title: How to get Pandas column multiindex names as a list
body: <p>I have the following CSV data:</p>
  
  <pre><code>id,gene,celltype,stem,stem,stem,bcell,bcell,tcell
  id,gene,organs,bm,bm,fl,pt,pt,bm
  134,foo,about_foo,20,10,11,23,22,79
  222,bar,about_bar,17,13,55,12,13,88
  </code></pre>
  
  <p>And I can successfully summarize them this way:</p>
  
  <pre><code>import pandas as pd
  df = pd.read_csv("http://dpaste.com/1X74TNP.txt",header=None,index_col=[1,2]).iloc[:, 1:]
  
  df.columns = pd.MultiIndex.from_arrays(df.ix[:2].values)
  df = df.ix[2:].astype(int)
  df.index.names = ['cell', 'organ']
  df = df.reset_index('organ', drop=True)
  
  result = df.groupby(level=[0, 1], axis=1).mean()
  result = result.stack().replace(np.nan, 0).unstack()
  result = result.swaplevel(0,1, axis=1).sort_index(axis=1)
  </code></pre>
  
  <p>Which looks like: </p>
  
  <pre><code>In [341]: result
  Out[341]:
          bm               fl               pt
       bcell stem tcell bcell stem tcell bcell stem tcell
  cell
  foo      0   15    79     0   11     0  22.5    0     0
  bar      0   15    88     0   55     0  12.5    0     0
  </code></pre>
  
  <p>My question is, from <code>result</code> how can I get the column index of the first level as list:</p>
  
  <pre><code>['bm','fl','pt']
  </code></pre>