[Python-ideas] PEP 572: Assignment Expressions (post #4)

[Guido van Rossum]

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 <mertz at 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.peters at gmail.com> wrote:
>
>> [David Mertz <mertz at 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 ;-)
>>
>
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-- 
--Guido van Rossum (python.org/~guido)
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