title: Reindexing a specific level of a MultiIndex dataframe
body: <p>I have a DataFrame with two indices and would like to reindex it by one of the indices.</p>
  
  <pre><code>from pandas_datareader import data
  import matplotlib.pyplot as plt
  import pandas as pd
  
  # Instruments to download
  tickers = ['AAPL']
  
  # Online source one should use
  data_source = 'yahoo'
  
  # Data range
  start_date = '2000-01-01'
  end_date = '2018-01-09'
  
  # Load the desired data
  panel_data = data.DataReader(tickers, data_source, start_date, end_date).to_frame()
  panel_data.head()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/l1QKH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/l1QKH.png" alt="Screenshot"></a></p>
  
  <p>The reindexing goes as follows:</p>
  
  <pre><code># Get just the adjusted closing prices
  adj_close = panel_data['Adj Close']
  
  # Gett all weekdays between start and end dates
  all_weekdays = pd.date_range(start=start_date, end=end_date, freq='B')
  
  # Align the existing prices in adj_close with our new set of dates
  adj_close = adj_close.reindex(all_weekdays, method="ffill")
  </code></pre>
  
  <p>The last line gives the following error:</p>
  
  <pre><code>TypeError: '&lt;' not supported between instances of 'tuple' and 'int'
  </code></pre>
  
  <p>This is because the DataFrame index is a list of tuples:</p>
  
  <pre><code>panel_data.index[0]
  </code></pre>
  
  <pre>(Timestamp('2018-01-09 00:00:00'), 'AAPL')</pre>
  
  <p>Is it possible to reindex <code>adj_close</code>? By the way, if I don't convert the Panel object to a DataFrame using <code>to_frame()</code>, the reindexing works as it is. But it seems that Panel objects are deprecated...</p>