title: Pandas Date Functionality: Extracting Period Index information as String
body: <p>I have a dataframe(df_qtr) in the following format:</p>
  
  <pre><code>Index      2008Q4     2009Q1    2009Q2 
   Ex       85833.33   85433.33   85766.66
   Kc      153200.00  150533.33  149666.66
   Al       83766.66   84200.00   84333.33
   Jk      107933.33  113600.00  115366.66
   Ar      151200.00  147133.33  143900.00
   Ax      139300.00  137333.33  132700.00
  </code></pre>
  
  <p>The column headers are <strong>period indexes</strong> created by consolidating Monthly data (dates were column headers) using the following code:</p>
  
  <p><code>In[1]: df_qtr = df.groupby(pd.PeriodIndex(df.columns, freq='q'), axis=1).mean()</code></p>
  
  <p>I would like to extract the <strong>column headers as strings</strong> as:</p>
  
  <p><code>Out[1]: ['2008Q4', '2009Q1', '2009Q2']</code></p>
  
  <p>The closest I have come is converting it back to timestamp and then string:</p>
  
  <pre><code> In[2]: list(df_qtr.columns.to_datetime().astype(str))
   Out[2]: Index(['2008-10-01', '2009-01-01', '2009-04-01']
  </code></pre>
  
  <p>But the same does not work with a Period Index and I get the following error:</p>
  
  <p><code>ValueError: Cannot cast PeriodIndex to dtype &lt;U0</code></p>
  
  <p>I can convert the list above to a Period Index again and then convert it to string but isn't there an easier workaround to this?</p>