issues: 173612265

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173612265 MDU6SXNzdWUxNzM2MTIyNjU= 988 Hooks for custom attribute handling in xarray operations 1217238 open 0     24 2016-08-27T19:48:22Z 2020-04-05T18:19:11Z   MEMBER      

Over in #964, I am working on a rewrite/unification of the guts of xarray's logic for computation with labelled data. The goal is to get all of xarray's internal logic for working with labelled data going through a minimal set of flexible functions which we can also expose as part of the API.

Because we will finally have all (or at least nearly all) xarray operations using the same code path, I think it will also finally become feasible to open up hooks allowing extensions how xarray handles metadata.

Two obvious use cases here are units (#525) and automatic maintenance of metadata (e.g., cell_methods or history fields). Both of these are out of scope for xarray itself, mostly because the specific logic tends to be domain specific. This could also subsume options like the existing keep_attrs on many operations.

I like the idea of supporting something like NumPy's __array_wrap__ to allow third-party code to finalize xarray objects in some way before they are returned. However, it's not obvious to me what the right design is. - Should we lookup a custom attribute on subclasses like __array_wrap__ (or __numpy_ufunc__) in NumPy, or should we have a system (e.g., unilaterally or with a context manager and xarray.set_options) for registering hooks that are then checked on all xarray objects? I am inclined toward the later, even though it's a little slower, just because it will be simpler and easier to get right - Should these methods be able to control the full result objects, or only set attrs and/or name? - To be useful, do we need to allow extensions to take control of the full operation, to support things like automatic unit conversion? This would suggest something closing to __numpy_ufunc__, which is a little more ambitious than what I had in mind here.

Feedback would be greatly appreciated.

CC @darothen @rabernat @jhamman @pwolfram

 
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