title: Pandas: Calculate Median of Group over Columns
body: <p>Given the following data frame:</p>
  
  <pre><code>import pandas as pd
  
  df = pd.DataFrame({'COL1': ['A', 'A','A','A','B','B'], 
                     'COL2' : ['AA','AA','BB','BB','BB','BB'],
                     'COL3' : [2,3,4,5,4,2],
                     'COL4' : [0,1,2,3,4,2]})
  df
      COL1    COL2    COL3    COL4
  0    A       AA      2       0
  1    A       AA      3       1
  2    A       BB      4       2
  3    A       BB      5       3
  4    B       BB      4       4
  5    B       BB      2       2
  </code></pre>
  
  <p>I would like, as efficiently as possible (i.e. via groupby and lambda x or better), to find the median of columns 3 and 4 for each distinct group of columns 1 and 2.</p>
  
  <p>The desired result is as follows:</p>
  
  <pre><code>    COL1    COL2    COL3    COL4  MEDIAN
  0    A       AA      2       0    1.5
  1    A       AA      3       1    1.5
  2    A       BB      4       2    3.5
  3    A       BB      5       3    3.5
  4    B       BB      4       4    3
  5    B       BB      2       2    3
  </code></pre>
  
  <p>Thanks in advance!</p>