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

Michael Sullivan sully at msully.net
Tue Apr 16 03:48:20 EDT 2019 
On Mon, Apr 15, 2019 at 8:12 PM Nathaniel Smith <njs at pobox.com> wrote:

> 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?
>
> Nothing else in typing does any type of runtime enforcement, so I'd be
reluctant to start here.

One approach would be doing something like this (maybe in a support module):
if typing.TYPE_CHECKING:
    from typing import final
else:
    from alsoruntime import final

So that at checking time, the typechecker would use the typing final but at
runtime we'd get something that does enforcement.
(And for the pre-3.6 case, you could maybe use something like
six.add_metaclass in order to specify the metaclass as a decorator.)

I can add this as an example to the PEP.

-sully


> -n
>
> [1] https://github.com/willingc/pep-communication/issues/1
> [2] https://stackoverflow.com/a/3949004/1925449
>
> --
> Nathaniel J. Smith -- https://vorpus.org
>
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