`then_enter_with(None)` makes the stub return `None` instead of an enterable context manager

[SyntaxColoring]

If you have production code that looks like this:

with foo.bar(bar_arg) as baz:
    ...

Then its Decoy test might do:

bar_arg_captor = matchers.Captor()
decoy.when(foo.bar(bar_arg_captor())).then_enter_with("baz value")

But sometimes the with statement has no as clause:

with foo.bar(bar_arg):
    ...

I expected this to work:

bar_arg_captor = matchers.Captor()
decoy.when(foo.bar(bar_arg_captor())).then_enter_with(None)

But this causes the production code's call to foo.bar(bar_arg) to return None. It should instead return a context manager that returns None when it's entered.

It looks to me like this is an unintentional side-effect of the way StubBehavior works internally:

decoy/decoy/stub_store.py

Lines 7 to 14 in c23d145

	class StubBehavior(NamedTuple):
	"""A recorded stub behavior."""
	return_value: Optional[Any] = None
	context_value: Optional[Any] = None
	error: Optional[Exception] = None
	action: Optional[Callable[..., Any]] = None
	once: bool = False

It can't distinguish between being configured with a context_value that's None versus not being configured with a context_value at all.

To solve a similar problem, the implementation of dataclasses uses a special MISSING sentinel value that's distinct from None. Maybe a similar pattern could work here.

[SyntaxColoring]

An easy workaround is to provide some arbitrary dummy value to then_enter_with(), like then_enter_with("<value unused>").

[mcous]

Shoot, we need a sentinel for this to work.

Stubbing a None context value is interesting, though, because it seems to me like it would be difficult to construct a test that ensures a stubbed context manager is actually used. Since there's no value to interact with, a stubbed None can't validate proper usage. Though it would definitely have usage when combined with tests that do test the functionality to ensure unrelated tests can pass.

I think the better workaround here is to return a CM mock with its __enter__ etc. stubbed out, as in: https://mike.cousins.io/decoy/advanced/context-managers/#general-context-managers

# don't do this
decoy.when(mock_dep.get_context()).then_enter_with("<dummy_value>")

# do this
mock_cm = decoy.mock()
decoy.when(mock_dep.get_context()).then_return(mock_cm)
# ...
# pretty sure this will work, will test it out though
decoy.verify(
    mock_cm.__enter__(),
    # ...
)