start() vs. run()

      I'm reading a Blog. But a rather familiar question occurred to me, "What's the difference between run() method and start() method in thread?". Let's test it with the following code:

#FileName: threadingDemo.py
#Python Version: 2.7
#Authour: lxw
#Date: 2015-1-28
#Usage: Demos for threading.

import time
import random

from threading import Thread

class MyThread(Thread):
    '''
    Definition of my own Thread class.
    '''

    def __init__(self, name):
        Thread.__init__(self)    # essential
        self.name = name
        
    def run(self):
        amount = random.randint(3, 10)
        time.sleep(amount)
        outStr = "Thread {0} sleeps {1} seconds. Thread {0} has finished.".format(self.name, amount)
        print(outStr)

def main():
    threads = []
    #Create 5 objects of MyThread.
    for index in range(5):
        th = MyThread(str(index))
        threads.append(th)

    print(threads)
    #Run the objects of MyThread.
    for index in range(5):
        threads[index].run()

if __name__ == "__main__":
    main()
else:
    print("Imported as a module")

      Note the code on Line 38. The output of the program is(Thread 0-Thread 4 never disorder.):

[<MyThread(0, initial)>, <MyThread(1, initial)>, <MyThread(2, initial)>, <MyThread(3, initial)>, <MyThread(4, initial)>]
Thread 0 sleeps 9 seconds. Thread 0 has finished.
Thread 1 sleeps 6 seconds. Thread 1 has finished.
Thread 2 sleeps 8 seconds. Thread 2 has finished.
Thread 3 sleeps 7 seconds. Thread 3 has finished.
Thread 4 sleeps 9 seconds. Thread 4 has finished.

Let's make some modifications around Line 38.

    for index in range(5):
        threads[index].start()

      The new output of the program becomes(it differs every time you run the program according to the time each thread sleeped):

[<MyThread(0, initial)>, <MyThread(1, initial)>, <MyThread(2, initial)>, <MyThread(3, initial)>, <MyThread(4, initial)>]
Thread 4 sleeps 4 seconds. Thread 4 has finished.
Thread 0 sleeps 8 seconds. Thread 0 has finished.
Thread 3 sleeps 8 seconds. Thread 3 has finished.
Thread 2 sleeps 9 seconds. Thread 2 has finished.
Thread 1 sleeps 10 seconds. Thread 1 has finished.

      Now, do you understand what's the difference between run() method and start() method in thread?

start()：

      用start()来启动线程，真正实现了多线程运行，这时无需等待run方法体代码执行完毕而直接继续执行下面的代码。

通过调用Thread类的start()来启动一个线程，这时此线程处于就绪（可运行）状态，并没有运行，一旦得到cpu

时间片，就开始执行run()，这里run()称为线程体，它包含了要执行的这个线程的内容，run方法运行结束，此

线程随即终止。

run()：

      run()只是Thread类的一个普通方法，如果直接调用run()，程序中依然只有主线程这一个线程，其程序执行路径

还是只有一条，还是要顺序执行，还是要等待run方法体执行完毕后才可继续执行下面的代码，这样就没有达到多线

程的目的。

总结：

      调用start()方可启动线程，而run()只是thread的一个普通方法，还是在主线程里执行。把需要并行处理的代码放在

run()中，start()启动线程将自动调用run().

Reference:

python并发:对线程的介绍

start()和run()方法的区别