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

[Ethan Furman]

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.)

--
~Ethan~