On Wed, 1 Dec 2021 at 06:19, Chris Angelico <ros...@gmail.com> wrote:
>
> I've just updated PEP 671 https://www.python.org/dev/peps/pep-0671/
> with some additional information about the reference implementation,
> and some clarifications elsewhere.
>
> *PEP 671: Syntax for late-bound function argument defaults*
>
> Questions, for you all:
>
> 1) If this feature existed in Python 3.11 exactly as described, would
> you use it?
Probably not. Mainly because I don't have any real use for it rather
than because I have any inherent problem with it (but see below, the
more I thought about it the more uncomfortable with it I became).
> 2) Independently: Is the syntactic distinction between "=" and "=>" a
> cognitive burden?
>
> (It's absolutely valid to say "yes" and "yes", and feel free to say
> which of those pulls is the stronger one.)
Not especially. The idea of having both late-bound and early-bound
parameters is probably more of a cognitive burden than the syntax.
> 3) If "yes" to question 1, would you use it for any/all of (a) mutable
> defaults, (b) referencing things that might have changed, (c)
> referencing other arguments, (d) something else?
N/A, except to say that when you enumerate the use cases like this,
none of them even tempt me to use this feature. I think that the only
thing I might use it for is to make it easier to annotate defaults (as
f(a: list[int] => []) rather than as f(a: list[int] | None = None).
So I'll revise my answer to (1) and say that I *might* use this, but
only in a way it wasn't intended to be used in, and mostly because I
hate how verbose it is to express optional arguments in type
annotations. (And the fact that the annotation exposes the sentinel
value, even when you want it to be opaque). I hope I don't succumb and
do that, though ;-)
> 4) If "no" to question 1, is there some other spelling or other small
> change that WOULD mean you would use it? (Some examples in the PEP.)
Not really. It addresses a wart in the language, but on consideration,
it feels like the cure is no better than the disease.
> 5) Do you know how to compile CPython from source, and would you be
> willing to try this out? Please? :)
Sorry, I really don't have time to, in the foreseeable future. If I
did have time, one thing I would experiment with is how this interacts
with typing and tools like pyright and mypy (yes, I know type checkers
would need updating for the new syntax, so that would mostly be a
thought experiment) - as I say, I'd expect to annotate a function with
an optional list argument defaulting to an empty list as f(a:
list[int] => []), which means that __annotations__ needs to
distinguish between this case and f(a: list[int]) with no default.
>>> def f(a: list[int]): pass
...
>>> f.__annotations__
{'a': list[int]}
>>> def f(a: list[int] => []): pass
...
>>> f.__annotations__
???
> I'd love to hear, also, from anyone's friends/family who know a bit of
> Python but haven't been involved in this discussion. If late-bound
> defaults "just make sense" to people, that would be highly
> informative.
Sorry, I don't have any feedback like that. What I can say, though, is
I'd find it quite hard to express the question, in the sense that I'd
struggle to explain the difference between early and late bound
parameters to a non-expert, much less explain why we need both. I'd
probably just say "it's short for a default of None and a check" which
doesn't really capture the point...
> Any and all comments welcomed. I mean, this is python-ideas after
> all... bikeshedding is what we do best!
I hope this was useful feedback.
Paul
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