python - pandas sort lambda function

Question

Given a dataframe a with 3 columns, A , B , C and 3 rows of numerical values. How does one sort all the rows with a comp operator using only the product of A[i]*B[i]. It seems that the pandas sort only takes columns and then a sort method.
I would like to use a comparison function like below.

f = lambda i,j: a['A'][i]*a['B'][i] < a['A'][j]*a['B'][j]

Answer 1

There are at least two ways:

Method 1

Say you start with

In [175]: df = pd.DataFrame({'A': [1, 2], 'B': [1, -1], 'C': [1, 1]})

You can add a column which is your sort key

In [176]: df['sort_val'] = df.A * df.B

Finally sort by it and drop it

In [190]: df.sort_values('sort_val').drop('sort_val', 1)
Out[190]: 
   A  B  C
1  2 -1  1
0  1  1  1

Method 2

Use numpy.argsort and then use .ix on the resulting indices:

In [197]: import numpy as np

In [198]: df.ix[np.argsort(df.A * df.B).values]
Out[198]: 
   A  B  C
0  1  1  1
1  2 -1  1