Direct vs indirect [was Re: why () is () and [] is [] work in other way?]

[Adam Skutt]

On Apr 27, 1:06 pm, Steven D'Aprano <steve
+comp.lang.pyt... at pearwood.info> wrote:
> On Thu, 26 Apr 2012 04:42:36 -0700, Adam Skutt wrote:
> > On Apr 26, 5:10 am, Steven D'Aprano <steve
> > +comp.lang.pyt... at pearwood.info> wrote:
> >> But I was actually referring to something more fundamental than that.
> >> The statement "a is b" is a *direct* statement of identity. "John is my
> >> father." "id(a) == id(b)" is *indirect*: "The only child of John's
> >> grandfather is the parent of the mother-in-law of my sister-in-law"
> >> sort of thing. (Excuse me if I got the relationships mixed up.)
>
> > Again, the fact that you somehow think this absurd family tree is
> > relevant only shows you're fundamentally confused about what object
> > oriented identity means.  That's rather depressing, seeing as I've given
> > you a link to the definition.
>
> Perhaps you failed to notice that this "absurd" family tree, as you put
> it, consists of grandparent+parent+sibling+in-law. What sort of families
> are you familiar with that this seems absurd to you?

No, I noticed, but who talks like that?  It's not remotely comparable
to the sort of difference we're talking about.

>
> I think you have inadvertently demonstrated the point I am clumsily
> trying to make. Even when two expressions are logically equivalent, the
> form of the expressions make a big difference to the comprehensibility of
> the text.

And if we were talking about 30, 20, 5, maybe even 2 line function
versus it's name, you might have a point. We're not talking about such
things though, and it's pretty disingenuous to pretend otherwise.
Yet, that's precisely what you did with your absurd family
relationship.

> Which would you rather read?
>
>    for item in sequence[1:]: ...
>
>    for item in sequence[sum(ord(c) for c in 'avocado') % 183:]: ...
>
> The two are logically equivalent, so logically you should have no
> preference between the two, yes?

No, they're not logically equivalent.  The first won't even execute,
as sequence is undefined.  You need two lines in the first case.

> A statement is "direct" in the sense I mean if it explicitly states the
> thing you intend it to state.

And in the case of the two ways to compare identity, both statements
state exactly what I intend to state.  They're synonyms.

>
> "a is b" is a direct test of whether a is b. (Duh.)
>
> "id(a) == id(b)" is an indirect test of whether a is b, since it requires
> at least three indirect steps: the knowledge of what the id() function
> does, the knowledge of what the == operator does, and the knowledge that
> equal IDs imply identity.

The problem is that using 'is' correctly requires understanding all of
those three things.

Adam