使用连接的python中具有类和不具有类的多线程

Question

I am practicing in python multithreaded environment for my project. I have developed(probably copied :)) a sample program which will use thread join method so main thread will wait till other threads finish, using threading model.

import threading
import time


class MyThread (threading.Thread):
    def __init__(self, thread_id, name, counter):
        threading.Thread.__init__(self)
        self.threadID = thread_id
        self.name = name
        self.counter = counter

    def run(self):
        print "Starting " + self.name
        threadLock.acquire()
        print_time(self.name, self.counter, 3)
        # Free lock to release next thread
        threadLock.release()


def print_time(thread_name, delay, counter):
    while counter:
        time.sleep(delay)
        print "%s: %s" % (thread_name, time.ctime(time.time()))
        counter -= 1

threadLock = threading.Lock()

thread1 = MyThread(1, "Thread-1", 1)
thread2 = MyThread(2, "Thread-2", 2)

thread1.start()
thread2.start()

print 'waiting to finish the thread'

thread1.join()
thread2.join()

print "Exiting Main Thread"

output of the above :

Starting Thread-1
waiting to finish the threadStarting Thread-2

Thread-1: Tue Nov 18 16:31:04 2014
Thread-1: Tue Nov 18 16:31:05 2014
Thread-1: Tue Nov 18 16:31:06 2014
Thread-2: Tue Nov 18 16:31:08 2014
Thread-2: Tue Nov 18 16:31:10 2014
Thread-2: Tue Nov 18 16:31:12 2014
Exiting Main Thread

Process finished with exit code 0

When I comment these lines of join method

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

Then I get the output as

Starting Thread-1
waiting to finish the threadStarting Thread-2

Exiting Main Thread
Thread-1: Tue Nov 18 16:32:31 2014
Thread-1: Tue Nov 18 16:32:32 2014
Thread-1: Tue Nov 18 16:32:33 2014
Thread-2: Tue Nov 18 16:32:35 2014
Thread-2: Tue Nov 18 16:32:37 2014
Thread-2: Tue Nov 18 16:32:39 2014

Process finished with exit code 0

Which is expected

Now I have written one more code as

import time
import threading


def printer():
    for _ in range(5):
        print 'Hello'
        time.sleep(1)

thread1 = threading.Thread(target=printer())
thread2 = threading.Thread(target=printer())
thread3 = threading.Thread(target=printer())
thread1.start()
thread2.start()
thread3.start()
print 'Bye Bye'

For this I have not used join() method of threads, But I am getting output as

Hello
Hello
:
Hello
:
Hello
:
Hello
Bye Bye

Process finished with exit code 0

And with using join() also I am getting same output.

As per my understanding of threads and joins, when I haven't used the join() statement 'Bye Bye' should be printed in the middle somewhere and not at the last.

Is there any behavior which changes for classes and simple functions in python?

Answer 1

target should be a function, not the result of a function (unless that result is a function). eg. target=printer . Note the absence of brackets.

What you have effectively done is run printer three times. Then started three threads which have nothing to do. Then printed bye bye.