title: How to change the columns name from a tuple to string?
body: <p>I have used <code>pd.pivot_table</code> in pandas dataframe, and the columns names becomes tuples like <code>('A1', 'B1'), ('A1', 'B2')...</code> and I want them to be like <code>'A1_B1', 'A1_B2'...</code></p>
  
  <p>I tried to use
  <code>df.columns.values[i] = df.columns.values[i][0] + '_' + df6.columns.values[i][1],</code>
  and tried rename as well. </p>
  
  <p>When I checked df.columns.values, the columns' names changed, but when I cannot use these names to do indexing. I am new to python, so might not know the difference between column names and column indices.</p>
  
  <p>Can anyone help me? Thanks!</p>