[Python-3000] Set literals (original) (raw)

Guido van Rossum guido at python.org
Mon Aug 28 22:14:17 CEST 2006

On 8/28/06, Georg Brandl <g.brandl at gmx.net> wrote:

Guido van Rossum wrote:

>> > Georg, can you do something about repr() of an empty set? This >> > currently produces "{}" while it should produce "set()". >> >> Right, forgot about that case. I'll correct that now. >> (Grr, I even mindlessly changed the unittest that would have caught it) > > Checkin? Done. It now also renders repr(frozenset()) as "frozenset()", which should cause no problems though.

Thanks -- looks good!

>> In the meantime, I played around with the peepholer and tried to copy >> the "for x in tupleorlist" optimization for sets. Results are in SF >> patch #1548082. >> >> >> Set comprehensions are not implemented. >> > >> > ETA? >> >> There are some points I'd like to have clarified first: >> >> * would it be wise to have some general listcomp <-> genexp >> cleanup first? This starts with the grammar, which currently is slightly >> different (see Grammar:79), and it looks like there's quite a lot of >> (almost) duplicated code in ast.c and compile.c too. > > I expec this cleanup to be quite a bit of work since the semantics are > seriously different. ([...] uses the surrounding scope for the loop > control variables.)

I didn't say that I wanted to champion that cleanup ;)

That's fine!

> However you might be able to just cleanup the grammar so they are > identical, that would be simpler I suspect.

Looking at the grammar, there's only testlistsafe left to kill, in favor of ortest like in generator expressions. The old rules are still needed.

Hm, it's been so long... Why?

Hm. Is the precedence in

x = lambda: 1 if 0 else 2 really obvious?

Yes if you think about how you would use it. Conditionally returning a lambda or something else is kind of rare. A lambda using a condition is kind of useful. :-)

>> * list comprehensions are special-cased because of the LISTAPPEND opcode. >> If there isn't going to be a special-cased SETADD, it's probably the >> easiest thing to transform {x for x in a} into set(x for x in a) in the >> AST step, with "set" of course always being the builtin set. > > Right. That might actually become a prototype for how to the list > translation as well.

Would this need a new opcode, or should generators be special-cased by BUILDSET?

Can't remember what BUILD_SET is.

Which doesn't seem like a good idea because it means that {(x for x in iterable)} == {x for x in iterable}

That should definitely not happen!

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

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