(Unit) Test python signal handler

Question

I have a simple Python service, where there is a loop that performs some action infinitely. On various signals, sys.exit(0) is called, which causes SystemExit to be raised and then some cleanup should happen if it can.

In a test, i.e. standard unittest.TestCase, I would like to test that this cleanup happens and the loop exits. However, I'm stuck on even getting the signal to be triggered / SystemExit to be raised.

# service.py
import signal
import sys
import time

def main():

    def signal_handler(signalnum, _):
        # How to get this to block to run in a test?
        sys.exit(0)

    signal.signal(signal.SIGINT, signal_handler)
    signal.signal(signal.SIGTERM, signal_handler)

    while True:
        try:
            print("Some action here")
            time.sleep(10)
        except SystemExit:
            # How to get this to block to run in a test?
            print("Some cleanup")
            break

if __name__ == '__main__':
    main()

How can the code enter the SystemExit handler / signal handler in the test environment? An alternative pattern would also be welcome.

Answer 1

You can trigger a SIGINT (or any signal) from another thread after some delay, which is received in the main thread. You can then assert on its effects just as in any other test, as below.

import os
import signal
import time
import threading
import unittest
from unittest.mock import (
    Mock,
    patch,
)

import service

class TestService(unittest.TestCase):

    @patch('service.print')
    def test_signal_handling(self, mock_print):

        pid = os.getpid()

        def trigger_signal():
            while len(mock_print.mock_calls) < 1:
                time.sleep(0.2)
            os.kill(pid, signal.SIGINT)

        thread = threading.Thread(target=trigger_signal)
        thread.daemon = True
        thread.start()

        service.main()

        self.assertEqual(mock_print.mock_calls[1][1][0], 'Some cleanup')


if __name__ == '__main__':
    unittest.main()