[Python-ideas] A comprehension scope issue in PEP 572

[Matt Arcidy]

On Sun, May 6, 2018 at 7:37 PM, Matt Arcidy <marcidy at gmail.com> wrote:
>> Personally, I'd still like to go back to := creating a statement-local
>> name, one that won't leak out of ANY statement. But the tide was
>> against that one, so I gave up on it.
>
> yes.
>
> I have some probably tangential to bad arguments but I'm going to make
> them anyways, because I think := makes the most sense along with SLNB.
>
> first, := vs post-hoc (e.g. where or given)
>
> base case:
> [ x for x in range(1) ]
> while obvious to all of us, reading left to right does not yield what
> x is till later.
> [ (x, y) for x in range(1) for y in range(1) ] doubly so.
> If x or y were defined above, it would not be clear until the right
> end if what contex they had.
>
> [ (x, y) for x in range(n) given y = f(n) ]
> i dont know what's the iterator till  after 'for'
>
> [ (x, y:=f(n) for x in range(n) ]
> At a minimum, I learn immediately that y is not the iterator.
> Slightly less cognitive load.
>
> it's not that one is better, or that either is unfamiliar, it's about
> having to hold a "promise" in my working memory, vs getting an
> immediate assignment earlier.  (it's a metric!)
>
> now my silly argument.
> ":" is like a "when" operator.
>
> if y==x:
keyboard fail.

if y == x:
   # execute when y == x

for x in y:
    # execute when x in y

class/def f:
    # execute when in f

x := 1  # x = 1 when in local scope

Stretch!  told you ti was silly.
>
>
>
>>
>> ChrisA
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