码迷,mamicode.com
首页 > 编程语言 > 详细

Python线程同步

时间:2018-10-03 20:46:05      阅读:148      评论:0      收藏:0      [点我收藏+]

标签:semaphore   href   sleep   elf   ase   format   notify   释放   ref   

线程执行

join与setDaemon

子线程在主线程运行结束后,会继续执行完,如果给子线程设置为守护线程(setDaemon=True),主线程运行结束子线程即结束;

如果join()线程,那么主线程会等待子线程执行完再执行。

 

 1 import threading
 2 import time
 3 
 4 
 5 def get_thread_a():
 6     print("get thread A started")
 7     time.sleep(3)
 8     print("get thread A end")
 9 
10 
11 def get_thread_b():
12     print("get thread B started")
13     time.sleep(5)
14     print("get thread B end")
15 
16 
17 if  __name__ == "__main__":
18     thread_a = threading.Thread(target=get_thread_a)
19     thread_b = threading.Thread(target=get_thread_b)
20     start_time = time.time()
21     thread_b.setDaemon(True)
22     thread_a.start()
23     thread_b.start()
24     thread_a.join()
25     
26     end_time = time.time()
27     print("execution time: {}".format(end_time - start_time))

 

thread_a是join,首先子线程thread_a执行,thread_b是守护线程,当主线程执行完后,thread_b不会再执行
执行结果如下:
get thread A started
get thread B started
get thread A end
execution time: 3.003199815750122

线程同步

当线程间共享全局变量,多个线程对该变量执行不同的操作时,该变量最终的结果可能是不确定的(每次线程执行后的结果不同),如:对count变量执行加减操作 ,count的值是不确定的,要想count的值是一个确定的需对线程执行的代码段加锁。

python对线程加锁主要有Lock和Rlock模块

Lock:

from threading import Lock

lock = Lock()
lock.acquire()
lock.release()

Lock有acquire()和release()方法,这两个方法必须是成对出现的,acquire()后面必须release()后才能再acquire(),否则会造成死锁

Rlock:

鉴于Lock可能会造成死锁的情况,RLock(可重入锁)对Lock进行了改进,RLock可以在同一个线程里面连续调用多次acquire(),但必须再执行相同次数的release()

from threading import RLock

lock = RLock()
lock.acquire()
lock.acquire()
lock.release()
lock.release()

condition(条件变量),线程在执行时,当满足了特定的条件后,才可以访问相关的数据

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内部有一把锁,默认是RLock,在调用wait()和notify()之前必须先调用acquire()获取这个锁,才能继续执行;当wait()和notify()执行完后,需调用release()释放这个锁,在执行with condition时,会先执行acquire(),with结束时,执行了release();所以condition有两层锁,最底层锁在调用wait()时会释放,同时会加一把锁到等待队列,等待notify()唤醒释放锁

wait() :允许等待某个条件变量的通知,notify()可唤醒

notify(): 唤醒等待队列wait()

执行结果:

B : Hi A, Lets start the conversation
A : Hello B,thats ok
B : How are you
A : Im fine,and you?
B : Im fine too
A : Nice to meet you
B : Nice to meet you,too
A : Thats all for today
B : Oh,goodbye

 

 

Semaphore(信号量)

 用于控制线程的并发数,如爬虫中请求次数过于频繁会被禁止ip,每次控制爬取网页的线程数量可在一定程度上防止ip被禁;文件读写中,控制写线程每次只有一个,读线程可多个。

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秒并发两个线程执行的情况,当调用一次semaphore.acquire()时,Semaphore的数量就减1,直至Semaphore数量为0时被锁上,当release()后Semaphore数量加1。Semaphore在本质上是调用的Condition,semaphore.acquire()在Semaphore的值为0的条件下会调用Condition.wait(), 否则将值减1,semaphore.release()会将Semaphore的值加1,并调用Condition.notify()

Semaphore源码

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()

 

Python线程同步

标签:semaphore   href   sleep   elf   ase   format   notify   释放   ref   

原文地址:https://www.cnblogs.com/FG123/p/9704158.html

(0)
(0)
   
举报
评论 一句话评论(0
登录后才能评论!
© 2014 mamicode.com 版权所有  联系我们:gaon5@hotmail.com
迷上了代码!