On Sun, Jan 16, 2022 at 04:43:36PM -0800, Christopher Barker wrote:
> I’m a bit confused — would adding a “literal” form for frozenset provide
> much, if any, of an optimization?
Yes. In at least some cases, it would avoid going through the song and
dance:
1. create a frozenset
2. convert the frozenset to a regular set
3. convert the regular set back to a frozenset
4. garbage collect the regular set
See the b.p.o. ticket referenced earlier, as well as the disassembled
code.
In other cases it would avoid:
1. create a set, tuple or list
2. create a frozenset
3. garbage collect the set, tuple or list
It would also avoid the name lookup of `frozenset`, and guarantee that
even if that name was shadowed or monkey-patched, you still get a
genuine frozenset.
(Just as [1, 2, 3] is guaranteed to return a genuine list, even if the
name "list" is deleted, shadowed or replaced.)
At the moment, sets and frozensets still share the same implementation,
but some years ago Serhiy suggested that he had some optimizations in
mind that would make frozensets smaller than regular sets.
> How often do folks need a frozen set literal? I don’t think I’ve ever used
> one.
If you are writing `if x in ("this", "that", "another", "more")` then
you probably should be using a frozenset literal, since membership
testing in sets is faster than linear search of a tuple.
I think that the CPython peephole optimizer actually replaces that
tuple with a frozenset, which is cool, but you can defeat that
optimization and go back to slow linear search by refactoring the code
and giving the targets a name:
targets = ("this", "that", "another", "more")
if x in targets: ...
> If we did, then f{‘this’: ‘that’} should make a frozen dict, yes?
We would have to get a frozen dict first, but if we did, that would be
an obvious syntax to use.
--
Steve
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