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

Guido van Rossum guido at python.org
Thu Apr 18 10:44:38 EDT 2019

Yes, please add this to the PEP in the rejected ideas section, with the motivation for rejection -- the example can show how to work around it.

On Tue, Apr 16, 2019 at 12:51 AM Michael Sullivan <sully at msully.net> wrote:

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 initsubclass, 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.TYPECHECKING: 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.addmetaclass 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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