Pandas: Calculate Median of Group over Columns

Question

Given the following data frame:

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

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.

The desired result is as follows:

    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

Thanks in advance!

Answer 1

You already had the idea -- groupby COL1 and COL2 and calculate median.

m = df.groupby(['COL1', 'COL2'])[['COL3','COL4']].apply(np.median)
m.name = 'MEDIAN'

print df.join(m, on=['COL1', 'COL2'])

  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     3.0
5    B   BB     2     2     3.0

Answer 2

df.groupby(['COL1', 'COL2']).median()[['COL3','COL4']]