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

Ethan Furman ethan at stoneleaf.us
Wed Apr 25 18:36:02 EDT 2018 
On 04/25/2018 03:15 PM, Ethan Furman wrote:
> On 04/25/2018 02:55 PM, Tim Peters wrote:
>
>> This becomes a question of seasoned judgment.  For example, here's a
>> real loop summing a series expansion, until the new terms become so
>> small they make no difference to the running total (a common enough
>> pattern in code slinging floats or decimals):
>>
>>          while True:
>>              old = total
>>              total += term
>>              if old == total:
>>                  return total
>>              term *= mx2 / (i*(i+1))
>>              i += 2
>>
>> 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.)

Having said that, since whomever mentioned reading ":=" as "which is", I'm good with ":=".

--
~Ethan~
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