title: python,numpy matrix must be 2-dimensional
body: <p>Why the line3 raise valueError‘ matrix must be 2-dimensional’</p>
  
  <pre><code>import numpy as np
  np.mat([[[1],[2]],[[10],[1,3]]])
  np.mat([[[1],[2]],[[10],[1]]])
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/PesXh.png" alt=""></p>