Add `.cat.to_enum()` for fast Categorical -> Enum casting about polars HOT 7 CLOSED

I can see a usecase though where you would want to perform operations first and then freeze the categories by casting from a Categorical to a Enum.

from polars.

It would be even nicer if the bins are correctly ordered when passed to enum

from polars.

Yes, I agree with @markxwang here. Enum is an orderded datatype, for cut I would much rather return an Enum directly than a categorical. See #13038.

from polars.

@c-peters I see you self-assigned. I have a working impl, if you haven't started already should I just submit a PR?

from polars.

After some more thought. We could allow this on Series (materialized data), but not on expressions. Categories are inside our Enum Datatype and are required for reasoning about the schema (correctness). If our Enum categories are dynamic then we can not reason up front if our query is correct which is an important property in our LogicalPlan.

from polars.

@c-peters I agree with you, and now I'm even hesitant to add this for Series, since the only reason is to workaround existing API issues where a categorical is returned instead of an enum.

Also, if we add an Enum as a return dtype to (for example) pl.cut, then we again are going to have a really hard time (read: impossible) propagating the schema in an expression context. What if someone uses pl.cut in a when/then and provides alternative values to what cut returns? Let's keep as categorical and reserve enums exclusively for static categoricals.

Do you agree here? I vote we just close and I'll cancel my PR.

from polars.

For pl.cut we do know the categories as they are the labels by the user or a derivative of the breaks parameter. So the schema would still be known after the operation.

But I agree, let's keep the Enums simple for now and exclusively use it for statics

from polars.