Warn instead of raising exception in context manager

[iforapsy]

Instead of raising an exception, warn when a pytest-mock mock is used as
a context manager. This lets pytest-mock mock objects that are used as
context managers, like threading.Lock, while still letting users know
that mocks returned from pytest-mock do not need their enter method
called to take effect.

Create a PytestMockWarning class so that the warning is easy to
pinpoint and handle.

#192

[nicoddemus]

Hey @iforapsy,

Thanks a lot for the PR, sorry for the delay!

The patch looks great, it shows you took great care on it, thanks.

One suggestion is to perhaps tweak the warning message a bit explaining more what's happening? Otherwise when using it as a context, the user might the impression that something is not correct.

---

Alternatively (and this just occurred to me now), perhaps we can have a different function, which operates like mocker.patch.object, but specific to test context managers and which doesn't emit a warning? Something like:

import threading

class Foo:

    CREATION_LOCK = threading.Lock()
    def create_widgets(self):
        with self.CREATION_LOCK:
            print('Creating object')


def test_create_widgets_acquires_lock(mocker):
    """Test that the creation lock is acquired."""
    lock_mock = mocker.patch.context_manager(Foo, "CREATION_LOCK", autospec=True)
    Foo().create_widgets()
    lock_mock.__enter__.assert_called()

What do you think?

[iforapsy]

Hi @nicoddemus,

No need to apologize. I understand that it's the holidays. I have not been the fastest responder either.

Thank you for your kind feedback. I like both your suggestions.

I made the warning message more explanatory and more newbie-friendly.

Though it won't be useful for my code base (as the CREATION_LOCK object is global in mine), I like the mocker.patch.context_manager function idea. It could be useful to others so I added that feature.

[nicoddemus]

Hi @iforapsy, just passing by to let you know I did not forget about this PR, I will probably have time to review/merge/make a new release tomorrow. Thanks again for the patience!

[nicoddemus]

Great PR, thanks @iforapsy!

[nicoddemus]

3.5.0 released 🎉