Return an array and missing value suitable for factorization.
An array suitable for factorization. This should maintain order
and be a supported dtype (Float64, Int64, UInt64, String, Object).
By default, the extension array is cast to object dtype.
The value in values to consider missing. This will be treated
as NA in the factorization routines, so it will be coded as
na_sentinal and not included in uniques. By default,
np.nan is used.
The values returned by this method are also used in