算法学习

1. threading.Semaphore(value=1) 线程信号量，可以用来控制线程线程的阻塞和释放

sm.acquire()  获取一个信号量，信号量-1，不够-1，则线程阻塞

sm.release()  释放一个信号量，信号量+1

示例如下，控制三个线程的执行顺序：

# -*- coding: utf-8 -*-
import threading
import time

sm0 = threading.Semaphore()
sm1 = threading.Semaphore(0)
sm2 = threading.Semaphore(0)

a = 0


def print1():
    global a
    while True:
        sm1.acquire()
        if a % 2 == 1:
            print(a, "----", a, threading.currentThread().name)
            a = a + 1
            time.sleep(1)
        sm0.release()


def print2():
    global a
    while True:
        sm2.acquire()
        if a % 2 == 0:
            print(a, "----", a, threading.currentThread().name)
            a = a + 1
            time.sleep(1)
        sm0.release()


def print0():
    global a
    while True:
        sm0.acquire()
        print(0, "----", a)
        time.sleep(1)
        if a % 2 == 1:
            sm1.release()
        else:
            sm2.release()


if __name__ == '__main__':
    t1 = threading.Thread(target=print1)
    t0 = threading.Thread(target=print0)
    t2 = threading.Thread(target=print2)
    t0.start()
    t1.start()
    t2.start()