Maintaining Other Columns During Groupby Operations

When performing a groupby operation on a pandas dataframe, it is often necessary to retain columns that are not involved in the grouping or aggregation process. By default, these other columns are dropped when the operation is complete. This can be problematic if the retained columns contain valuable information.

Consider the following data frame:

    item    diff   otherstuff
   0   1       2            1
   1   1       1            2
   2   1       3            7
   3   2      -1            0
   4   2       1            3
   5   2       4            9
   6   2      -6            2
   7   3       0            0
   8   3       2            9

If we were to group the data frame by the "item" column and find the minimum value of the "diff" column, the resulting data frame would look like this:

    item   diff
   0   1      1           
   1   2     -6           
   2   3      0                 

Notice that the "otherstuff" column has been dropped. To retain this column, we can use the idxmin() method to get the indices of the elements of minimum diff, and then select those:

>>> df.loc[df.groupby("item")["diff"].idxmin()]
   item  diff  otherstuff
1     1     1           2
6     2    -6           2
7     3     0           0

[3 rows x 3 columns]

Another method is to sort the data frame by the "diff" column, and then take the first element in each item group:

>>> df.sort_values("diff").groupby("item", as_index=False).first()
   item  diff  otherstuff
0     1     1           2
1     2    -6           2
2     3     0           0

[3 rows x 3 columns]

Both of these methods will produce the desired result, while retaining the "otherstuff" column. Keep in mind that the resulting indices may be different even though the row content is the same.