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

[Guido van Rossum]

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 __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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-- 
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him/his **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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