How can I index the last axis of a Numpy array if I don't know its rank in advance?

Here is what I want to do: Let a be a Numpy array of unknown rank. I want the slice of the last k elements of the last axis.

If a is 1D, I want

b = a[-k:]

If a is 2D, I want

b = a[:, -k:]

If a is 3D, I want

b = a[:, :, -k:]

and so on.

I want this to work regardless of the rank of a (as long as the rank is at least 1).

The fact that I want the last k elements in the example is irrelevant of course, the point is that I want to specify indices for whatever the last axis is when I don't know the rank of an array in advance.