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.)

On Sat, Apr 7, 2018 at 9:14 PM, Nick Coghlan <ncogh...@gmail.com> wrote: > On 8 April 2018 at 13:17, Tim Peters <tim.pet...@gmail.com> wrote: > > [Nick Coghlan <ncogh...@gmail.com>] > >> So I now think that having "start" as a parameter to one but not the > >> other, counts as a genuine API discrepancy. > > > > Genuine but minor ;-) > > Agreed :) > > >> Providing start to accumulate would then mean the same thing as > >> providing it to sum(): it would change the basis point for the first > >> addition operation, but it wouldn't change the *number* of cumulative > >> sums produced. > > > > That makes no sense to me. `sum()` with a `start` argument always > > returns a single result, even if the iterable is empty. > > > >>>> sum([], 42) > > 42 > > Right, but if itertools.accumulate() had the semantics of starting > with a sum() over an empty iterable, then it would always start with > an initial zero. > > It doesn't - it starts with "0+first_item", so the length of the > output iterator matches the number of items in the input iterable: > > >>> list(accumulate([])) > [] > >>> list(accumulate([1, 2, 3, 4])) > [1, 3, 6, 10] > > That matches the output you'd get from a naive O(n^2) implementation > of cumulative sums: > > data = list(iterable) > for stop in range(1, len(iterable)): > yield sum(data[:stop]) > > So if the new parameter were to be called start, then I'd expect the > semantics to be equivalent to: > > data = list(iterable) > for stop in range(1, len(iterable)): > yield sum(data[:stop], start=start) > > rather than the version Raymond posted at the top of the thread (where > setting start explicitly also implicitly increases the number of items > produced). > > That concern mostly goes away if the new parameter is deliberately > called something *other than* "start" (e.g. "prepend=value", or > "first=value"), but it could also be addressed by offering a dedicated > "yield_start" toggle, such that the revised semantics were: > > def accumulate(iterable, func=operator.add, start=0, > yield_start=False): > it = iter(iterable) > total = start > if yield_start: > yield total > for element in it: > total = func(total, element) > yield total > > That approach would have the advantage of making the default value of > "start" much easier to document (since it would just be zero, the same > as it is for sum()), and only the length of the input iterable and > "yield_start" would affect how many partial sums were produced. > > Cheers, > Nick. > > -- > Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > -- --Guido van Rossum (python.org/~guido)

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