Composing multiple configuration files

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 issue names no implementation files or tests. Start by reading mypy’s existing configuration-file handling and command-line override behavior, with particular attention to plugin settings. Done would mean a defined and tested way to compose a base configuration with project-specific overrides.