[Python-3000] set literals

[Michael Chermside]

Guido writes:
> I've also sometimes thought of unifying dicts and sets by implementing
> set operations on the keys of dicts only.
    [... much discussion ...]
> I'm still very much undecided but I don't want to rule this out for
> py3k. Perhaps I'll write up a PEP and see how it flies.

Playing with it, and PEPing it both sound fine, but I think DOING it
seems like a bad idea.

I see two advantages. One is public: it solves the issue of a set
literal. The other is private: it allows us to reuse the implementation.

Fixing the set literal just isn't sufficient justification, IMHO. And
as for the implementation, we care VERY much about perfectly tuning
the performance of the dict type, because its performance is so key to
the implementation of namespaces throughout Python. So I would not want
to accept any unnecessary restrictions on the implementation that might
constrain future optimizations of dict performance.

Besides, how difficult is it to maintain the existing C implementation
of set and frozenset (now that they're written and have been through
the wringer of being in a production release). It's not zero cost, but
it's also probably not THAT big.

Of course, that's the idea behind trying it out and even writing a PEP
-- then we'll see whether my guess (or yours!) is correct.

-- Michael Chermside