Return equivalent of `:` from function for indexing array

Question

I have a large array and a function that returns index lists into the array, i.e.,

import numpy

n = 500
a = numpy.random.rand(n)

def get_idx(k):
    # M         


        

Answer 1

You can use Ellipsis (translates to '...')

np.identity(2)[Ellipsis]
# array([[1.0, 0.0], [0.0, 1.0]])

Edit:

I feel I should mention one caveat: If you are sure this will only be used on 1d arrays, it's a perfectly fine and simple solution. However, in a public interface I wouldn't recommend it because it doesn't generalise correctly if a user uses the output of getidx for indexing a multi dimensional array.

For example A[getidx(k), ...] won't be safe anymore and the semantics of A[getidx(k), someslice] will be inconsistent if A has three or more dims.

That said, slice(None) isn't perfect in this respect either, for exammple A[getidx(k), getidx(k)] will behave inconsistently.

arange(n) looks like it's always giving the correct behaviour, but of course it's wasteful in most cases.