Re: Added atleast_nd, request for clarification/cleanup of atleast_3d
2016-07-06 16:26:49 GMT
On Jul 5, 2016 11:21 PM, "Ralf Gommers" <ralf.gommers <at> gmail.com> wrote:
> On Wed, Jul 6, 2016 at 7:06 AM, Nathaniel Smith <njs <at> pobox.com> wrote:
>> On Jul 5, 2016 9:09 PM, "Joseph Fox-Rabinovitz" <jfoxrabinovitz <at> gmail.com> wrote:
>> > Hi,
>> > I have generalized np.atleast_1d, np.atleast_2d, np.atleast_3d with a
>> > function np.atleast_nd in PR#7804
>> > (https://github.com/numpy/numpy/pull/7804).
>> > As a result of this PR, I have a couple of questions about
>> > `np.atleast_3d`. `np.atleast_3d` appears to do something weird with
>> > the dimensions: If the input is 1D, it prepends and appends a size-1
>> > dimension. If the input is 2D, it appends a size-1 dimension. This is
>> > inconsistent with `np.atleast_2d`, which always prepends (as does
>> > `np.atleast_nd`).
>> > - Is there any reason for this behavior?
>> > - Can it be cleaned up (e.g., by reimplementing `np.atleast_3d` in
>> > terms of `np.atleast_nd`, which is actually much simpler)? This would
>> > be a slight API change since the output would not be exactly the same.
>> Changing atleast_3d seems likely to break a bunch of stuff...
>> Beyond that, I find it hard to have an opinion about the best design for these functions, because I don't think I've ever encountered a situation where they were actually what I wanted. I'm not a big fan of coercing dimensions in the first place, for the usual "refuse to guess" reasons. And then generally if I do want to coerce an array to another dimension, then I have some opinion about where the new dimensions should go, and/or I have some opinion about the minimum acceptable starting dimension, and/or I have a maximum dimension in mind. (E.g. "coerce 1d inputs into a column matrix; 0d or 3d inputs are an error" -- atleast_2d is zero-for-three on that requirements list.)
>> I don't know how typical I am in this. But it does make me wonder if the atleast_* functions act as an attractive nuisance, where new users take their presence as an implicit recommendation that they are actually a useful thing to reach for, even though they... aren't that. And maybe we should be recommending folk move away from them rather than trying to extend them further?
>> Or maybe they're totally useful and I'm just missing it. What's your use case that motivates atleast_nd?
> I think you're just missing it:) atleast_1d/2d are used quite a bit in Scipy and Statsmodels (those are the only ones I checked), and in the large majority of cases it's the best thing to use there. There's a bunch of atleast_2d calls with a transpose appended because the input needs to be treated as columns instead of rows, but that's still efficient and readable enough.
I know people *use* it . What I'm confused about is in what situations you would invent it if it didn't exist. Can you point me to an example or two where it's "the best thing"? I actually had statsmodels in mind with my example of wanting the semantics "coerce 1d inputs into a column matrix; 0d or 3d inputs are an error". I'm surprised if there are places where you really want 0d arrays converted into 1x1, or want to allow high dimensional arrays to pass through - and if you do want to allow high dimensional arrays to pass through, then transposing might help with 2d cases but will silently mangle high-d cases, right?
_______________________________________________ NumPy-Discussion mailing list NumPy-Discussion <at> scipy.org https://mail.scipy.org/mailman/listinfo/numpy-discussion