ENH: Categorical.from_union

[jreback]

xref #13361

• support union w Series/CategoricalIndex as well as Categorical BUG: union_categorical with Series and cat idx #14199
• add ignore_order to ignore the raising on an ordered Categorical (and just have it work) ENH union_categoricals supports ignore_order GH13410 #15219
• do we want to put this in the pd namespace (or change its name). Consider Categorical.from_union(...)

[jreback]

cc @JanSchulz
cc @chris-b1
@jorisvandenbossche
@shoyer

[jreback]

I think the location is fine. This mostly is part of a developer/extender API, e.g. used internally by other parts of pandas and other packages (e.g. dask), rather than in an of itself useful to a regular user.

[jankatins]

+1 for adding a Categorical.from_union(*cats, ignore_order=False) instead of pd.xxx() -> IMO it shouldn't be exposed as top level API and from_union() is a nice equivalent to from_codes().

[jreback]

@chris-b1 this was partially closed by #14191 ?