Pandas sum column values according to another columns value

One-liner code to sum Pandas second columns according to same values in the first column.

df2 = df1.groupby(df1.columns[0])[df1.columns[1]].sum().reset_index()

For example, applying to a table listing pipe diameters and lenghts, the command will return total lenghts according to each unique diameters.

This functionality is similar to excel's pivot table sum.