[Python-ideas] anyone need a frozenset or bytearray literal?

[Guido van Rossum]

I know of many use cases for frozenset({...}) and I think a hack along
those lines is fine -- but what's the common use for bytearray(b"...") or
bytearray((...))? Almost invariably a bytearray is created empty or as a
given number of zeros. I certainly wouldn't want to burden the tuple type
with a to_bytearray() method, the types are unrelated (unlike set and
frozenset).

On Wed, Jul 11, 2018 at 6:03 PM, Robert Vanden Eynde <
robertvandeneynde at hotmail.com> wrote:

> {1,2,7}.freeze() or {1,2,7}.to_frozenset() seem very natural and if this
> can be optimized to avoid the copy, it's perfect.
> For bytearray, one use case would be to optimise bytearray([1,2,7,2]) in
> something like [1,2,7,2].to_byterray().
> About bytes, one could have (1,2,7,2).to_bytes() instead of
> bytes((1,2,7,2)) because b'\x01\x02\x07\x02' is long and boring.
> What about variables in the values {1,2,x}.freeze() should work too ?
> bytes((1,2,7,x)) is not writable as a b string and creates a copy.
>
>
> Le jeu. 12 juil. 2018 à 02:24, Chris Angelico <rosuav at gmail.com> a écrit :
>
>> On Thu, Jul 12, 2018 at 10:13 AM, Gregory P. Smith <greg at krypto.org>
>> wrote:
>> >
>> > On Wed, Jul 11, 2018 at 4:45 PM Jelle Zijlstra <
>> jelle.zijlstra at gmail.com>
>> > wrote:
>> >> 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.
>>
>> Be careful: a bytearray is mutable, so this isn't open to very many
>> optimizations. A .freeze() method on sets would allow a set display to
>> become a frozenset "literal", stored as a constant on the
>> corresponding function object, the way a tuple is; but that's safe
>> because the frozenset doesn't need to concern itself with identity,
>> only value. Example:
>>
>> def f(x):
>>     a = (1, 2, 3) # can be optimized
>>     b = (x, 4, 5) # cannot
>>     c = [6, 7, 8] # cannot
>>
>> Disassemble this or look at f.__code__.co_consts and you'll see (1, 2,
>> 3) as a single constant; but the others have to be built.
>>
>> +1 on set.freeze(); +0 on bytes.to_bytearray().
>>
>> ChrisA
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-- 
--Guido van Rossum (python.org/~guido)
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