title: How to convert "index" to a "string"?
body: <p>Here is a working example code:</p>
  
  <pre><code>data = {'name': ['Joe', 'Mike', 'Jack', 'Hack', 'David', 'Marry', 'Wansi', 'Sidy', 'Jason', 'Even'],
  
          'age': [25, 32, 18, np.nan, 15, 20, 41, np.nan, 37, 32],
  
          'gender': [1, 0, 1, 1, 0, 1, 0, 0, 1, 0],
  
          'isMarried': ['yes', 'yes', 'no', 'yes', 'no', 'no', 'no', 'yes', 'no', 'no']}
  labels = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j']
  
  df = pd.DataFrame(data, index=labels)
  print(df)
  print("---------------------------")
  obj = df[df["age"]&gt;40].index.format()
  print("obj is",type(obj))
  </code></pre>
  
  <p>I hope obj as a string (), but the above result is list().
  What should I do to correct it ?</p>