Python: Sum values in DataFrame if other values match between DataFrames

Question

I have two dataframes of different length like those:

DataFrame A:

FirstName    LastName
Adam         Smith
John         Johnson

Da

Answer 1

df_b_a = (pd.merge(df_b, df_a, left_on=['FirstName', 'LastName'], right_on=['First', 'Last'], how='left')
                .groupby(by=['First', 'Last'], as_index=False)['Value'].sum())

print(df_b_a)

    First   Last    Value
0   Adam    Smith   5.7
1   John    Johnson     2.5