python - Pandas DataFrame merge summing column

Question

I'm trying to merge two DataFrames summing columns value.

>>> print(df1)
   id name  weight
0   1    A       0
1   2    B      10
2   3    C      10

>>> print(df2)
   id name  weight
0   2    B      15
1   3    C      10

I need to sum weight values during merging for similar values in the common column.

merge = pd.merge(df1, df2, how='inner')

So the output will be something like following.

   id name  weight
1   2    B      25
2   3    C      20

question from:https://stackoverflow.com/questions/23361218/pandas-dataframe-merge-summing-column

Answer 1

This solution works also if you want to sum more than one column. Assume data frames

>>> df1
   id name  weight  height
0   1    A       0       5
1   2    B      10      10
2   3    C      10      15
>>> df2
   id name  weight  height
0   2    B      25      20
1   3    C      20      30

You can concatenate them and group by index columns.

>>> pd.concat([df1, df2]).groupby(['id', 'name']).sum().reset_index()
   id name  weight  height
0   1    A       0       5
1   2    B      35      30
2   3    C      30      45