numpy array slice using None

Question

This had me scratching my head for a while. I was unintentionally slicing an array with None and getting something other than an error (I expected an error). Instead, it returns an array with an extra dimension.

>>> import numpy
>>> a = numpy.arange(4).reshape(2,2)
>>> a
array([[0, 1],
       [2, 3]])
>>> a[None]
array([[[0, 1],
        [2, 3]]])

Is this behavior intentional or a side-effect? If intentional, does anyone have some rationale for it?

Answer 1

Using None is equivalent to using numpy.newaxis, so yes, it's intentional. In fact, they're the same thing, but, of course, newaxis spells it out better.

The docs

A related SO question.