Mypy plugin that prevents abstract classes from having an `__init__` method.

no_init_for_abc.py

from __future__ import annotations

import typing

from mypy.nodes import FuncDef
from mypy.plugin import ClassDefContext, Plugin

__all__ = ["NoInitForABCPlugin"]


class NoInitForABCPlugin(Plugin):
    """
    Mypy-plugin that enforces abstract classes (inheriting from `abc.ABC`) to
    not define an `__init__` method.

    We want abstract classes to be a pure abstract interface and not define any
    fields. Ideally, they should also not define any non-abstract methods, but
    this is less of a problem.

    Some literature for the justification:
    - "Nothing is something", Sandy Metz  - https://www.youtube.com/watch?v=OMPfEXIlTVE&t=1s
    - "Subclassing in Python Redux", Hynek Schlawack - https://hynek.me/articles/python-subclassing-redux/
    """

    def get_base_class_hook(
        self, fullname: str
    ) -> typing.Callable[[ClassDefContext], None]:
        return self.no_init_check

    def no_init_check(self, ctx: ClassDefContext) -> None:
        for base in ctx.cls.info.bases:
            if base.type.fullname == "abc.ABC":
                for node in ctx.cls.defs.body:
                    if isinstance(node, FuncDef) and node.name == "__init__":
                        ctx.api.fail(
                            "Abstract classes (inheriting from abc.ABC) "
                            "should not define an __init__ method",
                            ctx.cls,
                        )
                        break


def plugin(version: str) -> typing.Type[NoInitForABCPlugin]:
    return NoInitForABCPlugin