[Python-Dev] (name := expression) doesn't fit the narrative of PEP 20

[Guido van Rossum]

On Wed, Apr 25, 2018 at 3:15 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 04/25/2018 02:55 PM, Tim Peters wrote:
>
>> To my eyes, this is genuinely harder to follow, despite its relative
>> brevity:
>>
>>          while total != (total := total + term):
>>              term *= mx2 / (i*(i+1))
>>              i += 2
>>          return total
>>
>> So I wouldn't use binding expressions in that case.  I don't have a
>> compelling head argument for _why_ I find the latter spelling harder
>> to follow, but I don't need a theory to know that I in fact do.
>>
>
> I know why I do:  I see "while total != total" and my gears start
> stripping.  On the other hand,
>
>   while total != (total + term as total):
>      ...
>
> I find still intelligible.  (Yes, I know "as" is dead, just wanted to
> throw that out there.)
>

The problem with either variant is that they hinge on subtle left-to-right
evaluation rules. Python tries to promise left-to-right evaluation "except
when it doesn't apply", e.g. in assignments the RHS is typically evaluated
before subexpressions in the LHS: a[f()] = g() calls g() before f().

The example is supposed to load the left operand to != on the stack before
loading the right operand, but the rule that says the left operand is
evaluated before the right operand is much weaker than other evaluation
order rules (like the rule stating that the arguments are evaluated before
the function is called -- and before you laugh, in Algol-60 that wasn't
always the case).

This argument applies regardless of which syntactic form you use, and no
matter what we choose, the PEP will have to clarify evaluation order in
more cases than the current reference manual. (IIRC Nathaniel brought this
up.)

-- 
--Guido van Rossum (python.org/~guido)
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