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

[Chris Angelico]

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