Re: ENH: More user-friendly way to pad a specific axis

On Tue, 2025-06-24 at 21:06 +0000, Carlos Martin wrote:
> It is common to want to pad an array along a *specific* axis.
> Examples:
> 
> -
> https://stackoverflow.com/questions/72106542/how-to-pad-the-i-j-axes-of-a-3d-np-array-without-padding-its-k-axis
> -
> https://stackoverflow.com/questions/56076094/zero-pad-ndarray-along-axis
> -
> https://stackoverflow.com/questions/66225047/padding-a-3-dimensional-numpy-array-with-the-medians-alongside-specific-axis
> -
> https://stackoverflow.com/questions/19349410/how-to-pad-with-zeros-a-tensor-along-some-axis-python
> -
> https://stackoverflow.com/questions/74966041/how-to-pad-a-specific-dimension-of-a-numpy-array
> 
> Doing so with
> [`numpy.pad`](https://numpy.org/doc/stable/reference/generated/numpy.
> pad.html) requires constructing a list of pairs of length equal to
> the ndim of the array, with exactly one of those pairs at the right
> position containing the desired pad widths. This can be verbose and
> clumsy when there are several axes.
> 
> I propose a new, more user-friendly way to pad along a specific axis
> (or axes): Let the `pad_width` argument accept a *dictionary* whose
> keys are axes and whose values are the `(before, after)` pair (or
> perhaps single number) for the corresponding axis. Example:
> 
> ```python3
> # before
> np.pad(array, [(0, 0), (0, 0), (1, 2), (0, 0), (0, 0)])
> # after
> np.pad(array, {-3: (1, 2)})
> ```


Seems like a pretty reasonable extension,  I wonder how many other
similar functions do we have, and does it extend to that as well?
I think right now it is a bit more typical to just have an `axis=`
kwarg to choose in two arguments.
(Which I agree is not as a nice of an API.)

So I guess that is a mildly in favor from me when it comes to accepting
such a change, but I am not sure if it will be quite enough to overcome
friction of adding new API.

- Sebastian

> 
> This should require only minor modification to the implementation of
> `numpy.pad`. If others like this idea, I can create a PR for it.
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