On Wed, May 9, 2018 at 8:42 PM, Chris Angelico <ros...@gmail.com> wrote:
> On Thu, May 10, 2018 at 1:33 PM, Guido van Rossum <gu...@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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