Ending a looping thread using Events in Python 3.5

Question

I am having trouble with stopping a looping thread. I use an event to stop the loop but the output is different than I expected. I wrote the following code

import threading 
from time import sleep

class foo(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)
        self.stop_event = threading.Event()

    def run(self):
        while not self.stop_event.is_set():
            print('1')
            sleep(1)
            print('2')
            sleep(1)
        print('done')

class bar(threading.Thread):
    def __init__(self):
       threading.Thread.__init__(self)
       self.run_thread()


    def run_thread(self):
       th1 = foo()
       th1.start()
       sleep(4)
       th1.stop_event.set()

prgm = bar()
prgm.run_thread()

Gives me the output:

1
2
1
2
1
done
2
1
2
done

I expected the word 'done' to be printed onces when the while loop ends. But for some reason 'done' is printed twice. Am I using Events wrong or is the thread started multiple times?

I added print(''hallo') before the while (inside the thread). The output prints "hallo" twice. So for some reason the thread is started twice. — Jan van Tuijn
– Jan van Tuijn, Commented Mar 23, 2017 at 20:12

ForceBru · Accepted Answer · 2017-03-23 20:14:13Z

1

You already do self.run_thread() in the constructor of bar (i.e. bar.__init__):

class bar(...):
    def __init__(self):
       threading.Thread.__init__(self)

       # !!!!
       self.run_thread()

So, when you create an instance of bar:

stuff = bar()

You're executing bar.run_thread as well. Your current code executes this function twice:

prgm = bar() # executed here
prgm.run_thread() # and here

As a result, you get some unexpected behavior.

answered Mar 23, 2017 at 20:14

ForceBru

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1 Comment

Jan van Tuijn Over a year ago

Oops. Thanks for the answer

stovfl · Accepted Answer · 2017-03-23 20:08:35Z

0

Missing two points:

Add if __name__ == '__main__': before prgm = bar()
Add sleep(0.1)A before each run...

answered Mar 23, 2017 at 20:08

stovfl

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1 Comment

Jan van Tuijn Over a year ago

How would adding "if name == 'main':" improve the code? Other than improving the overall 'design' of the code.

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