Provide message of expected exception when timeout happens

[abotalov]

Currently if both timeout_seconds and expected_exception are used, only information that timeout expired is printed.
It would be good to add exception message of expected exception to TimeoutExpired message.

[vmalloc]

Can you please provide an example (not necessarily working) code snippet and what you expect to happen with it that currently does not happen? I'm not sure I'm following correctly...

[abotalov]

Well, I'd like to perform assert until it succeeds or timeout happens.

def assertion()
  assert window.url() == 'some_string'
wait(assertion, timeout_seconds=10, expected_exceptions=AssertionError)

Similar feature exists in pytest-qt (https://pytest-qt.readthedocs.io/en/latest/wait_until.html). There, if timeout happens, an expected exception is raised instead of TimeoutError (https://github.com/pytest-dev/pytest-qt/blob/master/pytestqt/qtbot.py#L520). The benefit is that a user sees a message that helps to understand why timeout has happened.

waiting library may however still raise TimeoutExpired but a message can be improved to include a message of happened expected exception.

Currently I see only something like:

TimeoutExpired: Timeout of 10 seconds expired waiting for <function assertion at 0x1031bdca2>

[vmalloc]

expected_exceptions in this case means "exceptions that are allowed to happen while polling", so they are ignored... I don't see the logic behind raising them on timeout....

[abotalov]

The use case is described above. Could you propose a better way to do it?
Yes, these exceptions are allowed to happen until timeout. But once timeout happened, the message is useful for debugging.

[vmalloc]

I see...

What if you had some adapter to achieve this, like:

waiting.wait(waiting.succeeds(func, expected_exceptions=(...)))

Then in this use case I can make the "default" message formatting consider this object type and print the last exception encountered on it

[abotalov]

Ah, I just got that a solution I proposed before wouldn't fully work. As successful assertion returns None rather than True. But it could be worked around by returning True from a func.
Will the mechanism that you wrote about work by raising a specific exception that is catched in wait?

[vmalloc]

I think it will be a specific subclass of a new class which waiting recognized, and which can be queried for the exact reason for not succeeding

[abotalov]

The following appears to work fine for me:

def assertion()
  assert window.url() == 'some_string'
  return True
wait(assertion, timeout_seconds=10, expected_exceptions=AssertionError)

together with a code change like abotalov@0d43a52 appears to work.
The code change only outlines idea - it would be better to pass a message as a third parameter and to build a better message in TimeoutExpired.