[Python-3000] Set literal

Adam Olsen rhamph at gmail.com
Fri Jan 25 07:40:12 CET 2008

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".

Adam Olsen, aka Rhamphoryncus

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