title: how to add add new row for each distinct ID in pandas?
body: <p>I have a datframe like this:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'Car_ID': ['B332', 'B332', 'B332', 'C315', 'C315', 'C315', 'C315', 'C315', 'F310', 'F310'], \
                      'Date': ['2018-03-15', '2018-03-14', '2018-03-12', '2018-03-15', '2018-03-14', '2018-03-13', \
                               '2018-03-12', '2018-03-11', '2018-03-10', '2018-03-09'], \
                      'Driver': ['Alex', 'Alex', 'Alex', 'Sara', 'Sara', 'Sara', 'Sara', 'Sara', 'Franck','Franck'], \
                    'Info': ["Group_B", "Group_B", "Group_B", "Group_C", "Group_C", "Group_C", "Group_C", "Group_C", "Group_F", "Group_F"]})
  df
  
      Car_ID  Date        Driver  Info
  0   B332    2018-03-15  Alex    Group_B
  1   B332    2018-03-14  Alex    Group_B
  2   B332    2018-03-12  Alex    Group_B
  3   C315    2018-03-15  Sara    Group_C
  4   C315    2018-03-14  Sara    Group_C
  5   C315    2018-03-13  Sara    Group_C
  6   C315    2018-03-12  Sara    Group_C
  7   C315    2018-03-11  Sara    Group_C
  8   F310    2018-03-10  Franck  Group_F
  9   F310    2018-03-09  Franck  Group_F
  </code></pre>
  
  <p>I want to add new row before each distinct <code>Car_ID</code> like this:</p>
  
  <pre><code>    Car_ID  Date        Driver  Info
  0   B332    2018-03-15  Alex    Group_B
  1   B332    2018-03-14  Alex    Group_B
  2   B332    2018-03-12  Alex    Group_B
  3   B332    2018-03-12  Alex    Changed
  4   C315    2018-03-15  Sara    Group_C
  5   C315    2018-03-14  Sara    Group_C
  6   C315    2018-03-13  Sara    Group_C
  7   C315    2018-03-12  Sara    Group_C
  8   C315    2018-03-11  Sara    Group_C
  9   C315    2018-03-11  Sara    Changed
  10  F310    2018-03-10  Franck  Group_F
  11  F310    2018-03-09  Franck  Group_F
  12  F310    2018-03-09  Franck  Changed
  </code></pre>
  
  <p>How could I do this job by <code>shift()</code>?</p>
  
  <p>Thanks</p>