[Python-Dev] Please reject or postpone PEP 526

[Koos Zevenhoven]

It looks like you are trying to make sense of this, but unfortunately
there's some added mess and copy&paste-like errors regarding who said
what. I think no such errors remain in what I quote below:

On Mon, Sep 5, 2016 at 3:10 PM, Steven D'Aprano <steve at pearwood.info> wrote:
>
> [Koos Zevenhoven]
>> >> How is it going to help that these are equivalent within one checker,
>> >> if the meaning may differ across checkers?
>
> Before I can give an answer to your [Koos'] question, I have to
> understand what you see as the problem here.

The problem was that suggested restrictive addition into PEP 526 with
no proper justification, especially since the PEP was not supposed to
restrict the semantics of type checking. I was asking how it would
help to add that restriction. Very simple. Maybe some people got
confused because I did want to *discuss* best practices for type
checking elsewhere.

> I *think* that you are worried that two different checkers will disagree
> on what counts as a type error. That given the same chunk of code:

In the long term, I'm worried about that, but there's nothing that PEP
526 can do about it at this point.

> [Nick Coghlan]
>> > For typechecker developers, it provides absolute clarity that the
>> > semantics of the new annotations should match the behaviour of
>> > existing type comments when there's an initialiser present,
>
> [Koos]
>> I understood that, but what's the benefit?
>
> Are you asking what is the benefit of having three forms of syntax for
> the same thing?

No, still the same thing: What is the benefit of that particular
restriction, when there are no other restrictions? Better just leave
it out.

> The type comment systax is required for Python 2 and backwards-
> compatibility. That's a given.

Sure, but all type checkers will not have to care about Python 2.

> The variable annotation syntax is required because the type comment
> syntax is (according to the PEP) very much a second-best solution. See
> the PEP:
>
> https://www.python.org/dev/peps/pep-0526/#id4
>
> So this is a proposal to create a *better* syntax for something which
> already exists. The old version, using comments, cannot be deprecated or
> removed, as it is required for Python 3.5 and older.

Right.

> Once we allow
>
>     x: T = value
>
> then there benefit in also allowing:
>
>     x: T
>     x = value
>
> since this supports some of the use cases that aren't well supported by
> type comments or one-line variable annotations. E.g. very long or deeply
> indented lines, situations where the assignment to x is inside an
> if...else branch, or any other time you wish to declare the type of the
> variable before actually setting the variable.

Sure.

> [Nick]
>> > For folks following along without necessarily keeping up with all the
>> > nuances, it makes it more explicit what Guido means when he says "PEP
>> > 526 does not make a stand on the
>> > behavior of type checkers (other than deferring to PEP 484)."
>
> [Koos]
>> What you are proposing is exactly "making a stand on the behavior of
>> type checkers", and the examples you provide below are all variations
>> of the same situation and provide no justification for a general rule.
>
> I'm sorry, I don't understand this objection. The closest I can get to
> an answer would be:
>
> A general rule is better than a large number of unconnected, arbitrary,
> special cases.

A general rule that does not solve a problem is worse than no rule.


-- Koos


>
> --
> Steve
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+ Koos Zevenhoven + http://twitter.com/k7hoven +