How to implement a watchdog timer in Python?

Question

I would like to implement a simple watchdog timer in Python with two use cases:

• Watchdog ensures that a function doesn't execute longer than x seconds
• Watchdog ensures that certain regularly executed function does indeed execute at least every y seconds

How do I do that?

Answer 1

Just publishing my own solution to this:

from threading import Timer

class Watchdog(Exception):
    def __init__(self, timeout, userHandler=None):  # timeout in seconds
        self.timeout = timeout
        self.handler = userHandler if userHandler is not None else self.defaultHandler
        self.timer = Timer(self.timeout, self.handler)
        self.timer.start()

    def reset(self):
        self.timer.cancel()
        self.timer = Timer(self.timeout, self.handler)
        self.timer.start()

    def stop(self):
        self.timer.cancel()

    def defaultHandler(self):
        raise self

Usage if you want to make sure function finishes in less than x seconds:

watchdog = Watchdog(x)
try:
  # do something that might take too long
except Watchdog:
  # handle watchdog error
watchdog.stop()

Usage if you regularly execute something and want to make sure it is executed at least every y seconds:

import sys

def myHandler():
  print "Whoa! Watchdog expired. Holy heavens!"
  sys.exit()

watchdog = Watchdog(y, myHandler)

def doSomethingRegularly():
  # make sure you do not return in here or call watchdog.reset() before returning
  watchdog.reset()

Answer 2

signal.alarm() sets a timeout for your program, and you can call it in your main loop, and set it to the greater of the two times you are prepared to tolerate:

import signal
while True:
    signal.alarm(10)
    infloop()

Answer 3

Here is a wdt I use in my app without class. It has no way to stop it:

from threading import Event, Thread

def wdt(time, callback):
    # a reset flag
    reset_e = Event()
    # a function to reset the wdt
    def reset(): reset_e.set()
    # the function to run in a differen thread
    def checker():
        # check if reset flag is set.
        # wait for specified time to give chance to reset.
        while reset_e.wait(time):
            # it was set in time. clear and wait again
            reset_e.clear()
        # time run out.
        callback()
    # the event is not set by default. Set it
    reset()
    # create and start the wdt
    t = Thread(target=checker)
    t.start()
    # return the resetter
    return reset


# The callback to run if wdt is not reset
def bark():
    print('woof')       

# Test
import time
reset = wdt(1.0, bark)
time.sleep(0.9)
reset()
time.sleep(2.0)