title: python - Pandas - Dataframe.set_index - how to keep the old index column
body: <p>I have this <code>Dataframe</code>:</p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({'Hugo' : {'age' : 21, 'weight' : 75},
                     'Bertram': {'age' : 45, 'weight' : 65},
                     'Donald' : {'age' : 75, 'weight' : 85}}).T
  df.index.names = ['name']
  
  
           age  weight
  name                
  Bertram   45      65
  Donald    75      85
  Hugo      21      75
  </code></pre>
  
  <p>I want to change the index to the column <code>'age'</code>:</p>
  
  <pre><code>df.set_index('age', inplace=True)
  
       weight
  age        
  45       65
  75       85
  21       75
  </code></pre>
  
  <p>The old index-column name gets lost. Is there a way to change the index without losing the original index-column and getting the old column as 'normal' column again, so that it looks like this?</p>
  
  <pre><code>     name       weight
  age        
  45   Bertram    65
  75   Donald     85
  21   Hugo       75
  </code></pre>