Reload Application on SIGHUP

A common convention in the Linux world is to handle a SIGHUP signal, by reloading the current runtime within the same process ID (pid). The following example demonstrates how you might achieve that:

import signal
from time import sleep
from cement.core.foundation import CementApp
from cement.core.controller import CementBaseController, expose
from cement.core.exc import CaughtSignal


class MyController(CementBaseController):
    class Meta:
        label = 'base'

    @expose(hide=True)
    def default(self):
        print('Inside MyController.default()')

        ### inner loop where the application logic happens
        while True:
            print('Inside Inner Loop')
            sleep(5)

class MyApp(CementApp):
    class Meta:
        label = 'myapp'
        base_controller = MyController


with MyApp() as app:
    ### outer loop where signals are handles, and application reload
    ### happens

    keep_alive = True
    while keep_alive is True:
        try:
            app.run()
        except CaughtSignal as e:
            app.log.warning(e.msg)
            if e.signum in [signal.SIGHUP]:
                app.log.warning('Reloading MyApp')
                app.reload()
                keep_alive = True
            else:
                app.exit_code = 1
                keep_alive = False

Running the application shows the inner loop in action:

$ python myapp.py
Inside MyController.default()
Inside Inner Loop
Inside Inner Loop
Inside Inner Loop
Inside Inner Loop
Inside Inner Loop
Inside Inner Loop
Inside Inner Loop

If we grab the PID in another terminal, and send it a SIGHUP signal we can see it reload:

$ ps auxw | grep [m]yapp
derks            1317   0.0  0.1  2425792  12988 s001  S+   10:03PM   0:00.11 python myapp.py

In the original terminal (running our app) we see that the app reloads and the inner loop continues:

Caught signal 1
WARNING: Reloading myapp
Inside MyController.default()
Inside Inner Loop
Inside Inner Loop

However, the PID remains the same:

$ ps auxw | grep [m]yapp
derks            1317   0.0  0.1  2425792  13012 s001  S+   10:03PM   0:00.11 python myapp.py

If you’d like to see even more detail on what Cement is doing during the reload try adding --debug.