Question

In Numpy (and Python in general, I suppose), how does one store a slice-index, such as (…,0,:), in order to pass it around and apply it to various arrays? It would be nice to, say, be able to pass a slice-index to and from functions.

Answer 1

Python creates special objects out of the slice syntax, but only inside the square brackets for indexing. You can either create those objects by hand (in this case, (...,0,:) is (Ellipsis, 0, slice(None, None, None)), or you can create a little helper object:

class ExtendedSliceMaker(object):
    def __getitem__(self, idx):
        return idx

>>> ExtendedSliceMaker()[...,0,:]
(Ellipsis, 0, slice(None, None, None))