[Python-ideas] PEP 572: about the operator precedence of :=

[Guido van Rossum]

On Wed, May 9, 2018 at 8:42 PM, Chris Angelico <rosuav at gmail.com> wrote:

> On Thu, May 10, 2018 at 1:33 PM, Guido van Rossum <guido at python.org>
> wrote:
> > (I vaguely recall this has been brought up before, but I'm too lazy to
> find
> > the subtread. So it goes.)
> >
> > PEP 572 currently seems to specify that when used in expressions, the
> > precedence of `:=` is lower (i.e. it binds more tightly) than all
> operators
> > except for the comma. I derive this from the single example `stuff = [[y
> :=
> > f(x), x/y] for x in range(5)]`.
> >
> > From this it would follow that `f(a := 1, a)` is equivalent to `a = 1;
> f(1,
> > 1)`, and also that `(a := 1, a)` is equivalent to `a = 1; (1, 1)`.
> (Although
> > M.A.L. objected to this.)
> >
> > But what should `a := 1, 1` at the top level (as a statement) do? On the
> one
> > hand, analogy with the above suggest that it is equivalent to `a = 1; (1,
> > 1)`. But on the other hand, it would be really strange if the following
> two
> > lines had different meanings:
> >
> >     a = 1, 1   # a = (1, 1)
> >     a := 1, 1  # a = 1; (1, 1)
> >
> > I now think that the best way out is to rule `:=` in the top level
> > expression of an expression statement completely (it would still be okay
> > inside parentheses, where it would bind tighter than comma).
>
> I would have := bind more tightly than the comma. Consider:
>
> a = 1, x := 2, 3
>
> IMO the only sane interpretation is "x = 2; a = 1, 2, 3". Effectively,
> the := operator does not like to play with commas; we've already ruled
> out "a, b := range(2)" as a means of unpacking, so it makes more sense
> to have that simply mean "b = range(2); a, b".
>

Oh, I hadn't even though of combining the two in one statement. That
example is truly horrible (on first skim I didn't even notice it used two
different assignment operators!) and strengthens my confidence that we
should just disallow an un-parenthesized `:=` operator at the top level,
where now the top level includes the RHS of a classic assignment.

-- 
--Guido van Rossum (python.org/~guido)
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