Categorical: let unique only return used categories

[jankatins]

Discussion see here: #8559 (comment)

[jreback]

put a release note referincing that issue. I think this is already in the API.rst ? (maybe just check doc-string too).

[jankatins]

Categorical.unique is not in api.rst, only methods under Series.cat are.

re release note: under API change or bug fix?

[jreback]

ahh, so maybe add to Series.cat methods? I forgot how we are calling this

s.unique() of course does it, but I suppose s.cat.unique() should be allowed as well?

This 'changes' the API slightly so API change it is. (maybe put a short explanation), example if you feel it is waranted.

[jankatins]

ok, added it under api change (without an example)

I don't think any more methods should be added to Series.cat: simple calling s.unique() (or s.min() or s.describe()) is enough. After the last discussion about what is API and what is not, I started to think of all methods in Categorical as "for compatibility with numpy" and I think that's the right way to think about them. If a users knows better they can look at the docstrings, but I don't think pandas should do any API guarantee whether these methods stay or change or be removed.

[jreback]

add a reference to the original issue here

[jreback]

thanks!