title: How to add a new row to an empty numpy array
body: <p>Using standard Python arrays, I can do the following:</p>
  
  <pre><code>arr = []
  arr.append([1,2,3])
  arr.append([4,5,6])
  # arr is now [[1,2,3],[4,5,6]]
  </code></pre>
  
  <p>However, I cannot do the same thing in numpy. For example:</p>
  
  <pre><code>arr = np.array([])
  arr = np.append(arr, np.array([1,2,3]))
  arr = np.append(arr, np.array([4,5,6]))
  # arr is now [1,2,3,4,5,6]
  </code></pre>
  
  <p>I also looked into <code>vstack</code>, but when I use <code>vstack</code> on an empty array, I get:</p>
  
  <pre><code>ValueError: all the input array dimensions except for the concatenation axis must match exactly
  </code></pre>
  
  <p>So how do I do append a new row to an empty array in numpy?</p>