[Numpy-discussion] Added atleast_nd, request for clarification/cleanup of atleast_3d

[Nathaniel Smith]

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?

-n
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