Breaking a while loop from other method inside the same class in Python

Question

I am wondering if I can run a while loop inside a method of a Python class that can be stopped from another method.

For example, something like this:

from time import sleep

class example():

    global recording

    def __init__(self):
        pass

    def start(self):
        global recording
        recording = True
        while recording:
            print(1)

    def stop(self):
        global recording
        recording = False
        print("SLEEEPINGGGGGGGGGGG")

a = example()
a.start()
sleep(0.5)
a.stop()

But, it does not work, the loop does not stop.

EDIT Since I do not want to create a Thread outside the class, I just tried this, but it doesn't work either.

from time import sleep
import threading

class example():

    def __init__(self):
        self.is_running = False

    def start(self):
        self.is_running = True
        self.loop_thread = threading.Thread(target=self.main_loop)

    def main_loop(self):
        while self.is_running:
            sleep(0.5)
            print(1)

    def stop(self):
        self.is_running = False
        print("SLEEEPINGGGGGGGGGGG")

a = example()
a.start()
sleep(3)
a.stop()

you would need some threading for that. and use a member data instead of a global! — Jean-François Fabre
– Jean-François Fabre ♦, Commented Jan 24, 2018 at 13:21

Jean-François Fabre · Accepted Answer · 2018-01-24 13:49:13Z

7

a.start() is an infinite loop. Since nothing runs at the same time, it just runs and never reaches the next statements.

You need to create a thread, like this

import time,threading

class example():

    def __init__(self):
        self.__recording = False

    def start(self):
        self.__recording = True
        while self.__recording:
            time.sleep(1)
            print(1)

    def stop(self):
        self.__recording = False

a = example()
t = threading.Thread(target=a.start)
t.start()
time.sleep(5)
a.stop()
t.join()

note that I have used a member variable instead of a global. When the start method sees that the variable is True, it quits the loop. job done.

That works because I'm using sleep(). If you're running a pure python CPU intensive job that won't work because of python GIL

As suggested in comments, you can go one step further and inherit from threading.Thread method instead:

import time,threading

class example(threading.Thread):

    def __init__(self):
        threading.Thread.__init__(self,target=self.run)

        self.__recording = False

    def run(self):
        self.__recording = True
        while self.__recording:
            time.sleep(1)
            print(1)

    def join(self):
        self.__recording = False
        threading.Thread.join(self)

a = example()
a.start()
time.sleep(5)
a.join()

Note that stop method is now replaced by join which signals the recording thread to stop then calls the parent join method. So when you join you stop the loop automatically first.

edited Jan 24, 2018 at 13:49

answered Jan 24, 2018 at 13:34

Jean-François Fabre♦

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7 Comments

Sven Marnach Over a year ago

It's also common to subclass Thread and override the run() method to achieve this.

Jean-François Fabre Over a year ago

@SvenMarnach that's even better, yes.

Víctor Martínez Over a year ago

@SvenMarnach Could you please provide me a simple example?

Víctor Martínez Over a year ago

@Jean-FrançoisFabre Thank you! But I'm facing another problem. I just want to encapsulate all in the class, in order to just need to call start() and stop() without creating Threads outside it. I tried to do it using your code, but I could not figure it. I have updated the comment, can you revise it?

Jean-François Fabre Over a year ago

I've updated my answer following Sven comments, I think that's the same thing you're asking for (you can rename join to stop BTW)

|

jusx · Accepted Answer · 2018-01-24 14:45:20Z

1

You have to start your thread after initiating it:

from time import sleep
import threading

class example():

    def __init__(self):
        self.is_running = False

    def start(self):
        self.is_running = True
        self.loop_thread = threading.Thread(target=self.main_loop)
        self.loop_thread.start() # <--- you forgot to start your thread

    def main_loop(self):
        while self.is_running:
            sleep(0.5)
            print(1)

    def stop(self):
        self.is_running = False
        print("SLEEEPINGGGGGGGGGGG")

a = example()
a.start()
sleep(3)
a.stop()

answered Jan 24, 2018 at 14:45

jusx

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