Class decorator for enforcing the use of `with` statement for an object

context_only.py

from typing import Type, ContextManager, TypeVar

T = TypeVar("T")


def context_only(cls: Type[T]) -> Type[ContextManager[T]]:
    def _default_enter(self):
        return self

    def _default_exit(self, exc_type, exc_val, exc_tb):
        return False

    context_members = dict(cls.__dict__)

    context_enter = context_members.pop("__enter__", _default_enter)
    context_exit = context_members.pop("__exit__", _default_exit)

    context_cls = type(cls.__name__, cls.__bases__, context_members)

    class _ContextManager:
        def __init__(self, *args, **kwargs):
            self.__context: T = context_cls(*args, **kwargs)

        def __enter__(self) -> T:
            return context_enter(self.__context)

        def __exit__(self, exc_type, exc_val, exc_tb):
            return context_exit(self.__context, exc_type, exc_val, exc_tb)

    return _ContextManager


@context_only
class Printer:
    line: int
    name: str

    def __init__(self, name: str):
        self.line = 1
        self.name = name

    def print(self, thing: str) -> None:
        print(f"{self.name}:{self.line:4}: {thing}")
        self.line += 1


def main():
    with Printer("Log") as printer:
        printer.print("Hello, World!")
        printer.print("Goodbye, World.")


if __name__ == '__main__':
    main()