title: What does [::-1] actually do in numpy?
body: <p>Say I have something like</p>
  <pre><code>import numpy as np
  a = np.array([10,20,30,40,50,60])
  
  # this will get the indices of elements in reverse sorted order
  a.argsort()[::-1]
  </code></pre>
  <p>I can imagine that <code>-1</code> specifies the direction, but what does the <code>::</code> operator do?
  Is this a numpy thing or a python thing in general?</p>