python多线程

1 .线程创建

使用Thread创建

import threading
import time


def a_thread(a):
    print("a_thread started")
    time.sleep(2)
    print("a_thread end")

def b_thread(b):
    print("b_thread started")
    time.sleep(4)
    print("b_thread end")

if __name__ == '__main__':
    thread1 = threading.Thread(target=a_thread, args=("",))
    thread2 = threading.Thread(target=b_thread, args=("",))
    
    start_time = time.time()
    thread1.start()
    thread2.start()

    print("last time: {}".format(time.time() - start_time))

运行结果

a_thread started
b_thread started
last time: 0.00026798248291015625
a_thread end
b_thread end

需要注意last time很短，是因为时间打印在主线程中，整个例子中a，b，主三个线程并行。所以主线程并没有等待a，b两个线程运行完毕。

继承Thread方式

# 对于io操作 多线程和多进程性能差别不大
# 1. 通过thread实例化
import threading
import time


def a_thread(a):
    print("a_thread started")
    time.sleep(2)
    print("a_thread end")

def b_thread(b):
    print("b_thread started")
    time.sleep(4)
    print("b_thread end")


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

    def run(self):
        print("{} started".format(self.name))
        time.sleep(2)
        print("{} end".format(self.name))

if __name__ == '__main__':
    thread1 = A_thread("a_thread")
    thread2 = threading.Thread(target=b_thread, args=("",))

    # 设置守护线程
    # thread2.setDaemon(True)


    start_time = time.time()
    thread1.start()
    thread2.start()

    # thread1.join()
    # thread2.join()

    print("last time: {}".format(time.time() - start_time))

运行结果

a_thread started
b_thread started
last time: 0.0005140304565429688
a_thread end
b_thread end

2.守护线程

上面的例子可以看出main函数主线程走完后，a，b两个线程还在执行
现实场景中可能需要主线程走完，某些子线程就自动关停，如jvm中的gc，当java程序关闭后，gc直接关闭，我们把这种线程称为守护线程（Daemon）

python中实现守护线程比较简单，直接调用setDaemon方法，如下

# 对于io操作 多线程和多进程性能差别不大
# 1. 通过thread实例化
import threading
import time


def a_thread(a):
    print("a_thread started")
    time.sleep(2)
    print("a_thread end")

def b_thread(b):
    print("b_thread started")
    time.sleep(4)
    print("b_thread end")

if __name__ == '__main__':
    thread1 = threading.Thread(target=a_thread, args=("",))
    thread2 = threading.Thread(target=b_thread, args=("",))

    # 设置守护线程
    thread2.setDaemon(True)


    start_time = time.time()
    thread1.start()
    thread2.start()

    print("last time: {}".format(time.time() - start_time))

运行结果

a_thread started
b_thread started
last time: 0.0003409385681152344
a_thread end

可以看到b线程设置为守护线程后，当主线程运行结束，它也随之结束，没有继续执行。

3. 线程加入到当前线程（join）

同样1中的例子，有时需要在一个线程中等待另外的线程执行后才继续执行，如 例子中主线程等待a，b执行完毕才继续执行，则需要使用join方法

import threading
import time


def a_thread(a):
    print("a_thread started")
    time.sleep(2)
    print("a_thread end")

def b_thread(b):
    print("b_thread started")
    time.sleep(4)
    print("b_thread end")

if __name__ == '__main__':
    thread1 = threading.Thread(target=a_thread, args=("",))
    thread2 = threading.Thread(target=b_thread, args=("",))
    
    start_time = time.time()
    thread1.start()
    thread2.start()

    thread1.join()
    thread2.join()

    print("last time: {}".format(time.time() - start_time))

运行结果

a_thread started
b_thread started
a_thread end
b_thread end
last time: 4.008444786071777