Add capteesys capture fixture to bubble up output to --capture handler

[ayjayt]

potentially closes #12081(and more!)

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Explanation

To set global capture behavior, we set --capture=tee-sys so that pytest can a) capture output for printing reports and b) dump output to our terminals. Both of these things are useful.

Passing capture-type fixtures in our tests will block global capture behavior, but allow us to analyze captured output inside the test.

Unfortunately, capture fixtures don't support anything like --capture=tee-sys- until now. This PR adds a new fixture: capteesys which captures output for analysis in tests but also pushes the output to whatever is set by --capture=. It allows the output to "bubble up" to the next capture handler.

Docs build, tests pass.

My Approach (Engineering)

How the system works right now:

Right now, when you include a fixture (syscap, fdcap, etc), that fixture initializes the CaptureFixture wrapper and passes it the class of the implementation it wants to use, one of:

class NoCapture(...): ... # eg. __init__ takes file descriptor #
class SysCapture(...): ... # eg. __init__ takes file descriptor # + several options
class SysCaptureBinary(...): ...
class SysCapture(...): ...
class FDCapture(...): ...
class FDCaptureBinary(...): ...

self.captureclass = ... # whatever was passed

which the wrapper will instantiate as an object later (depending on fixture scope) as such:

def _start(...):
    ...
    out = self.captureclass(1) # captureclass set to one of above classes
    err = self.captureclass(2)

There is no current way to pass other options to the constructor, which would enable other features, such as tee.

Solution:

Discarded Solutions

• Make that each Capture class accept a tee argument- most will discard it (FDCapture probably could accept it). Most of the time it would be nonsensical and maybe misleading to developers or users.

• Create a conditional switch for the CaptureFixture wrapper which changes the initialization process based on the actual constructor- But it seems bad to create individual branches in base/containing classes for all possible children/attributes.

Chosen Solution

Since the CaptureFixture class is already initialized by passing the class of the implementation, you just add a new configure dictionary:

capture_fixture = CaptureFixture(SysCapture, request, _ispytest=True)
# to
capture_fixture = CaptureFixture(SysCapture, request, config=dict(tee=True), _ispytest=True)

Which is then expanded when initializing the implementation:

out = self.captureclass(1, **self._config)

It requires very few code changes. This explanation was longer.

To create the capteesys fixture, I just copied the capsys fixture but added the config dictionary as part of its setup.

[ayjayt]

It may be better to use "impl_kwargs" instead of "config" or something, in the future maybe have a "impl_args" that expands with * to allow positional arguments and a "impl_kwargs" that expands with **.

[ayjayt]

this fixture is almost 100% a copy of the capsys one above it, since a tee-sys is just a sys w/ an extra argument (tree=True)

[ayjayt]

I've created a polyfill to add to conftest.py to use this feature and warn if/when it is finally released:

@pytest.fixture(scope="function")
def capteesys(request):
    from _pytest import capture
    import warnings
    if hasattr(capture, "capteesys"):
        warnings.warn(( "You are using a polyfill for capteesys, but this"
                        " version of pytest supports it natively- you may"
                        f" remove the polyfill from your {__file__}"),
                        DeprecationWarning)
        # Remove next two lines if you don't want to ever switch to native version
        yield request.getfixturevalue("capteesys")
        return
    capman = request.config.pluginmanager.getplugin("capturemanager")
    capture_fixture = capture.CaptureFixture(capture.SysCapture, request, _ispytest=True)
    def _inject_start():
        self = capture_fixture # closure seems easier than importing Type or Partial
        if self._capture is None:
            self._capture = capture.MultiCapture(
                in_ = None,
                out = self.captureclass(1, tee=True),
                err = self.captureclass(2, tee=True)
            )
            self._capture.start_capturing()
    capture_fixture._start = _inject_start
    capman.set_fixture(capture_fixture)
    capture_fixture._start()
    yield capture_fixture
    capture_fixture.close()
    capman.unset_fixture()

[gvwilson]

👍 this would be very useful (albeit in rather esoteric contexts :-) )

[nicoddemus]

Looks good, thanks @geopozo!