[Python-Dev] PEP 572: Assignment Expressions

[David Mertz]

I do think the pronunciation issue that Greg notices is important.  I teach
Python for most of my living, and reading and discussing code segments is
an important part of that.  When focussing on how Python actually *spells*
something, you can't always jump to the higher-level meaning of a
construct.  For some complex expression—whether or not "binding
expressions" are added—sometimes it makes sense to give a characterization
of the *meaning* of the expression, but other times you want to say aloud
the entire spelling of the expression.

Although feelings are mixed about this, I like the "dunder" contraction for
this purpose.  It's less of a mouthful to say "dunder-init" than
"underscore-underscore-init-underscore-underscore" aloud.  And once you
learn that shorthand, it's unambiguous.

I think I'd pronounce:

    if (diff := x - x_base) and (g := gcd(diff, n)) > 1:
        return g

As:

"If diff bound to x minus x_base (is non-zero), and g bound to gcd of diff
comma n is greater than 1, return g"

But having a convention for pronouncing this would be nice, rather than it
being my idiosyncrasy.


On Mon, Apr 23, 2018 at 8:23 PM, Tim Peters <tim.peters at gmail.com> wrote:

> [Tim]
> >> if (diff := x - x_base) and (g := gcd(diff, n)) > 1:
> >>     return g
>
> [Greg Ewing <greg.ewing at canterbury.ac.nz>]
> > My problem with this is -- how do you read such code out loud?
>
> In the message in which I first gave that example:
>
>     if the diff isn't 0 and gcd(diff, n) > 1, return the gcd.
>    That's how I _thought_ of it from the start.
>
> In my mind, `x - x_base` doesn't even exist except as a low-level
> definition of what "diff" means.  It's different for the other test:
> _there_ `g` doesn't exist except as a shorthand for "the gcd".  In one
> case it's the name that's important to me, and in the other case the
> expression.  The entire function from which this came is doing all
> arithmetic modulo `n`, so `n` isn't in my mind either - it's a
> ubiquitous part of the background in this specific function.
>
> But you did ask how_I_ would read that code ;-)  Anyone else is free
> to read it however they like.  I naturally read it in the way that
> makes most sense to me in its context.
>
>
> > From my Pascal days I'm used to reading ":=" as "becomes". So
> > this says:
> >
> >    "If diff becomes x - base and g becomes gcd(diff, n) is
> >     greater than or equal to 1 then return g."
> >
> > But "diff becomes x - base" is not what we're testing!
>
> I don't really follow that.  In Python,
>
>     if f() and g > 1:
>
> first tests whether `f()` "is truthy", regardless of whether it does
> or doesn't appear in a binding expression.  Because this code is
> working with integers, there's an _implied_ "!= 0" comparison.
>
>
> > That makes it sound like the result of x - base may or may not
> > get assigned to diff, which is not what's happening at all.
>
> Then I suggest the problem you're having doesn't stem from the binding
> expression, but from that you're omitting to fill in the != 0 part:
> if you're not thrown by "greater than 1", I can't see how you can be
> thrown by "not zero".
>
>
> > The "as" variant makes more sense when you read it as an
> > English sentence:
> >
> >    if ((x - x_base) as diff) and ...
> >
> >    "If x - x_base (and by the way, I'm going to call that
> >     diff so I can refer to it later) is not zero ..."
>
> So read the original as "if diff (which is x - x_base) is not zero ...".
>
> Regardless, Guido has already said "as" is DOA (Dead On Arrival)
> (illustrating that it's also common enough in English to give a short
> name before its long-winded meaning ;-) ).
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