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

[Chris Angelico]

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".

ChrisA