāng)滿足了特定的條件后，才可以訪問相關(guān)的數(shù)據(jù)

import threading

def get_thread_a(condition):
 with condition:
  condition.wait()
  print("A : Hello B,that's ok")
  condition.notify()
  condition.wait()
  print("A : I'm fine,and you?")
  condition.notify()
  condition.wait()
  print("A : Nice to meet you")
  condition.notify()
  condition.wait()
  print("A : That's all for today")
  condition.notify()

def get_thread_b(condition):
 with condition:
  print("B : Hi A, Let's start the conversation")
  condition.notify()
  condition.wait()
  print("B : How are you")
  condition.notify()
  condition.wait()
  print("B : I'm fine too")
  condition.notify()
  condition.wait()
  print("B : Nice to meet you,too")
  condition.notify()
  condition.wait()
  print("B : Oh,goodbye")

if __name__ == "__main__":
 condition = threading.Condition()
 thread_a = threading.Thread(target=get_thread_a, args=(condition,))
 thread_b = threading.Thread(target=get_thread_b, args=(condition,))
 thread_a.start()
 thread_b.start()

Condition內(nèi)部有一把鎖，默認(rèn)是RLock，在調(diào)用wait()和notify()之前必須先調(diào)用acquire()獲取這個(gè)鎖，才能繼續(xù)執(zhí)行；當(dāng)wait()和notify()執(zhí)行完后，需調(diào)用release()釋放這個(gè)鎖，在執(zhí)行with condition時(shí)，會(huì)先執(zhí)行acquire()，with結(jié)束時(shí)，執(zhí)行了release()；所以condition有兩層鎖，最底層鎖在調(diào)用wait()時(shí)會(huì)釋放，同時(shí)會(huì)加一把鎖到等待隊(duì)列，等待notify()喚醒釋放鎖

wait() ：允許等待某個(gè)條件變量的通知，notify()可喚醒

notify()：喚醒等待隊(duì)列wait()

執(zhí)行結(jié)果：

B : Hi A, Let's start the conversation
A : Hello B,that's ok
B : How are you
A : I'm fine,and you?
B : I'm fine too
A : Nice to meet you
B : Nice to meet you,too
A : That's all for today
B : Oh,goodbye

Semaphore(信號(hào)量)

用于控制線程的并發(fā)數(shù)，如爬蟲中請求次數(shù)過于頻繁會(huì)被禁止ip，每次控制爬取網(wǎng)頁的線程數(shù)量可在一定程度上防止ip被禁；文件讀寫中，控制寫線程每次只有一個(gè)，讀線程可多個(gè)。

import time
import threading


def get_thread_a(semaphore,i):
 time.sleep(1)
 print("get thread : {}".format(i))
 semaphore.release()


def get_thread_b(semaphore):
 for i in range(10):
  semaphore.acquire()
  thread_a = threading.Thread(target=get_thread_a, args=(semaphore,i))
  thread_a.start()


if __name__ == "__main__":
 semaphore = threading.Semaphore(2)
 thread_b = threading.Thread(target=get_thread_b, args=(semaphore,))
 thread_b.start()

上述示例了每隔1秒并發(fā)兩個(gè)線程執(zhí)行的情況，當(dāng)調(diào)用一次semaphore.acquire()時(shí)，Semaphore的數(shù)量就減1，直至Semaphore數(shù)量為0時(shí)被鎖上，當(dāng)release()后Semaphore數(shù)量加1。Semaphore在本質(zhì)上是調(diào)用的Condition，semaphore.acquire()在Semaphore的值為0的條件下會(huì)調(diào)用Condition.wait(), 否則將值減1，semaphore.release()會(huì)將Semaphore的值加1，并調(diào)用Condition.notify()

Semaphore源碼

def acquire(self, blocking=True, timeout=None):
  if not blocking and timeout is not None:
   raise ValueError("can't specify timeout for non-blocking acquire")
  rc = False
  endtime = None
  with self._cond:
   while self._value == 0:
    if not blocking:
     break
    if timeout is not None:
     if endtime is None:
      endtime = _time() + timeout
     else:
      timeout = endtime - _time()
      if timeout <= 0:
       break
    self._cond.wait(timeout)
   else:
    self._value -= 1
    rc = True
  return rc

def release(self):
  with self._cond:
   self._value += 1
   self._cond.notify()

以上就是這篇文章的全部內(nèi)容了，希望本文的內(nèi)容對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，如果有疑問大家可以留言交流，謝謝大家對(duì)腳本之家的支持。

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