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

2019-04-18 Thread 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  wrote:

> On Mon, Apr 15, 2019 at 8:12 PM Nathaniel Smith  wrote:
>
>> On Mon, Apr 15, 2019 at 5:00 PM Michael Sullivan 
>> 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?)*

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

2019-04-16 Thread Michael Sullivan 
On Mon, Apr 15, 2019 at 8:12 PM Nathaniel Smith  wrote:

> On Mon, Apr 15, 2019 at 5:00 PM Michael Sullivan  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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Re: [Python-Dev] PEP 591 discussion (final qualifier) happening at typing-sig@

2019-04-15 Thread Nathaniel Smith 
On Mon, Apr 15, 2019 at 5:00 PM Michael Sullivan  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
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[Python-Dev] PEP 591 discussion (final qualifier) happening at typing-sig@

2019-04-15 Thread Michael Sullivan 
I've submitted PEP 591 (Adding a final qualifier to typing) for discussion
to typing-sig [1].

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

Full text at https://www.python.org/dev/peps/pep-0591/

-sully

[1] https://mail.python.org/mailman3/lists/typing-sig.python.org/
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