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

Answer 2

I would use merge with keys:

For this solution taking your dataframe A as dfA & dataframe as dfB

   df_result =  pd.merge(dfA, dfB, how='inner', on=['Department','Speciality','TargetMonth'])

This will put the dataframes together based on the keys: ['Department','Speciality','TargetMonth'] and will result in a dataframe where the the keys appear in both dataframes (how = 'inner').

I.E. if there is a key in dfB that is:

   {'Department': 'IT','Speciality':'Furniture','TargetMonth':2019-1}

This value will not appear in dataframe df_result. More Information can be found here - https://pandas.pydata.org/pandas-docs/stable/user_guide/merging.html

Then the solution using Pandas vectorization:

   df_result['Result'] = df_result['Capacity'] - df_result['Required']