Subtract two Pandas DataFrames joined on multiple column values

Question

I am trying to subtract the values of a column in DataFrame A, from the values from a column in DataFrame B, but only if multiple column values are equal to each other.

Answer 1

This is why the Index is so useful, subtraction will be aligned on the indices (both rows and columns).

dfA = dfA.set_index(['Department', 'Speciality', 'TargetMonth'])
dfB = dfB.set_index(['Department', 'Speciality', 'TargetMonth'])

dfA.sub(dfB.rename(columns={'Required': 'Capacity'}), fill_value=0)

                                   Capacity
Department Speciality TargetMonth          
IT         Servers    2019-1             50
Sales      Cars       2019-1             50
                      2019-2              0
           Furniture  2019-1             60