[Python-3000] Set literal

[Collin Winter]

On Jan 24, 2008 10:40 PM, Adam Olsen <rhamph at gmail.com> wrote:
> On Jan 24, 2008 5:12 PM, Raymond Hettinger <python at rcn.com> wrote:
> > > Looking over the code base, frozensets are used rarely.
> > > So I don't think this is warranted.
> >
> > There is no shortage for perfect use cases in the form:
> >
> >    if urlext in {'html', 'xml', 'php'}:
> >          . . .
> >
> >
> > If the curly braces are taken to mean a frozenset,
> > then the peepholer can code the whole thing as a
> > compile time constant.  It is fast and clean.
> > You need not even be aware that you're using a
> > frozenset.
>
> Is there any reason preventing it from being optimized even with a
> mutable set?  urlext only interacts with the items in the set, and
> doesn't get to see the set itself, so I don't see why the "in"
> operator couldn't trigger transforming it into a constant.
>
> The same argument applies to list literals.  The only limitation I see
> is whether or not calling hash() on urlext is considered an essential
> part of the semantics - and I'm inclined to say "no".

You're asking if it would be possible to automatically turn "x in [a,
b, c]" into "x in {a, b, c}"? If so, the answer is no: x may be
unhashable.

Collin Winter