mypy.ini

[mypy]
plugins = myplug.py

plugins.py

from mypy.plugin import Plugin, FunctionContext
from mypy.subtypes import is_subtype
from mypy.types import AnyType, LiteralType
from typing import Any


class Special(AnyType):
    def __repr__(self) -> str:
        return "Special Subtype Type"


class MyPlugin(Plugin):
    def get_function_hook(self, fullname: str) -> Any:
        if "check_sub" in fullname:
            return self.fn_hook
        return None

    def fn_hook(self, ctx: FunctionContext) -> Any:
        if len(ctx.arg_types) == 2:
            t1, t2 = ctx.arg_types[0][0], ctx.arg_types[1][0]
            str_type = ctx.api.named_generic_type("builtins.str", [])
            if is_subtype(t1, t2):
                return LiteralType(value="YES", fallback=str_type)
            else:
                return LiteralType(value="NO", fallback=str_type)

        return ctx.default_return_type


def plugin(version: str) -> type[MyPlugin]:
    return MyPlugin

test.py

from typing import reveal_type


class A: ...
class B(A): ...
class C(B): ...


def check_sub(x, y): ...


def f() -> None:
    reveal_type(check_sub(B(), A()))  # plugin sees B <: A
    reveal_type(check_sub(C(), A()))  # plugin sees C <: A
    reveal_type(check_sub(A(), B()))  # not a subtype