[Python-ideas] Short form for keyword arguments and dicts

[Nick Coghlan]

On 27 June 2013 22:10, Joshua Landau <joshua.landau.ws at gmail.com> wrote:
> I would *never* have thought that:
>
>     func(*, arg): pass
>
> meant
>
>     func(arg=arg): pass

It doesn't mean that, since a bare keyword-only argument *must* be
passed in when calling the function (as it has no default value).

> despite that I sort of get the *very slight* (but not really even
> that) symmetry (now that you've explained it).

The "*" in the keyword-only parameter syntax still refers to
positional argument packing. It just leaves the destination undefined
to say "no extra positional arguments are allowed" rather than "any
extra positional arguments go here".

The parameters that appear after that entry are keyword only solely by
a process of elimination - if all the positional arguments have been
consumed (or disallowed), then any parameters that are left *have* to
be passed as keywords, since positional and keyword are the only two
argument passing options we offer.

On the calling side, we've actually toyed with the idea of removing
the argument ordering restrictions, because they imply a parallel with
the parameter definitions that doesn't actually exist. We haven't
though, as it's significant extra work for no compelling benefit.

As far as this whole thread goes, though, we're still in the situation
where the simplest mechanisms Python currently has to extract a submap
from a mapping are a dict comprehension and operator.itemgetter. These
proposed changes to function calling syntax then target a very niche
case of that broader problem, where the mapping is specifically the
current local variables and the destination for the submap is a
function call with like named parameters. This is *not* the calibre of
problem that prompts us to make Python harder to learn by adding new
syntax.

Cheers,
Nick.

--
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia