(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

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()