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

[Kirill Balunov]

2018-04-08 8:19 GMT+03:00 Nick Coghlan <ncoghlan at gmail.com>:

> 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]
>
>
It is a fair point! But the usual way to understand how to use an
additional argument, is to try it or to look examples in the documentation.

Concerning the topic of relationship between `sum` and `accumulate` I have
another point of view. If `start` means something to the `sum`, there are
no grounds for believing that it should mean the same for
`accumulate`, these functions are not really comparable and fundamentally
different. The closest `sum`s friend is `functools.reduce` which uses
`initial` instead of `start`. ( the documentation uses `initializer` and
the docstring uses `initial`, as for me I prefer the `initial`) and so
there is already a discrepancy.

Having said this I think that it is no matter how it will be named `start`
or `initial` or `first_result` or `first`, which is more suitable. I would
prefer `initial` to be the same as in `itertoolz` package. Regarding the
second point, should it yield one more element if provided - I think
everyone here agrees that yes.

With kind regards,
-gdg
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