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

Use DataFrame.merge + DataFrame.groupby:

new_df=( dfa.merge(dfb.groupby(['First','Last'],as_index=False).Value.sum() ,
                   left_on='LastName',right_on='Last',how='left')
            .drop('Last',axis=1) )
print(new_df)

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to join for both columns:

new_df=( dfa.merge(dfb.groupby(['First','Last'],as_index=False).Value.sum() ,
              left_on=['FirstName','LastName'],right_on=['First','Last'],how='left')
            .drop(['First','Last'],axis=1) )
print(new_df)

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Output:

  FirstName LastName  Value
0      Adam    Smith    5.7
1      John  Johnson    2.5