html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/987#issuecomment-242912131,https://api.github.com/repos/pydata/xarray/issues/987,242912131,MDEyOklzc3VlQ29tbWVudDI0MjkxMjEzMQ==,4992424,2016-08-27T11:34:28Z,2016-08-27T11:34:28Z,NONE,"@joonro, I think there's a strong case to be made about returning a `DataArray` with some metadata appended. Referring to the latest [draft of the CF Metadata Conventions](http://cfconventions.org/cf-conventions/cf-conventions.html), there is a clear way to indicate when operations such as `mean`, `max`, or `min` have been applied to a variable by using the [**cell_methods**](http://cfconventions.org/cf-conventions/cf-conventions.html#cell-methods) attribute. 

It might be more prudent to add this attribute whenever we apply these operations to a `DataArray` (or perhaps variable-wise when applied to a `Dataset`). That way, there is a clear reason to not return a scalar - the documentation of what operations were applied to produce that final result.

I can whip up a working example/pull request if people think this is a direction to go. I'd probably build a decorator which handles inspection of the operator name and arguments and uses that to add the **cell_methods** attribute, that way people can add the same functionality to homegrown methods/operators.
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