Composing multiple configuration files

[cihati]

Feature

Ability to specify a base configuration file and another one to override/extend behavior.

Pitch

We have a monorepo with Gradle. There are a few different Python projects and each of them define their linters either fully duplicated or only with slight differences. I'd like to standardize this. For mypy, I'd like to have a base configuration file where I define common settings, and then make it possible to have projects overriding/extending behavior.

For instance, one project uses Django, and it has django-stubs plugin for mypy. If I put this into the base config, then mypy complains on the non-Django projects as the plugin becomes unusable. I do know that I can override some configuration behavior through command line arguments ("Settings override mypy’s built-in defaults and command line flags can override settings.") but I'm not sure if it's possible for plugin settings anyways.

[The-Compiler]

We have a similar situation in qutebrowser (cc @toofar), where we want to support different major versions of a library via something like:

if machinery.USE_PYQT5:
    from PyQt5.QtCore import *
elif machinery.USE_PYQT6:
    from PyQt6.QtCore import *

Stubs are available for both PyQt5 and PyQt6, so we'd like to run mypy twice, once with --always-true=USE_PYQT5 --always-false=USE_PYQT6, and once with the opposite.

There are other wrappers over PyQt5/PyQt6 which share the same approach, and usually ship a command-line tool on top of mypy to generate flags to pass to it: qts (cc @altendky), qtpy.

However, it'd be nice to have a default setting for that (so that mypy can be run, even from e.g. an editor integration, without needing a wrapper). Then there could be a config snippet overriding those again.

Unfortunately, in our case, even having a default in the config and overriding it on the commandline won't work properly, as:

mypy: error: You can't make a variable always true and always false (IS_QT5, IS_QT6, USE_PYQT5, USE_PYQT6)

but I suppose that's different from what this issue describes.

[ruslaniv]

Yes, this is required for monorepos badly!
It would be great to implement it like Ruff does, where you can keep the main config file in the root and then project specific configs in the project folders and then just do:

extend = "../mypy.ini"

[Avasam]

I'll add that even across multiple repositories, this can enable shared based configuration through git submodules.

[ericmburgess]

Yes, please! Lack of this capability is the only thing in the way of my practice of keeping a scratch folder in each repo, which will be totally ignored by git and mypy. It's important that I can configure this without modifying anything that is checked in to source control. Currently I have simply duplicated the (git tracked) .mypy.ini with a git-ignored mypy.ini (no dot) file, and added my personal ignores. But this is brittle since I can forget to update my replacement config when the real one is changed.