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

[Stephan Houben]

Hi all,

While we are at it, could

{"a":1, "b":2}.freeze()

perhaps create a MappingProxyType?

I've a gist at
https://gist.github.com/stephanh42/d277170dd8a3a2f026c272a4fda15396
with a stand-alone freeze function which attempts to convert objects to a
read-only
version, and dict -> MappingProxyType is one of the transforms.

Stephan


2018-07-12 7:33 GMT+02:00 Gregory P. Smith <greg at krypto.org>:

>
>
> On Wed, Jul 11, 2018 at 6:13 PM Guido van Rossum <guido at python.org> wrote:
>
>> 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).
>>
>
> Agreed, bytearray(b'...') should be way less common.  I don't immediately
> have a use for that beyond merely than disliking the copy from temporary
> bytes object and gc behind the scenes.  I could find a practical use for it
> in micropython where ram is extremely limited, but that VM could already
> implement such a compile time optimization on its own.  The concept of the
> optimization that'd be required just seemed similar to that of frozenset to
> me.
>
> frozenset is the one that led me down this train of thought as I was
> looking at code declaring a bunch on constants.
>
> -gps
>
>
>> 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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>
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