Deprecate pytest_plugins in non-toplevel conftests

[nicoddemus]

We must deprecate this somehow before being able to remove it.

After this issue is merged, we should create a task for removal associated with a proper milestone.

[pytestbot]

GitMate.io thinks the contributor most likely able to help you is @RonnyPfannschmidt.

[brianmaissy]

I'll take this.

Just so I understand the full picture, why do we want to deprecate this? Are there implementation considerations, behavioral considerations, or just for good practice for staying organized?

[RonnyPfannschmidt]

@brianmaissy pytest_plugins always has global effects, so plug-ins added somewhere in a directory tree leak to the complete project, which is undesirable and source of hard to comprehend bugs

[brianmaissy]

ah i see, makes sense

[brianmaissy]

@nicoddemus @RonnyPfannschmidt While trying to test this deprecation I ran into a problem. Since DeprecationWarnings are ignored by default, no user will ever see this warning, unless we don't use an actual DeprecationWarning. The warning filters on the command line and in the ini files don't take effect during the initialization stages of importing conftests, so even if a user was aware of this and wanted to receive DeprecationWarnings from pytest they wouldn't have a way. I assume you've run into this problem before, but I wasn't able to find an example in the code. Am I missing something?

Am I supposed to use the _warn method of the PytestPluginManager? I didn't understand the different codes used there (I1, I2).

[RonnyPfannschmidt]

@brianmaissy the starting point is declaring a RemovedInPytest4 wanring instance in _pytest.deprecated

[brianmaissy]

That I understand, but if I raise it in config.py while importing conftests, it will just get ignored by the python warnings filter

[brianmaissy]

I guess they could change the warnings filter in the top-level conftest. Still, it seems like deprecation warnings should be more visible