+1 I would appreciate being able to create a frozenset using f{1, 2, 3}.
This will also make the `repr` of frozensets shorter.On Sun, Jan 16, 2022 at 10:33 AM Steven D'Aprano <st...@pearwood.info> wrote: > Inspired by this enhancement request: > > https://bugs.python.org/issue46393 > > I thought it might be time to revist the idea of a frozenset display. > This has been discussed a few times before, such as here: > > https://mail.python.org/pipermail/python-ideas/2018-July/051902.html > > We have displays for the most important builtin data structures: > > - lists [1, 2, 3] > - tuples (1, 2, 3) > - dicts {1: 0, 2: 0, 3: 0} > - sets {1, 2, 3} > > (as well as literals for ints, floats, strings and bytes) > > but not for frozensets. So the only way to guarantee that you have a > frozenset is to explicitly call the builtin, which is not only a runtime > call rather than a compile-time operation, but can be monkey-patched or > shadowed. > > CPython already has some neat optimizations in place to use frozensets > instead of sets, for example: > > >>> import dis > >>> dis.dis("x in {1, 2, 3}") > 1 0 LOAD_NAME 0 (x) > 2 LOAD_CONST 0 (frozenset({1, 2, 3})) > 4 CONTAINS_OP 0 > 6 RETURN_VALUE > > and the compiler can build frozensets of literals as a constant. > Ironically, this means that actually making a frozenset explicitly does > far more work than needed: > > >>> dis.dis("frozenset({1, 2, 3})") > 1 0 LOAD_NAME 0 (frozenset) > 2 BUILD_SET 0 > 4 LOAD_CONST 0 (frozenset({1, 2, 3})) > 6 SET_UPDATE 1 > 8 CALL_FUNCTION 1 > 10 RETURN_VALUE > > Got that? To create a frozenset of literals, first the compiler creates > a frozenset constant containing what you wanted. Then at runtime, it: > > - looks up frozenset in globals and builtins; > - loads the pre-prepared frozenset (which is exactly what we want); > - creates a new set from that frozenset; > - calls the frozenset() function on that set to create a new frozenset > that duplicates the pre-prepared one; > - and finally garbage-collects the temporary set. > > So to get the frozenset we want, we start with the frozenset we want, > and make an unnecessary copy the long way o_O > > If you didn't know that every step in that song and dance routine was > necessary, it would seem ludicrously wasteful. > > If we had a frozenset display, we could avoid most of that work, > optimizing that down to a LOAD_CONST like this: > > >>> dis.dis('(1, 2, 3)') > 1 0 LOAD_CONST 0 ((1, 2, 3)) > 2 RETURN_VALUE > > It seems to me that all of the machinery to make this work already > exists. The compiler already knows how to create frozensets at > compile-time, avoiding the need to lookup and call the frozenset() > builtin. All we need is syntax for a frozenset display. > > How does this work for you? > > f{1, 2, 3} > > > -- > Steve > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/GRMNMWUQXG67PXXNZ4W7W27AQTCB6UQQ/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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