title: How to rename the column names post flattening
body: <p>This is </p>
  
  <pre><code>    Date      Group   Value   Duration
  2018-01-01      A      20       30
  2018-02-01      A      10       60
  2018-03-01      A      25       88    
  2018-01-01      B      15      180
  2018-02-01      B      30      210
  2018-03-01      B      25      238    
  </code></pre>
  
  <p>I want to pivot the above <code>df</code></p>
  
  <p>My Approach:</p>
  
  <pre><code> df_pivot = dealer_f.pivot_table(index='Group',columns='Date',fill_value=0)
   df_pivot.columns = dealer_f_pivot.columns.map('_'.join)
   ff_pivot = dealer_f_pivot.reset_index()
  </code></pre>
  
  <p>I am getting an error as <code>TypeError: sequence item 1: expected str instance, int found</code></p>
  
  <p>If I simply follow <code>reset_index</code> then I get the column names as <code>('Value',2018-01-01),('Value',2018-02-10)</code> etc. </p>
  
  <p>I want to <code>flatten</code> the columns so that my output looks like below</p>
  
  <pre><code>df_pivot.columns.tolist()
  ['2018-01-01_Value','2018-02-01_Value',.....'2018-01-01_Duration',...]
  </code></pre>
  
  <p>Any clue? Or where I am missing?</p>