[Python-ideas] optional parameters

[Guido van Rossum]

This thread is derailing quickly. :-)

Does anyone remember why the marker pattern was invented? (That's a
rhetorical question, in case you wondered. :-)


On Wed, Apr 16, 2014 at 11:22 AM, Zachary Ware <
zachary.ware+pyideas at gmail.com> wrote:

> On Wed, Apr 16, 2014 at 12:09 PM, Yury Selivanov
> <yselivanov.ml at gmail.com> wrote:
> > Hello,
> >
> > There is a very common pattern for creating optional arguments
> > when you can't use None:
> >
> > _optional = object()
> > def foo(*, arg1='spam', arg3=None, arg4=_optional):
> >     if arg4 is _optional:
> >         # caller didn't pass *anything* for arg4
> >     else:
> >         # caller did pass some (maybe None) value for arg4
> >
> > It's a bit annoying to create this marker objects, and also,
> > if you try to render a signature of such function, you'll get
> > something like:
> >
> > "(*, arg1='spam', arg3=None, arg4=<object object at 0x104be7080>)"
> >
> > What if we add a standard marker for this use-case:
> > functools.optional or inspect.Parameter.optional?
>
> Has Ellipsis (...) been suggested and rejected for this use before?
> It's not an absolutely perfect fit, but it looks better to me:
>
> def foo(*, arg1='spam', arg3=None, arg4=...):
>     if arg4 is Ellipsis:
>         # caller didn't pass anything for arg4
>         ...
>     else:
>         # caller passed something, possibly None, for arg4
>         ...
>
> The signature would look like "(*, arg1='spam', arg3=None,
> arg4=Ellipsis)", unless inspect.Signature.__str__ special-cased
> Ellipsis to be "..." (or "<optional>" if we officially blessed
> Ellipsis for the purpose).
>
> --
> Zach
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>



-- 
--Guido van Rossum (python.org/~guido)
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