摘要：信號量使用前執行后發現不斷在輸出使用后通過執行上面的代碼，我們發現一次只能輸出三個數字，控制訪問并發量事件等待后，才能往下執行定義五個線程重置，使得起到阻塞作用啟動所有線程等待喚醒所有線程等待喚醒所有線程

本文講python中的四種鎖
Lock互斥鎖
使用前

num = 0


def a():
    global num
    for _ in range(10000000):
        num += 1


def b():
    global num
    for _ in range(10000000):
        num += 1


if __name__ == "__main__":
    t1=Thread(target=a)
    t1.start()
    t2=Thread(target=b)
    t2.start()
    t1.join()
    t2.join()
    print(num)    #基本永遠會小于20000000

使用后

num = 0


def a(lock):
    global num
    for _ in range(1000000):
        with lock:
            num += 1


def b(lock):
    global num
    for _ in range(1000000):
        with lock:
            num += 1


if __name__ == "__main__":
    lock = threading.Lock()
    t1=Thread(target=a, args=(lock,))
    t1.start()
    t2=Thread(target=b, args=(lock,))
    t2.start()
    t1.join()
    t2.join()
    print(num)    #永遠會輸出20000000

RLock重用鎖

#在之前的代碼中永遠不可能出現鎖在沒釋放之前重新獲得鎖，但rlock可以做到，但只能發生在一個線程中，如：
num = 0


def a(lock):
    with lock:
        print("我是A")
        b(lock)


def b(lock):
    with lock:
        print("我是b")

if __name__ == "__main__":
    lock = threading.Lock()
    t1 = Thread(target=a, args=(lock,))
    t1.start()    #會發生死鎖，因為在第一次還沒釋放鎖后，b就準備上鎖，并阻止a釋放鎖

使用后

if __name__ == "__main__":
    lock = threading.RLock()    #只需要改變鎖為RLock程序馬上恢復
    t1 = Thread(target=a, args=(lock,))
    t1.start()

Condition同步鎖

#這個程序我們模擬甲乙對話
Jlist = ["在嗎", "干啥呢", "去玩兒不", "好吧"]
Ylist = ["在呀", "玩兒手機", "不去"]


def J(list):
    for i in list:
        print(i)
        time.sleep(0.1)


def Y(list):
    for i in list:
        print(i)
        time.sleep(0.1)


if __name__ == "__main__":
    t1 = Thread(target=J, args=(Jlist,))
    t1.start()
    t1.join()
    t2 = Thread(target=Y, args=(Ylist,))
    t2.start()
    t2.join()    #上面的程序輸出后發現效果就是咱們想要的，但是我們每次輸出后都要等待0.1秒，也無法正好確定可以拿到時間片的最短時間值，并且不能保證每次正好都是另一個線程執行。因此，我們用以下方式，完美解決這些問題。

使用后

Jlist = ["在嗎", "干啥呢", "去玩兒不", "好吧"]
Ylist = ["在呀", "玩兒手機", "不去","哦"]


def J(cond, list):
    for i in list:
        with cond:
            print(i)
            cond.notify()
            cond.wait()


def Y(cond, list):
    for i in list:
        with cond:
            cond.wait()
            print(i)
            cond.notify()


if __name__ == "__main__":
    cond = threading.Condition()
    t1 = Thread(target=J, args=(cond, Jlist))
    t2 = Thread(target=Y, args=(cond, Ylist))
    t2.start()
    t1.start()    #一定保證t1啟動在t2之后,因為notify發送的信號要被t2接受到，如果t1先啟動，會發生阻塞。

Seamplore信號量
使用前

class B(threading.Thread):
    def __init__(self, name):
        super().__init__()
        self.name = name

    def run(self):
        time.sleep(1)
        print(self.name)


class A(threading.Thread):
    def __init__(self):
        super().__init__()

    def run(self):
        for i in range(100):
            b = B(i)
            b.start()


if __name__ == "__main__":
    a = A()
    a.start()    #執行后發現不斷在輸出

使用后

class B(threading.Thread):
    def __init__(self, name, sem):
        super().__init__()
        self.name = name
        self.sem = sem

    def run(self):
        time.sleep(1)
        print(self.name)
        sem.release()


class A(threading.Thread):
    def __init__(self, sem):
        super().__init__()
        self.sem = sem

    def run(self):
        for i in range(100):
            self.sem.acquire()
            b = B(i, self.sem)
            b.start()


if __name__ == "__main__":
    sem = threading.Semaphore(value=3)
    a = A(sem)
    a.start()    #通過執行上面的代碼，我們發現一次只能輸出三個數字，sem控制訪問并發量

Event事件

import time
import threading


class MyThread(threading.Thread):
    def __init__(self, name, event):
        super().__init__()
        self.name = name
        self.event = event

    def run(self):
        print("Thread: {} start at {}".format(self.name, time.ctime(time.time())))
        # 等待event.set()后，才能往下執行
        self.event.wait()
        print("Thread: {} finish at {}".format(self.name, time.ctime(time.time())))


event = threading.Event()

# 定義五個線程
threads = [MyThread(str(i), event) for i in range(1,5)]

# 重置event，使得event.wait()起到阻塞作用
event.clear()

# 啟動所有線程
[t.start() for t in threads]

print("等待5s...")
time.sleep(5)

print("喚醒所有線程...")
event.set()

# output:
"""
Thread: 1 start at Sun May 13 20:38:08 2018
Thread: 2 start at Sun May 13 20:38:08 2018
Thread: 3 start at Sun May 13 20:38:08 2018
Thread: 4 start at Sun May 13 20:38:08 2018

等待5s...

喚醒所有線程...
Thread: 1 finish at Sun May 13 20:38:13 2018
Thread: 4 finish at Sun May 13 20:38:13 2018
Thread: 2 finish at Sun May 13 20:38:13 2018
Thread: 3 finish at Sun May 13 20:38:13 2018
"""