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devel / comp.lang.python / Re: Why operations between dict views return a set and not a frozenset?

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o Re: Why operations between dict views return a set and not aMarco Sulla

1
Re: Why operations between dict views return a set and not a frozenset?

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From: Marco.Su...@gmail.com (Marco Sulla)
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Subject: Re: Why operations between dict views return a set and not a
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 by: Marco Sulla - Tue, 11 Jan 2022 18:49 UTC

Ok... so I suppose, since you're inviting me to use dis and look at the
bytecode, that are you talking about constants in assembly, so const in C?
Sorry for the confusion, I'm not so skilled in C and I know nearly nothing
about assembly. Furthermore I never look at the bytecode of any language
before, so I simply didn't understand you.

I think this is what you mean:

>>> dis.dis("for _ in {1, 2}: pass")
1 0 SETUP_LOOP 12 (to 14)
2 LOAD_CONST 3 (frozenset({1, 2}))
4 GET_ITER
>> 6 FOR_ITER 4 (to 12)
8 STORE_NAME 0 (_)
10 JUMP_ABSOLUTE 6
>> 12 POP_BLOCK
>> 14 LOAD_CONST 2 (None)
16 RETURN_VALUE
>>> a = {1, 2}
>>> dis.dis("for _ in a: pass")
1 0 SETUP_LOOP 12 (to 14)
2 LOAD_NAME 0 (a)
4 GET_ITER
>> 6 FOR_ITER 4 (to 12)
8 STORE_NAME 1 (_)
10 JUMP_ABSOLUTE 6
>> 12 POP_BLOCK
>> 14 LOAD_CONST 0 (None)
16 RETURN_VALUE

On Tue, 11 Jan 2022 at 01:05, Chris Angelico <rosuav@gmail.com> wrote:

> On Tue, Jan 11, 2022 at 10:26 AM Marco Sulla
> <Marco.Sulla.Python@gmail.com> wrote:
> >
> > On Wed, 5 Jan 2022 at 23:02, Chris Angelico <rosuav@gmail.com> wrote:
> > >
> > > On Thu, Jan 6, 2022 at 8:01 AM Marco Sulla <
> Marco.Sulla.Python@gmail.com> wrote:
> > > >
> > > > On Wed, 5 Jan 2022 at 14:16, Chris Angelico <rosuav@gmail.com>
> wrote:
> > > > > That's an entirely invisible optimization, but it's more than just
> > > > > "frozenset is faster than set". It's that a frozenset or tuple can
> be
> > > > > stored as a function's constants, which is a massive difference.
> > > >
> > > > Can you explain this?
> > >
> > > Play around with dis.dis and timeit.
> >
> > ? I don't understand. You're talking about function constants. What
> > are they? I can't dig deep into something if I can't know what it is.
> > Maybe are you talking about function default values for parameters?
>
> No, I'm talking about constants. Every function has them.
>
> > Of course. You can use a proxy and slow down almost everything much
> > more. Or you can simply create a version of the mutable object with
> > fewer methods, as more or less frozenset is. I checked the
> > implementation, no fast iteration is implemented. I do not understand
> > why in `for x in {1, 2, 3}` the set is substituted by a frozenset.
>
> Constants. Like I said, play around with dis.dis, and explore what's
> already happening. A set can't be a constant, a frozenset can be.
> Constants are way faster than building from scratch.
>
> Explore. Play around. I'm not going to try to explain everything in detail.
>
> If you're delving into the details of the C implementation of the
> dictionary, I would have expected you'd already be familiar with the
> way that functions behave.
>
> ChrisA
> --
> https://mail.python.org/mailman/listinfo/python-list
>

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