>>> import pandas as pd
>>> import numpy as np
>>> df = pd.DataFrame([[1, 2], [3, 4]], columns=['f1', 'f2'], index=['r1', 'r2'])
>>> df
    f1  f2
r1   1   2
r2   3   4

>>> for cn in df.columns:
...     diff_dat.append(abs(df[cn]['r1'] - df[cn]['r2']))
...
>>> diff_dat
[2, 2]
>>> df.append(pd.DataFrame([diff_dat], index=['r3'], columns=df.columns))
    f1  f2
r1   1   2
r2   3   4
r3   2   2