Question

from threading import Thread
import time
print 'start of script'

class MyThread(Thread):
    def run(self):
        for i in xrange(10):
            print 'thread->', '['+self.name+']', '('+str(i)+')'
            time.sleep(2)


for i in range(3):
    my_thread = MyThread()
    my_thread.name = i
    my_thread.start()

print 'end of script'

>>> ================================ RESTART ================================
>>> 
start of script
thread->thread->thread->end of script   
[0][1][2]
>>>    (0)(0)(0)


thread->thread->thread->   [0][2][1]   (1)(1)(1)


thread->thread-> thread-> [0] [2] [1] (2) (2)
(2)

thread-> thread->thread->[0]   [2][1](3)  
(3)(3)

thread-> thread->thread->[0]   [2][1](4)  
(4)(4)

thread-> thread->[0]thread->   [2](5)[1] 
 (5)(5)

thread-> [0]thread->thread->   (6)[2][1]
  (6)(6)

thread-> thread->[0]thread->   [2](7)[1] 
 (7)(7)

thread-> thread->[0] thread-> [2] (8) [1]
(8) 
(8)
thread->thread-> thread-> [0] [2] [1] (9) (9)
(9)


>>> 

As you can see I’m printing ‘start of script’ first, then executing multiple threads, and then printing ‘end of script’

Yet, ‘end of script’ gets printed right after I execute the first thread, instead of after all the threads have completed. How can I prevent this?

The jumbled nature of the output is expected and in fact desired as these threads are supposed to be executing simultaneously…

I’m on Windows 7 python 2.7 btw…

Answer 1

You want to add .join(), since by default there’s no reason for your main program to block until the threads are finished:

my_threads = []
for i in range(3):
    my_thread = MyThread()
    my_thread.name = i
    my_thread.start()
    my_threads.append(my_thread)

for t in my_threads:
    t.join()

print 'end of script'