title: Convert 'numpy.ndarray' to float64 failed in python?
body: <p>This is my code below:</p>
  
  <pre><code>a = np.array([[1,2,3]])
  b = np.array([[2,2,2]])
  x = np.dot(b,a.T)
  print(type(x))
  </code></pre>
  
  <p>This give me <code>'numpy.ndarray'</code>,I want to convert <code>'numpy.ndarray'</code> to float64:</p>
  
  <pre><code>x = x.astype(np.float64)
  print(type(x))
  </code></pre>
  
  <p>but it returned <code>'numpy.ndarray'</code>, why doesn't work?</p>