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

[Guido van Rossum]

On 5/22/06, Collin Winter <collinw at gmail.com> wrote:
> On 5/22/06, Josiah Carlson <jcarlson at uci.edu> wrote:
> > "Guido van Rossum" <guido at python.org> wrote:
> > > On 5/22/06, Collin Winter <collinw at gmail.com> wrote:
> > > > One thing I've run into several times in this process is the use of
> > > > *args as a dict key. As several others have mentioned, the "tuple ==
> > > > fixed-length, list == arbitrary-length" distinction means we now need
> > > > a way to create hashable lists. I propose a new, hashable frozenlist
> > > > built-in type to fill this niche.
> > > >
> > > > This type would subclass from list, overriding __getitem__,
> > > > __delitem__, __hash__ and other methods as appropriate.
> > > >
> > > > Thoughts? Does this need to go through a full PEP process?
> > >
> > > No, no, no!
> >
> > Or to clarify Guido: use a tuple.  Tuples are also arbitrary-length, and
> > serve the same purposes that any 'frozen list' equivalent would.
>
> In Python 2, sure, but if Python 3 introduces the idea that tuples
> should be used for fixed-length structures (since Guido has said that
> tuple[Number, Number] should be a 2-tuple of Numbers), then something
> else needs to fill the "hashable, arbitrary-length ordered container"
> niche.

No. This distinction is in your mind. The tuple type can easily fulfill both.

> The main use case I'm thinking of is parameterizing mapping types. Say
> I wanted a set of Number lists: in Python 2, I'd just convert the
> lists to tuples before adding them to the set. However, in Python 3,
> converting the lists to tuples will cause any annotations like
> set[list[Number]] to reject the candidate set. I'd like to avoid
> having annotations like set[tuple & EveryElementIs(Number)]. Also,
> before anyone proposes it, I don't think "just remove the offending
> annotations" is a viable solution.

You should be able to declare it as set[ImmutableSequence[Number]] and
a set containing tuples would match that.

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