[Numpy-discussion] ENH: Proposal to add atleast_nd function

[Joseph Fox-Rabinovitz]

On Fri, Feb 12, 2021, 09:32 Robert Kern <robert.kern at gmail.com> wrote:

> On Fri, Feb 12, 2021 at 5:15 AM Eric Wieser <wieser.eric+numpy at gmail.com>
> wrote:
>
>> > There might be some linear algebraic reason why those axis positions
>> make sense, but I’m not aware of it...
>>
>> My guess is that the historical motivation was to allow grayscale `(H,
>> W)` images to be converted into `(H, W, 1)` images so that they can be
>> broadcast against `(H, W, 3)` RGB images.
>>
>
> Correct. If you do introduce atleast_nd(), I'm not sure why you'd
> deprecate and remove the one existing function that *isn't* made redundant
> thereby.
>

`atleast_nd` handles the promotion of 2D to 3D correctly. The `pos`
argument lets you tell it where to put the new axes. What's unintuitive to
my is that the 1D case gets promoted to from shape `(x,)` to shape `(1, x,
1)`. It takes two calls to `atleast_nd` to replicate that behavior.

One modification to `atleast_nd` I've thought about is making `pos` refer
to the position of the existing axes in the new array rather than the
position of the new axes, but that's likely not a useful way to go about it.

- Joe


> --
> Robert Kern
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