[Python-Dev] PEP 591 discussion (final qualifier) happening at typing-sig@

[Nathaniel Smith]

On Mon, Apr 15, 2019 at 5:00 PM Michael Sullivan <sully at msully.net> wrote:
>
> I've submitted PEP 591 (Adding a final qualifier to typing) for discussion to typing-sig [1].

I'm not on typing-sig [1] so I'm replying here.

> Here's the abstract:
> This PEP proposes a "final" qualifier to be added to the ``typing``
> module---in the form of a ``final`` decorator and a ``Final`` type
> annotation---to serve three related purposes:
>
> * Declaring that a method should not be overridden
> * Declaring that a class should not be subclassed
> * Declaring that a variable or attribute should not be reassigned

I've been meaning to start blocking subclassing at runtime (e.g. like
[2]), so being able to express that to the typechecker seems like a
nice addition. I'm assuming though that the '@final' decorator doesn't
have any runtime effect, so I'd have to say it twice?

@typing.final
class MyClass(metaclass=othermod.Final):
    ...

Or on 3.6+ with __init_subclass__, it's easy to define a @final
decorator that works at runtime, but I guess this would have to be a
different decorator?

@typing.final
@alsoruntime.final
class MyClass:
    ...

This seems kinda awkward. Have you considered giving it a runtime
effect, or providing some way for users to combine these two things
together on their own?

-n

[1] https://github.com/willingc/pep-communication/issues/1
[2] https://stackoverflow.com/a/3949004/1925449

-- 
Nathaniel J. Smith -- https://vorpus.org