This adds some code (from iiasa/ixmp#317) that allows DataArray.to_series() to be called without invoking sparse.COO.todense() when that is the backing data type.

I'm aware this needs some improvement to meet the standard of the existing codebase, so I hope I could ask for some guidance on how to address the following points (including whom to ask about them): - [ ] Make the same improvement in {DataArray,Dataset}.to_dataframe(). - [ ] Possibly move the code out of dataarray.py to a more appropriate location (where?). - [ ] Possibly check for sparse.COO explicitly instead of xarray.core.pycompat.sparse_array_type. Other SparseArray subclasses, e.g. DOK, may not have the same attributes.

Standard items: - [ ] Tests added. - [x] Passes isort -rc . && black . && mypy . && flake8 (Sort of: these wanted to modify 7 files beyond the one I touched; didn't commit these changes.) - [ ] Fully documented, including whats-new.rst for all changes and api.rst for new API.

MCVE Code Sample

```python import numpy as np import pandas as pd import sparse import xarray as xr

Indices and raw data

foo = [f'foo{i}' for i in range(6)] bar = [f'bar{i}' for i in range(6)] raw = np.random.rand(len(foo) // 2, len(bar))

DataArray

a = xr.DataArray( data=sparse.COO.from_numpy(raw), coords=[foo[:3], bar], dims=['foo', 'bar'])

print(a.data.fill_value) # 0.0

Created from a pd.Series

b_series = pd.DataFrame(raw, index=foo[3:], columns=bar) \ .stack() \ .rename_axis(index=['foo', 'bar']) b = xr.DataArray.from_series(b_series, sparse=True)

print(b.data.fill_value) # nan

Works despite inconsistent fill-values

a + b a * b

Fails: complains about inconsistent fill-values

xr.concat([a, b], dim='foo') # ***

The fill_value argument doesn't help

xr.concat([a, b], dim='foo', fill_value=np.nan)

def fill_value(da): """Try to coerce one argument to a consistent fill-value.""" return xr.DataArray( data=sparse.as_coo(da.data, fill_value=np.nan), coords=da.coords, dims=da.dims, name=da.name, attrs=da.attrs, )

Fails: "Cannot provide a fill-value in combination with something that

already has a fill-value"

print(xr.concat([a.pipe(fill_value), b], dim='foo'))

If we cheat by recreating 'a' from scratch, copying the fill value of the

intended other argument, it works again:

a = xr.DataArray( data=sparse.COO.from_numpy(raw, fill_value=b.data.fill_value), coords=[foo[:3], bar], dims=['foo', 'bar']) c = xr.concat([a, b], dim='foo')

print(c.data.fill_value) # nan

But simple operations again create objects with potentially incompatible

fill-values

d = c.sum(dim='bar') print(d.data.fill_value) # 0.0 ```

Expected

concat() can be used without having to create new objects; i.e. the line marked *** just works.

Problem Description

Some basic xarray manipulations don't work on sparse.COO-backed objects.

xarray should automatically coerce objects into a compatible state, or at least provide users with methods to do so. Behaviour should also be documented, e.g. in this instance, which operations (here, .sum()) modify the underlying storage format in ways that necessitate some kind of (re-)conversion.

Output of xr.show_versions()