[Python-Dev] PEP 572: Now with 25% less reference implementation!

[Guido van Rossum]

Maybe annotations should get a brief mention in the Rejected Ideas section,
with your explanation here added. (And maybe my response.)

On Thu, Apr 19, 2018 at 11:31 PM, Chris Angelico <rosuav at gmail.com> wrote:

> On Fri, Apr 20, 2018 at 2:45 PM, Dmitry Malinovsky <damalinov at gmail.com>
> wrote:
> > Hello Chris, and thank you for working on this PEP!
> >
> > What do you think about using variable type hints with this syntax?
> > I tried to search through python-dev and couldn't find a single post
> > discussing that question.
> > If I missed it somehow, could you please include its conclusions into
> the PEP?
>
> I'm ignoring them for the sake of the PEP, because it'd complicate the
> grammar for little benefit. If someone wants to add an enhancement
> later, that's fine; but the proposal can stand without it, and with
> it, it'll make for even more noise in a line full of colons.
>
> > For instance, as I understand now the parser will fail on this snippet:
> >
> >     while data: bytes := stream.read():
> >         print("Received data:", data)
> >
> > Do brackets help?
> >
> >     while (data: bytes := stream.read()):
> >         print("Received data:", data)
> >
> > IIUC, in 3.7 It is invalid syntax to specify a type hint for a for loop
> item;
> > should brackets help? Currently they don't:
> >
> >     Python 3.7.0b3+ (heads/3.7:7dcfd6c, Mar 30 2018, 21:30:34)
> >     [Clang 9.0.0 (clang-900.0.39.2)] on darwin
> >     Type "help", "copyright", "credits" or "license" for more
> information.
> >     >>> for (x: int) in [1,2,3]:
> >       File "<stdin>", line 1
> >         for (x: int) in [1,2,3]:
> >           ^
> >     SyntaxError: invalid syntax
>
> And that's another good reason not to bother, at least for now. I'm
> not sure whether you can use a Py2-style type hint comment on a for
> loop, but if so, you should also be able to do it on a while loop or
> anything. Or, of course, you can just annotate the variable before the
> loop, if you want to.
>
> ChrisA
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-- 
--Guido van Rossum (python.org/~guido)
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