Clearly the PEP should spell out the precedence of :=. I'm sure Chris intended to give := the lowest possible precedence: the analogy with = (both in Python and in other languages), and the original design of the PEP, where the syntax was (<expr> as <var>), with mandatory parentheses. (Probably his implementation helps to guess his intention too, but I can't be bothered yet to clone and build it just to answer this question definitively. :-)
I find the analogy argument compelling, and I think we should just make it a style rule that parentheses should be used whenever it could be misread by a human who's not sure about the precedence. Too bad for that one example. On Thu, Apr 12, 2018 at 9:14 PM, David Mertz <me...@gnosis.cx> wrote: > Yes, I have not read all the iterations of the PEP, and none of them > extremely closely. I had thought it "obvious" that ':=' should have a very > high operator precedence. But of course if it doesn't then expressions like > the one I proposed could mean something quite different. > > I find the third hand argument rather compelling. All those potentially > required parentheses turn elegant looking code into an ugly mess. > > > On Thu, Apr 12, 2018, 11:56 PM Tim Peters <tim.pet...@gmail.com> wrote: > >> [David Mertz <me...@gnosis.cx>] >> > Yes, I should have added ternary expressions to if statements. I can >> > definitely see the use there. >> > >> > However, your example is not null checking. You'd have to modify it >> slightly >> > to get that: >> > >> > None if var:= function() is None else var.method() >> > >> > Still not bad looking. >> >> I couldn't find text in the PEP spelling out precedence, but there are >> two plausible ways that could be grouped: >> >> 1. None if (var:= function()) is None else var.method() >> >> which is what I believe you intended, and >> >> 2. None if var:= (function() is None) else var.method() >> >> which from earlier iterations of this thread I believe is the actual >> meaning - but isn't at all what was intended. >> >> The most clearly related example in the PEP appears to be: >> >> x = "default" if (eggs := spam().ham) is None else eggs >> >> which forced the intended meaning as in #1 above. >> >> While "assignment" is currently a statement type rather than "an >> operator", viewing the current situation as if "=" were an operator it >> has very low precedence, so it would be just as surprising at times to >> boost the precedence of ":=": >> >> if x := i+1: >> >> That is, in _that_ example, >> >> if x := (i+1): >> >> is "clearly intended", not the more tightly binding >> >> if (x ;= i) + 1: >> >> On the third hand, requiring parentheses all the time would also feel >> strained: >> >> while m := someregexp.match(somestring): >> >> is already impossible to misread. >> >> Annoying ;-) >> > > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > > -- --Guido van Rossum (python.org/~guido)
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