load-plugins not working if configured in setup.cfg [pylint.MASTER]

[mteiste]

load_plugins option does not work if defined in [pylint.MASTER] section of setup.cfg

Steps to reproduce

Given a config file setup.cfg

[pylint.MASTER]
load_plugins=pylint.extensions.bad_builtin
[pylint.DEPRECATED_BUILTINS]
bad-functions=print

and a python file a.py

print("Hello World!")

Current behavior

Result of `pylint --rcfile=setup.cfg a.py

************* Module a
a.py:1:0: C0114: Missing module docstring (missing-module-docstring)

-----------------------------------
Your code has been rated at 0.00/10

Expected behavior

************* Module a
a.py:1:0: C0114: Missing module docstring (missing-module-docstring)
a.py:1:0: W0141: Used builtin function 'print' (bad-builtin)

---------------------------------------------------------------------
Your code has been rated at -10.00/10

pylint --version output

Result of pylint --version output:

pylint 2.7.4
astroid 2.5.2
Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0]

[cdce8p]

If you use setup.cfg the correct config should be

[MASTER]
load-plugins=pylint.extensions.bad_builtin

[pylint.DEPRECATED_BUILTINS]
bad-functions=print

Notice that it's only [MASTER] not [pylint.MASTER]

You could also use pyproject.toml. In that case, this would be the correct config

[tool.pylint.MASTER]
load-plugins= "pylint.extensions.bad_builtin"

[tool.pylint.DEPRECATED_BUILTINS]
bad-functions= ["print"]

Here you shouldn't need to specify the --rcfile option.

[mteiste]

Not argument from me against using toml. Just that "setup.cfg" support is a bit patchy and confusing (using [pylint.master]as the section does work). Actually the only config values that are affected by this are load-plugins and init-hook.

I just assumed that only sections marked with pylint.* would be the ones pylint uses and it would ignore any other sections that are not specifically marked for pylint.

[mteiste]

There is also an other slightly nasty side effect from this. If you have setup.cfg with sections for other tools that use same option names those will be taken in by pylint. For example the following setup.cfg

[MASTER]
load-plugins=pylint.extensions.bad_builtin

[pylint.DEPRECATED_BUILTINS]
bad-functions=print

[pycodestyle]
ignore=E722,W503

has the effect of setting pylint's exclude to E722,W503, hence there is a lot of possibilities to have hard to find configuration problems when using setup.cfg.

[cdce8p]

I almost always use a pylintrc file for my pylint config, not setup.cfg
As for the pylint. prefix, I think this isn't used because it should be a separate file. Could be wrong though, maybe @Pierre-Sassoulas knows more?

[mteiste]

True, there are workarounds / arguably better options, just commenting that the current support for setup.cfg is slightly confusing. Apart from not ignoring sections not marked with pylint, using lowercase for all the pylint.sections actually works as expected (due to how the section title normalization is implemented in OptionManagerMixin::read_config_file()).

[Pierre-Sassoulas]

Thank you for the issue @mteiste, evidentely setup.cfg is not super well supported.

I did not modify a lot of configuration code, but I think we'd get some benefit of having a single standardized point of entry like flake8 (ask for pyproject.toml or setup.cfg support here, see how fast assotile closes the issue 😄 ). @AWhetter convinced me that actually merging #3769 would be shooting ourselves in the foot. Maybe we need to consider a breaking change in 3.0.0 where we only support .pylintrc and local pyproject.toml