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". ChrisA _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
