Inplace transformation pandas with groupby

Question

Would it be possible to mutate DataFrame inplace with groupby statement?

import pandas as pd
dt = pd.DataFrame({
                       


        

Answer 1

I think you can use transform which return Series same length and same index as df with substracting:

print (dt.groupby("LETTER")['VALUE'].transform('mean'))
0     5.0
1    13.5
2    13.0
3     5.0
4    13.5
Name: VALUE, dtype: float64

dt['NEW_COL'] = dt['VALUE'] - dt.groupby("LETTER")['VALUE'].transform('mean')
print (dt)
  LETTER  VALUE  NEW_COL
0      a     10      5.0
1      b     12     -1.5
2      c     13      0.0
3      a      0     -5.0
4      b     15      1.5