DEPR: inplace dtype-changing methods on Categorical

[jbrockmendel]

• Categorical.remove_unused_categories Deprecate inplace in Categorical.remove_unused_categories #37918
• Cateogrical.remove_categories Deprecate inplace in Categorical.remove_categories #37981
• Categorical.add_categories Deprecate inplace in Categorical.add_categories. #41118
• Categorical.reorder_categories Deprecate inplace in Categorical.reorder_categories. #41133
• Categorical.rename_categories Deprecate inplace in Categorical.rename_categories #41186
• Categorical.set_categories Deprecate inplace in Categorical.set_categories. #41307

The ability to alter an existing array's dtype is a footgun. We can return a new object without having to make a copy.

[jorisvandenbossche]

Also set_categories.

All those methods have a inplace=False keyword, so by default they don't actually change the existing array's dtype. And with inplace=True this can be expected? (it's a bit similar issue as with inplace deprecation discussion, but also here a copy=True/False keyword might be more appropriate ...)

[AlexKirko]

Hard to think of a use case where changing a Categorical dtype in place would be intended, but easy to think of use cases where it would lead to unexpected errors. Probably best to deprecate this and raise.

[OlehKSS]

Hi, I've created a pull request to solve this issue for Categorical.remove_unused_categories. Could you check it?
For the other methods, should I create separate pull requests for each one of them so as to keep them small?

[OlehKSS]

take

[jorisvandenbossche]

I am not sure we should deprecate the inplace ability for all of those. For example, for set_categories, or rename_categories, this operation can actually happen inplace, and can thus be beneficial.

[TomAugspurger]

Agreed. It can be expensive to copy all the data (codes) when not necessary.

[jbrockmendel]

Agreed. It can be expensive to copy all the data (codes) when not necessary.

i was thinking we'd get a view on the codes, just a new Categorical object. This should be cheap.