Gooey/docs/Gracefully-Stopping.md

Gracefully Stopping a Running Process

New in v1.0.9!

Contents:

• How to tell Gooey which shutdown signal to use
• How to catch KeyboardInterrupts
• How to catch general interrupt signals

By default, Gooey will kill the child process without any chance for cleanup. This guide will explain how to adjust that behavior so that you can detect when Gooey is attempting to close your process and use that signal to shutdown gracefully.

Basics: How to tell Gooey which shutdown signal to use:

You can control the signal Gooey sends while stopping your process via shutdown_signal decorator argument. Signal values come from the builtin signal python module. On linux, any of the available constants may be used as a value. However, on Windows, only CTRL_BREAK_EVENT, CTRL_C_EVENT and SIGTERM are supported by the OS.

import signal 
@Gooey(shutdown_signal=signal.CTRL_C_EVENT)
def main():
    ...

How to catch KeyboardInterrupts:

Keyboard interrupts are triggered in response to the CTRL_C_EVENT signal.

import signal 
@Gooey(shutdown_signal=signal.CTRL_C_EVENT)
def main():
    ...

Catching them in your code is really easy! They conveniently show up as top-level Exceptions. Just wrap your main logic in a try/except and you'll be able to catch when Gooey tries to shut down your process.

try
   # your code here
except KeyboardInterrupt: 
   # cleanup and shutdown or ignore 

How to catch general interrupt signals

Handling other signals is only slightly more involved than the CTRL_C_EVENT one. You need to install a handler via the signal module and tie it to the specific signal you want to handle. Let's use the CTRL_BREAK_EVENT signal as example.

import signal

# (1)
def handler(*args): 
    print("I am called in response to an external signal!")
    raise Exception("Kaboom!")

# (2) 
signal.signal(signal.SIGBREAK, handler)

# (3)
@Gooey(shutdown_signal=signal.CTRL_BREAK_EVENT)
def main():
    # your code here 
    # ... 

Here we setup a handler called handler (1). This function can do anything you want in response to the signal including ignoring it entirely. Next we tie the signal we're interested in to the handler (2). Finally, we tell Gooey to send the BREAK signal(3) when the stop button is clicked.

Note: pay close attention to the different constants used while specifying a handler (e.g. SIGBREAK) versus specifying which signal will be sent (e.g. CTRL_BREAK_SIGNAL).