title: AttributeError: 'numpy.ndarray' object has no attribute 'toList'
body: <p>I'm trying to append certain columns of <code>Pandas Data Frames</code> from <code>CSV</code> files into a numpy array. I have no idea how to instantiate an empty numpy array, so I'm testing it first with a list. </p>
  
  <pre><code>def windows(files):
      x = []
      for my_files in files:
          with open(os.path.join("/Users", "saqibali", "PycharmProjects", "sensorLogProject", "Data", my_files),
                'rU') as my_file:
              df = pd.DataFrame(columns=['timestamp', 'time skipped', 'x', 'y', 'z', 'label']).set_index('timestamp')
              for d in sliding_window(sample_difference(my_file), 500, 250):
                  df = df.append(d[['x', 'y', 'z']])
              x.append(df.values.toList())
      return x
  </code></pre>
  
  <p>I get the error in the title, and it doesn't make sense to me because <code>x</code> is a <code>list</code> and <code>df</code> is a <code>Data Frame</code>. </p>