CODEWITHSUNDEEP
pythonpython-2.7timerwhile-loopexit
Arbok
Arbok

Reputation: 63

Using a timer in a while true loop in Python

I have a problem in managing a infinite while loop in Python, in which I would insert a timer as a sort of "watchdog". I try to explain better: the script has to listen on a serial channel and wait for messages coming from sensors connected on the other side of the channel. I do this with a while True loop because the script must catch all the signals that are passing. However, even if it's probably unnecessary, i would like to insert a timer which is always reset every loop. So, if (for example) the loop get stuck for some reason, the timer will end and will exit the program. I thought i could do this in this way:

def periodicUpdate():
    exitTimer = threading.Timer(60.0, sys.exit())
    while True:
        exitTimer.start()
        readData()
        exitTimer.cancel()

Now, the problem is that when i start the script it immediatly exit. It seems that it reads sys.exit() before all the rest, and it doesn't respect the construct of the timer, but simply when periodicUpdate is called it exits. Why this happen?! I tried changing the syntax, putting sys.exit in an other function and call that function, i tried other solutions but always it behave in two ways: it exits or it behave as the timer doesn't exists. Can someone help me? Thanks very much

Upvotes: 0

Views: 8729

Answers (8)

Fabrizio
Fabrizio

Reputation: 457

It seems to me that none of the solutions proposed here actually works. This is an example of code. Would be great to know how to fix it. Also, please try it before answering.

import time, threading, sys

def _exit():
    print 'exiting'
    sys.exit()

def periodicUpdate():
    exitTimer = threading.Timer(2.0, _exit)
    while True:
        exitTimer.start()
        print 'go'
        time.sleep(5)
        exitTimer.cancel()


periodicUpdate()

The actual output is:

$python test.py
go
exiting
RuntimeError: threads can only be started once

Upvotes: 1

kirelagin
kirelagin

Reputation: 13626

It seems that it reads sys.exit() before all the rest

Sure it does. The second argument of threadint.Timer is a function that will be called. What you do is you actually call sys.exit and supply its return value (which is undefined, by the way, since the call terminates the program) as a function to be called. That's an error, since sys.exit's return value is not a function, but that doesn't matter, as the program is terminated anyway.

What you want to do is

exitTimer = threading.Timer(60.0, sys.exit) # note: no brackets

But that actually will not work, as sys.exit in the thread will terminate only that thread. In short, you can't use a watchdog to terminate a stuck while-loop if this loops is in your main thread. In your particular situation you should investigate the function you are using to read data, most likely it lets you set a read timeout.

Upvotes: 2

falsetru
falsetru

Reputation: 369424

threading.Timer(60.0, os._exit, [0])

Upvotes: 0

Samy Arous
Samy Arous

Reputation: 6814

I think your approach is not the best one, instead of running a timer that exits the application, why not simply timeout the function that reads the data:

Timeout on a function call

Upvotes: 2

Nasgar
Nasgar

Reputation: 951

Remove the () from sys.exit in timer call

def _exit()
    sys.exit()

def periodicUpdate():
    exitTimer = threading.Timer(60.0, _exit)
    while True:
        exitTimer.start()
        readData()
        exitTimer.cancel()

In you put the () the interpreter call this function if not the interpreter get a reference for this function.

Have fun :)

Edit: Use of _exit

Upvotes: 0

gukoff
gukoff

Reputation: 2250

def periodicUpdate():
    exitTimer = sys.exit(60.0, sys.exit())
    while sys.exit():
        sys.exit()
        readData()
        sys.exit()

if __name__ == "__main__":
    sys.exit()

Don't forget to import sys.

Upvotes: -2

Armali
Armali

Reputation: 19395

exitTimer = threading.Timer(60.0, sys.exit)

Upvotes: 0

bwind
bwind

Reputation: 715

Remove the parentheses. If you use sys.exit(), it will fire straight away. When you use sys.exit, it will pass the function as Timer's second argument.

Upvotes: 2

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