id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
520815068,MDU6SXNzdWU1MjA4MTUwNjg=,3509,"NEP 18, physical units, uncertainties, and the scipp library?",12912489,closed,0,,,3,2019-11-11T08:45:29Z,2022-09-09T13:10:47Z,2022-09-09T13:10:47Z,NONE,,,,"This is an idea and meant as a discussion starter on an potential route to providing support for physical units and propagation of uncertainties for `xarray`.

### Context

1. NEP 18 (which, as far as I understand, was pushed by you guys, for similar purposes) provides means to combine such features with `xarray` using `__array_function__`, if the underlying array implementation supports it.

2. I am working on [scipp](https://scipp.readthedocs.io/en/latest/), which (based on a decision I may or may not regret in the future) is reimplementing a lot of features that `xarray` provides, plus some additional features. Two of these features are physical units and propagation of uncertainties.

   - `scipp.Variable` is essentially equivalent to a `numpy` array with a unit, dimension labels, and an optional array of uncertainties. [*]
   - `scipp` implements basic arithmetic operations (and some more) for `scipp.Variable`, including efficient propagation of uncertainties.

[*] *Caveat: `scipp`'s current unit implementation is static an would probably need to be replaced for becoming useful for a wider audience.*

### Idea and questions

Can we implement the `__array_function__` protocol for `scipp.Variable` so it can be used with `xarray`? As far as I can tell this would simply be a lightweight wrapper.

- Did I understand `__array_function__` correctly?
- Is there anything else I should be aware of?
- Would anyone be interested in this?

This would amount to using the lower-level parts of `scipp` which is quite compact and can be extended to support more data types and more operations in a relatively simple manner (requiring recompilation, since it is written in C++).","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3509/reactions"", ""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue