[Python-ideas] Re: Generalized deferred computation in Python

On Thu, Jun 23, 2022 at 2:53 AM Chris Angelico <rosuav@gmail.com> wrote:

> On Thu, 23 Jun 2022 at 11:35, Joao S. O. Bueno <jsbueno@python.org.br>
> wrote:
> >
> > Martin Di Paola wrote:
> > > Three cases: Dask/PySpark, Django's ORM and selectq. All of them
> > > implement deferred expressions but all of them "compute" them in very
> > > specific ways (aka, they plan and execute the computation differently).
> >
> >
> > So - I've been hit with the "transparency execution of deferred code"
> dilemma
> > before.
> >
> > What happens is that: Python, at one point will have to "use" an object
> - and that use
> > is through calling one of the dunder methods. Up to that time, like,
> just writing the object name
> > in a no-operation line, does nothing. (unless the line is in a REPL,
> which will then call the __repr__
> > method in the object).
>
> Why are dunder methods special? Does being passed to some other
> function also do nothing? What about a non-dunder attribute?
>

Non-dunder attributes goes through obj.__getattribute__  at which point
evaluation
is triggered anyway.


>
> Especially, does being involved in an 'is' check count as using an object?
>

"is" is not "using', and will be always false or true as for any other
object.
Under this approach, the delayed object is a proxy, and remains a proxy,
so this would have side-effects in code consuming the object.
(extensions expecting strict built-in types might not work with a
proxy for an int or str) - but "is" comparison should bring 0 surprises.

>
> dflt = fetch_cached_object("default")
> mine = later fetch_cached_object(user.keyword)
> ...
> if mine is dflt: ... # "using" mine? Or not?
>
> Does it make a difference whether the object has previously been poked
> in some other way?
>

In this case, "mine" should be a proxy for the evaluation of the call
of "fetch_cached_object" which clearly IS NOT the returned
object stored in "dflt".

This is so little, or so much, surprising as verifying that "bool([])"
yields False:
it just follows the language inner workings, with not special casing.

Of course, this if this proposal goes forward - I am just pointing that the
existing mechanisms in the language can already support it in a way
with no modification. If "is" triggering the resolve is desired, or if
is desired the delayed object should  be replaced "in place", instead
of using a proxy, another approach would be needed - and
I'd favor the "already working" proxy approach I presented here.

(I won't dare touch the bike-shedding about the syntax on this, though)



> ChrisA
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