Reverse an arbitrary dimension in an ndarray

Question

I\'m working with an n-dimensional array, and I\'d like a way to reverse a numbered dimension. So rather than

rev = a[:,:,::-1]

I\'d like to be

Answer 1

Turns out this can be done with slice, for which : is shorthand in some contexts. The trick is to build the index object as a tuple of slices:

import numpy as np

def reverse(a, axis=0): 
    idx = [slice(None)]*len(a.shape)
    idx[axis] = slice(None, None, -1)
    return a[idx]

a = np.arange(24).reshape(2,3,4)
print reverse(a, axis=2)

With Ellipsis this can be collapsed into a one-liner:

a[[slice(None)]*axis + [slice(None, None, -1)] + [Ellipsis]]