Odd name shadowing in comprehension

[Terry Reedy]

On 10/22/2016 11:24 PM, Chris Angelico wrote:
> On Sun, Oct 23, 2016 at 2:14 PM, Steve D'Aprano
> <steve+python at pearwood.info> wrote:
>>>> But it seems that the first iterator (and only that one) is evaluated
>>>> in the parent context:
>>>
>>> Because the first iterator *can* always be evaluated.
>>
>> I don't understand what you mean by that. If I take you literally, it is
>> obviously not true:
>>
>> py> [x for x in garglebarblewarble]
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> NameError: name 'garglebarblewarble' is not defined
>>
>> but I'm sure you know that, so I don't understand what you mean by "always".
>
> AIUI, what he means is this:

Right.  You put it better than I did.

> 1) Sometimes, all the iterables can be evaluated in advance.
>
> dice_2d6 = [a+b for a in range(1,7) for b in range(1,7)]
>
> 2) But sometimes, subsequent iterables depend on the outer loop.
>
> triangle = [a+b for a in range(1, 7) for b in range(1, a+1)]
>
> So in case #2, you cannot evaluate the second range until you're
> actually in the loop - but in both cases, the first one can be
> pre-evaluated. Also, the most-outside loop's iterable gets evaluated
> exactly once, whereas inner loops might be evaluated more often (or
> less, for that matter).
>
> Only in extreme edge cases involving name scoping can the evaluation
> of the first iterable depend on whether it's inside or outside the
> invisible function.
>
> ChrisA
>


-- 
Terry Jan Reedy