title: Append lists to list of lists
body: <p>I want to append lists of dataframes in an existing list of lists:</p>
  
  <pre><code>df1 = pd.DataFrame({'A': ['A0', 'A1', 'A2', 'A3'],
                       'B': ['B0', 'B1', 'B2', 'B3'],
                       'C': ['C0', 'C1', 'C2', 'C3'],
                       'D': ['D0', 'D1', 'D2', 'D3']},
                      index=[0, 1, 2, 3])
  
  
  fr_list = [[] for x in range(2)]
  fr_list[0].append(df1)
  fr_list[0].append(df1)
  fr_list[1].append(df1)
  
  
  fr2 = [[] for x in range(2)]
  fr2[0].append(df1)
  fr2[1].append(df1)
  
  fr_list.append(fr2)  # &lt;-- here is the problem
  </code></pre>
  
  <p>Output: <code>fr_list = [[df1, df1], [df1], [fr2[0], fr2[1]]]</code> List contains <strong>3</strong> elements</p>
  
  <p>Expected: <code>fr_list = [[df1, df1, fr2[0]],[df1, fr2[1]]]</code> List contains <strong>2</strong> elements</p>