[Python-3000] Type parameterization (was: Re: Type annotations: annotating generators)

[Collin Winter]

On 5/19/06, Guido van Rossum <guido at python.org> wrote:
> On 5/18/06, Collin Winter <collinw at gmail.com> wrote:
> > I'd like to see the typechecking machinery ignore differences like
> > "real type" v "pseudo type", looking only at whether, zB, a
> > __parameterize__ method is present. Using __parameterize__ method (or
> > some other name) allows maximum flexibility with respect to type
> > parameterization.
> >
> > For example, the built-in list type might not allow the patterned
> > parameterization I asked about above. A user might choose to subclass
> > list to provide this, giving the subclass a __parameterize__ method
> > something like:
> >
> > @classmethod
> > def __parameterize__(cls, *vargs):
> >     ....
>
> I'm not sure what you're up to here -- maybe it's a reference to
> something you did in your own system? But I agree that that kind of
> subclassing (perhaps at the metaclass level) ought to be possible.

I'm just trying to flesh out how type parameterization will work under
the covers. What I'm proposing is that list[int] would desugar to
list.__parameterize__(int). In context:

def foo(a: int, b: list[int]) -> int

desugars to

def foo(a: int, b: list.__parameterize__(int)) -> int

The __parameterize__ methods take the parameter(s) as arguments and
return an object that can be used to verify that an object is a list
with int elements.

I've implemented a demo of what I'm driving at, using your
metaclass-with-__getitem__ method, taking a PatternedList class as a
quick example [1].

My goal with this is to keep user-defined annotation classes on the
same level as the built-in types, to prevent them from becoming
second-class citizens in this system.

Collin Winter

[1] - http://tinyurl.com/egabj