issues: 264517839

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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
264517839 MDU6SXNzdWUyNjQ1MTc4Mzk= 1625 Option for arithmetics to ignore nans created by alignment 6213168 closed 0     3 2017-10-11T09:33:34Z 2019-07-11T09:48:07Z 2019-07-11T09:48:07Z MEMBER      

Can anybody tell me if there is anybody who benefits from this behaviour? I can't think of any good use cases.

``` wallet = xarray.DataArray([50, 70], dims=['currency'], coords={'currency': ['EUR', 'USD']}) restaurant_bill = xarray.DataArray([30], dims=['currency'], coords={'currency': ['USD']}) with xarray.set_options(arithmetic_join="outer"): print(wallet - restaurant_bill)

<xarray.DataArray (currency: 2)> array([ nan, 40.]) Coordinates: * currency (currency) object 'EUR' 'USD' ```

While it is fairly clear why it can be desirable to have nan + not nan = nan as a default in arithmetic when the nan is already present in one of the input arrays, when the nan is introduced as part of an automatic align things become much less intuitive.

Proposal: - add a parameter to xarray.align, fillvalue=numpy.nan, which determines what will appear in the newly created array elements - change __add__, __sub__ etc. to invoke xarray.align(fillvalue=0) - change __mul__, __truediv__ etc. to invoke xarray.align(fillvalue=1)

In theory the setting could be left as an opt-in as set_options(arithmetic_align_fillvalue='neutral'), yet I wonder who would actually want the current behaviour?

 
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