title: Numpy dtype for list with mixed data types
body: <p>I have a list, <code>my_list</code>, with mixed data types that I want to convert into a numpy array. However, I get the error <code>TypeError: expected a readable buffer object</code>. See code below. I've tried to base my code on the  <a href="http://docs.scipy.org/doc/numpy/reference/arrays.dtypes.html" rel="noreferrer">NumPy documentation</a>.</p>
  
  <pre><code>my_list = [['User_0', '2012-2', 1, 6, 0, 1.0], ['User_0', '2012-2', 5, 6, 0, 1.0], ['User_0', '2012-3', 0, 0, 4, 1.0]]
  my_np_array = np.array(my_list, dtype='S30, S8, i4, i4, f32')   
  </code></pre>