title: Tranform multindex-key to column
body: <p>I've trying make the key of the index become a column for a while. I've tried <code>df.to_frame()</code> and other variations without success.</p>
  <p>For reproducibility:</p>
  <pre><code>nasdaq_list = [&quot;CBAK&quot;, &quot;CBAN&quot;, &quot;CBAY&quot;, &quot;CBDE&quot;, &quot;CBF&quot;, &quot;CBFV&quot;, &quot;CBIN&quot;, &quot;CBLI&quot;, &quot;CBMG&quot;, &quot;CBMX&quot;, &quot;CBNJ&quot;, &quot;CBNK&quot;, &quot;CBOE&quot;, &quot;CBPO&quot;, &quot;CBRL&quot;, &quot;CBRX&quot;, &quot;CBSH&quot;, &quot;CBSHP&quot;, &quot;CBST&quot;, &quot;CBSTZ&quot;, &quot;CCBG&quot;, &quot;CCCL&quot;, &quot;CCCR&quot;, &quot;CCIH&quot;, &quot;CCLP&quot;, &quot;CCMP&quot;, &quot;CCNE&quot;, &quot;CCOI&quot;, &quot;CCRN&quot;, &quot;CCUR&quot;, &quot;CCXI&quot;, &quot;CDC&quot;, &quot;CDK&quot;, &quot;CDNA&quot;, &quot;CDNS&quot;, &quot;CDTI&quot;, &quot;CDW&quot;, &quot;CDXS&quot;, &quot;CDZI&quot;, &quot;CECE&quot;, &quot;CECO&quot;, &quot;CELG&quot;, &quot;CELGZ&quot;, &quot;CEMI&quot;, &quot;CEMP&quot;, &quot;CENT&quot;, &quot;CENTA&quot;, &quot;CENX&quot;, &quot;CERE&quot;, &quot;CERN&quot;, &quot;CERS&quot;, &quot;CERU&quot;, &quot;CETV&quot;, &quot;CEVA&quot;, &quot;CFA&quot;, &quot;CFBK&quot;, &quot;CFFI&quot;, &quot;CFFN&quot;, &quot;CFGE&quot;, &quot;CFNB&quot;, &quot;CFNL&quot;, &quot;CFO&quot;, &quot;CFRX&quot;, &quot;CFRXW&quot;, &quot;CFRXZ&quot;, &quot;CG&quot;, &quot;CGEN&quot;, &quot;CGIX&quot;, &quot;CGNX&quot;, &quot;CGO&quot;, &quot;CHCI&quot;, &quot;CHCO&quot;, &quot;CHDN&quot;, &quot;CHEF&quot;, &quot;CHEV&quot;, &quot;CHFC&quot;, &quot;CHFN&quot;, &quot;CHI&quot;, &quot;CHKE&quot;, &quot;CHKP&quot;, &quot;CHLN&quot;, &quot;CHMG&quot;, &quot;CHNR&quot;, &quot;CHOP&quot;, &quot;CHRS&quot;, &quot;CHRW&quot;, &quot;CHSCM&quot;, &quot;CHSCN&quot;, &quot;CHSCO&quot;, &quot;CHSCP&quot;, &quot;CHTR&quot;, &quot;CHUY&quot;, &quot;CHW&quot;, &quot;CHXF&quot;, &quot;CHY&quot;, &quot;CHYR&quot;, &quot;CIDM&quot;, &quot;CIFC&quot;, &quot;CIMT&quot;, &quot;CINF&quot;]
  start = date(2017, 10, 1)
  end = date(2020, 6, 25)
  
  import yfinance as yf
  df = yf.download(nasdaq_list, start, end)
  
  df.tail()
  Out[84]: 
                  Adj Close                          Volume                            
                  CBAK   CBAN  CBAY CBDE CBF  ...    CHYR     CIDM CIFC CIMT     CINF
  Date                                        ...                                   
  2020-06-18       NaN  11.76  3.66  NaN NaN  ...    NaN  1518300  NaN  NaN  1166600
  2020-06-19       NaN  11.85  3.94  NaN NaN  ...    NaN  4842900  NaN  NaN  6750100
  2020-06-22       NaN  12.29  4.13  NaN NaN  ...    NaN  2542600  NaN  NaN  1774600
  2020-06-23       NaN  12.62  3.91  NaN NaN  ...    NaN  3552000  NaN  NaN  1342100
  2020-06-24       NaN  11.84  3.74  NaN NaN  ...    NaN  3288200  NaN  NaN  1531700
  
  [5 rows x 600 columns]
  </code></pre>
  <p>While I'm looking for something like:</p>
  <pre><code>Date       Symbol     Adj Close                  ...    Volume                                   
  2020-06-18 CBAK       NaN  11.76  3.66  NaN NaN  ...    NaN  1518300  NaN  NaN  1166600
  2020-06-18 CBAN       NaN  11.85  3.94  NaN NaN  ...    NaN  4842900  NaN  NaN  6750100
  2020-06-18 CBAY       NaN  12.29  4.13  NaN NaN  ...    NaN  2542600  NaN  NaN  1774600
  2020-06-18 CBDE       NaN  12.62  3.91  NaN NaN  ...    NaN  3552000  NaN  NaN  1342100
  2020-06-18 CBF        NaN  11.84  3.74  NaN NaN  ...    NaN  3288200  NaN  NaN  1531700
  
  [5 rows x 600 columns]
  </code></pre>
  <p>Notice that the date keep the same because all of these symbols represent the first date.</p>
  <p>Any help would be greatly appreciated.</p>