ply function column-by-column to the GroupBy object.
    Series.apply : Apply a function to a Series.
    DataFrame.apply : Apply a function to each row or column of a DataFrame.

    Notes
    -----

    .. versionchanged:: 1.3.0

        The resulting dtype will reflect the return value of the passed ``func``,
        see the examples below.

    Functions that mutate the passed object can produce unexpected
    behavior or errors and are not supported. See :ref:`gotchas.udf-mutation`
    for more details.

    Examples
    --------
    {examples}
    a%