On Wed, Jul 11, 2018 at 4:45 PM Jelle Zijlstra <jelle.zijls...@gmail.com>
wrote:
> 2018-07-11 16:25 GMT-07:00 Gregory P. Smith <g...@krypto.org>:
>
>> Completely obvious what it does, but it irritates my aesthetic
>> sensibilities every time I see:
>> frozenset({spam, eggs})
>>
>> Why? Because I assume under the hood that creates a set of spam and eggs
>> before calling frozenset to copy it into a new frozenset object before the
>> original set is garbage collected. Wasteful. This is in fact what happens
>> in CPython 3.7 today.
>>
>> I'd love to avoid this. I have no rational measured reason to believe it
>> even matters (thus seeding this on python-ideas and not elsewhere), even
>> though it would technically speed up frozenset creation.
>>
>> (a) detecting frozenset({}) as syntax to encode a frozenset in the python
>> bytecode would be somewhat magical. it could break the person unfortunate
>> enough to monkeypatch out the frozenset builtin (really? don't do that!).
>>
>> (b) abusing the concept of letter prefixes as we already have for strings
>> on {} syntax would be possible but not at all obvious to a reader:
>>
>> f{} or c{} or r{} perhaps. but then someone would want a frozendict.
>>
>> (c) adding a .freeze() method to sets which would raise an exception if
>> the set's refcount were > 1 and would mutate the type of the set object
>> into a frozenset object in place. refcount assertions are bad, not all VMs
>> need refcounts. The concept of a method that can mutate the type of the
>> underlying object in place is... unpythonic. even though technically
>> possible to implement within CPython.
>>
>> This could be done safely and without too much craziness if .freeze() on
> a set returned a new frozenset. The compiler could then safely optimize {a,
> set, literal}.freeze() into a frozenset literal, because methods on builtin
> types cannot be monkeypatched. There's been talk of a similar optimization
> on calls to .format() on string literals (not sure whether it's been
> implemented).
>
> Whether implementing that is a good use of anyone's time is a different
> question.
>
Neat optimization. I hadn't considered that. We do know for sure it is a
builtin type at that point.
If that were implemented, bytes objects could gain a to_bytearray() (along
the lines of the int.to_bytes() API) method that could be optimized away in
literal circumstances.
-gps
>
>> I'm -1 on all of my ideas above. But figured I'd toss them out there as
>> food for thought for the concept.
>>
>> We lived for years without even having a set literal in the first place.
>> So this isn't a big deal.
>>
>> frozenset is not the only base type that lacks a literals leading to
>> loading values into these types involving creation of an intermediate
>> throwaway object: bytearray. bytearray(b'short lived bytes object')
>>
>> I was going to suggest complex was in this boat as well, but upon
>> investigation we appear to do constant folding (or amazing parsingon that
>> so 0.3+0.6j does turn into a single LOAD_CONST instead of two consts and an
>> addition. Nice! Not that I expect practical code to use complex numbers.
>>
>> -gps
>>
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>>
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