Actually deprecate long standing features

[nicoddemus]

This issues proper warnings for:

• Compatible properties
• cached_setup
• Using special named objects to customize node objects during class creation.

This fixes #3616. It also supports a new type of changelog entry, "deprecations", so we can show Deprecations separated from Removals.

@RonnyPfannschmidt are other features you remember of that we should issue deprecation warnings? Would love to add to them as well.

Also it follows that we should not merge #3898, as now we have a proper way to turn the warnings into errors in pytest-4.0, to effectively remove them in 4.1. 👍

[RonnyPfannschmidt]

i had massive issue about one part of those warnings being triggered from pytest internals by accident which is why i deffered it on the old warnings system

[nicoddemus]

catch_warnings to the rescue. 😁

It won't show the warnings for this specific access, but I guess that's OK because the whole class will be kicked out anyway. 👍

[RonnyPfannschmidt]

this is a breaking change, not doing it is why i had massive trouble with the warnings

[RonnyPfannschmidt]

please move the actual warning texts/instances to the deprecation modules

also we should take a look at creating a minimal tool for creating warnings that will be formatted at display time

[nicoddemus]

please move the actual warning texts/instances to the deprecation modules

Argh I missed that point. Definitely, will do.

also we should take a look at creating a minimal tool for creating warnings that will be formatted at display time

Can you please clarify what you mean here?

[coveralls]

Coverage increased (+0.03%) to 93.913% when pulling ae8f369 on nicoddemus:warn-yield-and-compat-properties into 53c9124 on pytest-dev:features.

[RonnyPfannschmidt]

we do have more and more warnings where we format in code - its not nice to use them in code as one has to import the message and still pick the correct category

instead of warn(SomeWarningType(CONSTANT.format(foo))) i want to write warn(CONSTANT.format(foo))

[nicoddemus]

we do have more and more warnings where we format in code - its not nice to use them in code as one has to import the message and still pick the correct category

I suppose we can have format method in our warnings that returns a new instance of that warning, but applying the formatting.

Could be simply:

class PytestWarning:
    def format(self, *args, **kwargs):
        return type(self)(self.args[0].format(*args, **kwargs))

FUNCARG_PREFIX = RemovedInPytest4Warning(
    '{name}: declaring fixtures using "pytest_funcarg__" prefix is deprecated')

Usage:

warn(FUNCARG_PREFIX.format(name=f.name))

[RonnyPfannschmidt]

instead i would like to see a

@attr.s
class UnformattedWarning(object):
    category = attr.ib()
    template = attr.ib()

    def format(self, **kw):
        return self.category(self.template.format(**kw))

FUNCARG_PREFIX = UnformattedWarning(
   RemovedInPytest4Warning,
   '{name}: declaring fixtures using "pytest_funcarg__" prefix is deprecated',
)

that way we have a much better indicator of missuse

[nicoddemus]

Oh great idea. I will implement it then. 👍

[nicoddemus]

Implemented @RonnyPfannschmidt! The result is much better, thanks for the suggestion.

I also found one warning which was using the wrong category, I fixed and noted in the CHANGELOG. 👍

[RonnyPfannschmidt]

fabulous work, thanks