title: What does "three dots" in Python mean when indexing what looks like a number?
body: <p>What is the meaning of <code>x[...]</code> below?</p>
  
  <pre><code>a = np.arange(6).reshape(2,3)
  for x in np.nditer(a, op_flags=['readwrite']):
      x[...] = 2 * x
  </code></pre>