Function apply_multiple 

pub fn apply_multiple<F, DT, E>(
    function: F,
    expr: E,
    output_type: DT,
    returns_scalar: bool,
) -> Expr
where
    F: Fn(&mut [Column]) -> Result<Column, PolarsError> + 'static + Send + Sync,
    DT: Fn(&Schema<DataType>, &[Field]) -> Result<Field, PolarsError> + 'static + Send + Sync,
    E: AsRef<[Expr]>,

Available on crate feature lazy only.

Apply a function/closure over the groups of multiple columns. This should only be used in a group_by aggregation.

It is the responsibility of the caller that the schema is correct by giving the correct output_type. If None given the output type of the input expr is used.

This difference with map_multiple is that apply_multiple will create a separate Series per group.

• map_multiple should be used for operations that are independent of groups, e.g. multiply * 2, or raise to the power
• apply_multiple should be used for operations that work on a group of data. e.g. sum, count, etc.