[Python-ideas] Optional static typing -- the crossroads

[Guido van Rossum]

Doesn't seem a big deal. The only place where you'd see an implementation
of optional() would be in typing.py, and optional(None) is redundant anyway.


On Fri, Aug 15, 2014 at 10:51 AM, Petr Viktorin <encukou at gmail.com> wrote:

> On Fri, Aug 15, 2014 at 7:36 PM, Guido van Rossum <guido at python.org>
> wrote:
> > On Fri, Aug 15, 2014 at 10:19 AM, Petr Viktorin <encukou at gmail.com>
> wrote:
> >>
> >> On Fri, Aug 15, 2014 at 7:00 PM, Guido van Rossum <guido at python.org>
> >> wrote:
> >> > On Fri, Aug 15, 2014 at 9:48 AM, Petr Viktorin <encukou at gmail.com>
> >> > wrote:
> >> >>
> >> >> On Fri, Aug 15, 2014 at 5:55 PM, Guido van Rossum <guido at python.org>
> >> >> wrote:
> >> >> ...
> >> >> >> Also... Does None magically mean NoneType in type definitions?
> >> >> >
> >> >> > Yes.
> >> >>
> >> >> This would mean either that `(None | None) is None`, or that (x |
> >> >> None) is not always "optional x".
> >> >> And if type objects grow any other common functionality, None will
> >> >> have to support that as well.
> >> >
> >> >
> >> > Perhaps None itself should not implement any of this, and the __ror__
> >> > method
> >> > on ABCs should implement it. That way, None|Mapping and Mapping|None
> >> > would
> >> > both work, yet None|None would still be the TypeError it is today.
> >>
> >> ... and that (x|None) does not always mean "optional x".
> >> Is this case special enough?
> >
> >
> > I'm not following. The proposal seems to be to add __or__ and __ror__
> > methods to type itself requiring the other argument to be also a type, or
> > the special case None (which is a value, not a type).
>
> My concern is that if someone does programmatic type declaration
> manipulation/generation, there's now a special case to keep in mind.
> Instead of
>
>     def optional(t):
>         return t | None
>
> it's now:
>
>     def optional(t):
>         if t is None:
>             return t
>         else:
>             return t | None
>
> because unlike other type declarations, None doesn't have __or__, or
> any other operation that types will gain in the future as this
> proposal matures.
>
> But maybe this is will never be a valid use case?
>



-- 
--Guido van Rossum (python.org/~guido)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20140815/776456a6/attachment.html>