[Python-3000] Adaptation and type declarations

[Guido van Rossum]

On 4/10/06, Jim Jewett <jimjjewett at gmail.com> wrote:
> Every so often Guido talks about adding optional typing to python.
>
> Adaptation may offer the cleanest way to do this.
>
>
> Turning
>
>     def fn(a, b, c="default"): ...
>
> into any of
>
>     def fn(Seq a,  Index b, Text c="default"): ...
>     def fn(Seq(a),  Index(b), Text(c)="default"): ...
>
> or (wrapped version)
>
>     def fn(Seq a,
>            Index b,
>            Text c="default"): ...
>
> doesn't seem so awful.  (I'm not sure it is a net positive for
> readability, but I'm also not sure it isn't.)  I read the type
> information as "normally this is just an assertion, but I suppose some
> protocols might *make* it true for me."

I do think that we now have enough proposals on the table that require
specifying types (or other metadata!) for each argument of certain
function definitions that it's time to do something about this.

A bit more than a year ago I blogged extensively about this. The only
syntax that is acceptable to me is slightly different; the above would
look like

def fn(a: Seq, b: Index, c: Text = "default"): ...

where Seq, Index and Text can be expressions (the main problem with
the syntax you propose is that the type can't be much more than an
identifier before it gets ambiguous or unreadable).

A completely separate issue is what kind of objects Seq, Index and
Text would be; but that's a discussion we have separate from the
syntactic discussion.

I would imagine that instead of the currently proposed

@foo.register(list, int, str)
def foo_1(a, b, c): ...

we'd be allowed to write

@foo.register
def foo_1(a: list, b: int, c: str): ...

FWIW (to ward of immediate questions) the syntax for an argument would
be something like this: NAME [':' EXPR] ['=' EXPR] where currently it
is NAME ['=' EXPR]. The only defined semantics would be that the name,
the type and the default can all be inspected through the
__signature__ attribute on the function object.

--
--Guido van Rossum (home page: http://www.python.org/~guido/)