threading, SystemExit and PytestUnhandledThreadExceptionWarning

[sashkab]

Hello,

I have a test, which results in sys.exit call. Something similar to following code:

import sys
import threading


def test_sys_exit():
    def foo():
        sys.exit(42)

    t = threading.Thread(target=foo)
    t.start()
    t.join()

Recently pytest started warning about unhandled SystemExit exception. I made several attempts to resolve this warning, and was unable to. What would be a correct way to handle this case? I know, I can disable a plugin, but I'm pretty sure there should be a way to handle it properly, but I'm not seeing it... yet.

test_thread.py::test_sys_exit
  /Users/user/.virtualenvs/3/lib/python3.8/site-packages/_pytest/threadexception.py:75: PytestUnhandledThreadExceptionWarning: Exception in thread Thread-1

  Traceback (most recent call last):
    File "/usr/local/Cellar/python@3.8/3.8.13/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 932, in _bootstrap_inner
      self.run()
    File "/usr/local/Cellar/python@3.8/3.8.13/Frameworks/Python.framework/Versions/3.8/lib/python3.8/threading.py", line 870, in run
      self._target(*self._args, **self._kwargs)
    File "/Users/user/.tmp/test_thread.py", line 7, in foo
      sys.exit(42)
  SystemExit: 42

    warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))

-- Docs: https://docs.pytest.org/en/stable/warnings.html

Thanks.

[sashkab]

I probably should have mentioned, that my workaround for now is somewhat long and ugly:

import threading
import sys

import pytest


class SysExitThread(threading.Thread):
    """Pass exception from thread run to the parent"""

    def __init__(self, group=None, target=None, name=None, args=(), kwargs=None, *, daemon=None):
        super().__init__(group=group, target=target, name=name, args=args, kwargs=kwargs, daemon=daemon)
        self._exc = None

    def run(self):
        try:
            super().run()
        except SystemExit as exc:
            self._exc = exc

    def join(self, timeout=None):
        super().join(timeout=timeout)
        if self._exc:
            raise self._exc

def test_sys_exit():
    def foo():
        sys.exit(42)

    t = SysExitThread(target=foo)
    t.start()
    with pytest.raises(SystemExit) as exc:
      t.join()
    assert exc.typename == "SystemExit"

[RonnyPfannschmidt]

Pytest intentionally warns about this as threads that fail with exception indicate gruesome failure

[RonnyPfannschmidt]

@nicoddemus i took note that systemexit is ignored by the default, perhaps there should be a opt out for that specific case