title: A way to get the count of non zero values in a coo_matrix of pythons scipy module?
body: <p>I thought of using <code>coo_matrix.nonzero()</code> which returns a tuple of two arrays which contain the indices of the nonzero entrys in a given matrix. The example from the docs states:</p>
  
  <pre><code>&gt;&gt;&gt; from scipy.sparse import coo_matrix
  &gt;&gt;&gt; A = coo_matrix([[1,2,0],[0,0,3],[4,0,5]])
  &gt;&gt;&gt; nonzero_entrys = A.nonzero()
  (array([0, 0, 1, 2, 2]), array([0, 1, 2, 0, 2]))
  </code></pre>
  
  <p>Then I would do something like <code>len(nonzero_entrys[0])</code> but this seem like a diversion. Is there a better way I have overlooked in the docs?</p>