[Python-ideas] Start argument for itertools.accumulate() [Was: Proposal: A Reduce-Map Comprehension and a "last" builtin]

[Tim Peters]

Top-posting just to say I agree with Nick's bottom line (changing the
name to `first_result=`).  I remain just +0.5, although that is up a
notch from yesterday's +0.4 ;-)

--- nothing new below ---

On Sun, Apr 8, 2018 at 12:19 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 8 April 2018 at 14:31, Guido van Rossum <guido at python.org> wrote:
>> Given that two respected members of the community so strongly disagree
>> whether accumulate([], start=0) should behave like accumulate([]) or like
>> accumulate([0]), maybe in the end it's better not to add a start argument.
>> (The disagreement suggests that we can't trust users' intuition here.)
>
> The potential ambiguity I see is created mainly by calling the
> proposed parameter "start", while having it do more than just adjust
> the individual partial sum calculations by adding an extra partial
> result to the output series.
>
> If it's called something else (e.g. "first_result"), then the
> potential "sum(iterable, start=start)" misinterpretation goes away,
> and it can have Tim's desired effect of defining the first output
> value (effectively prepending it to the input iterable, without the
> boilerplate and overhead of actually doing so).
>
> A name like "first_result" would also make it clearer to readers that
> passing that parameter has an impact on the length of the output
> series (since you're injecting an extra result), and also that the
> production of the first result skips calling func completely (as can
> be seen in Tim's str coercion example).
>
> So where I'd be -1 on:
>
>     >>> list(accumulate(1, 2, 3))
>     [1, 3, 6]
>     >>> list(accumulate(1, 2, 3, start=0))
>     [0, 1, 3, 6]
>     >>> list(accumulate(1, 2, 3, start=1))
>     [1, 2, 4, 7]
>
> I'd be +1 on:
>
>     >>> list(accumulate(1, 2, 3))
>     [1, 3, 6]
>     >>> list(accumulate(1, 2, 3, first_result=0))
>     [0, 1, 3, 6]
>     >>> list(accumulate(1, 2, 3, first_result=1))
>     [1, 2, 4, 7]
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia