title: Pandas "diff()" with string
body: <p>How can I flag a row in a dataframe every time a column change its string value?</p>
  
  <p>Ex:</p>
  
  <p>Input</p>
  
  <pre><code>ColumnA   ColumnB
  1            Blue
  2            Blue
  3            Red
  4            Red
  5            Yellow
  
  
  #  diff won't work here with strings....  only works in numerical values
  dataframe['changed'] = dataframe['ColumnB'].diff()        
  
  
  ColumnA   ColumnB      changed
  1            Blue         0
  2            Blue         0
  3            Red          1
  4            Red          0
  5            Yellow       1
  </code></pre>