Return or break as a side effect in python mock?

Question

I have a multiprocessing application that has workers running in "while True" loops. For testing, I'd like to be able to mock sys.exit() in a way like this:

with mock.patch('sys.exit') as sys_mock:
  sys_mock.side_effect = break

or

with mock.patch('sys.exit') as sys_mock:
  sys_mock.side_effect = return

So I can break out of the loop and complete my test. Neither of these work, but is there a different way to do what I'm trying to accomplish?

Answer 1

You can use an Exception as side_effect to simulate sys.exit behavior without exit from your test.

side_effect documentation say:

This can either be a function to be called when the mock is called, an iterable or an exception (class or instance) to be raised.

So you can not use a statement like break or return but what you want to do is exit from your run cycle and that can be obtained by raise an Exception ... I hope you don't use a wild try-except in your thread's main cycle.

I wrote a simple example to test it, I used decorator patch syntax and put inline side_effect=Exception that make the test more readable:

import sys
import threading
import unittest
from unittest.mock import patch

class T(threading.Thread):
    def __init__(self,  *args, **kwargs):
        super(T, self).__init__(*args, **kwargs)
        self._interrupt = threading.Event()
        self.started = threading.Event() #Used to be sure that we test run() behavior 
        self.started.clear()
        self.terminated = False

    def interrupt(self):
        self._interrupt.set()

    def run(self, *args, **kwargs):
        self._interrupt.clear()
        self.started.set()
        while not self._interrupt.is_set():
            self._interrupt.wait(timeout=1)
        self.terminated = True
        sys.exit()


class TestInterrupt(unittest.TestCase):

    @patch("sys.exit", side_effect=Exception("Ignore it... just close thread"))
    def test_interrupt(self, mock_sys_exit):
        t = T()
        t.start()
        if not t.started.is_set():
            t.started.wait(timeout=0.2)
        self.assertTrue(t.started.is_set(), "t not started!")
        #Ok t is in run() main cycle: we can test interrupt
        t.interrupt()
        t.join(0.1)
        self.assertTrue(t.terminated)
        self.assertFalse(t.isAlive())