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/3570#issuecomment-692664177,https://api.github.com/repos/pydata/xarray/issues/3570,692664177,MDEyOklzc3VlQ29tbWVudDY5MjY2NDE3Nw==,10194086,2020-09-15T11:50:11Z,2020-09-15T11:50:11Z,MEMBER,"There are 3 types of failures:
- non-lazy evaluation
- missing alignment (in merge)
- not raising `DimensionalityError`

So its not trivial...

<details>

```Python traceback
FAILED xarray/tests/test_dask.py::test_lazy_array_equiv_merge[no_conflicts] - RuntimeError: Too many computes. Total: 1 > max: 0.
FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmin[True-x2-minindex2-maxindex2-nanindex2] - RuntimeError: Too many computes. Total: 2 > max: 1.
FAILED xarray/tests/test_dataarray.py::TestReduce2D::test_idxmax[True-x2-minindex2-maxindex2-nanindex2] - RuntimeError: Too many computes. Total: 2 > max: 1.
FAILED xarray/tests/test_dataset.py::TestDataset::test_dask_is_lazy - xarray.tests.UnexpectedDataAccess: Tried accessing data
FAILED xarray/tests/test_merge.py::TestMergeMethod::test_merge_broadcast_equals - ValueError: applied function returned data with unexpected number of dimensions. Received 0 dimension(s) but expected 1 dim...
FAILED xarray/tests/test_units.py::TestDataArray::test_fillna[int64-python_scalar-no_unit] - Failed: DID NOT RAISE <class 'pint.errors.DimensionalityError'>
... more of those
```
</details>

Potentially this could also be fixed in `Dataset.fillna`, however, the fill value would need to be `dtype` dependent, so also not trivial...

https://github.com/pydata/xarray/blob/66ab0ae4f3aa3c461357a5a895405e81357796b1/xarray/core/dataset.py#L4000
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,528060435
https://github.com/pydata/xarray/issues/3570#issuecomment-692574610,https://api.github.com/repos/pydata/xarray/issues/3570,692574610,MDEyOklzc3VlQ29tbWVudDY5MjU3NDYxMA==,10194086,2020-09-15T08:59:49Z,2020-09-15T09:58:49Z,MEMBER,"The problem is that xarray calls `where(isnull(data), data, other)` for *all* variables. It also uses `dataset_join=""left""`, thus `other` is `NaN` for all DataArrays that are not passed... I think `fillna` should be made a no-op `if notnull(data).any()`


https://github.com/pydata/xarray/blob/66ab0ae4f3aa3c461357a5a895405e81357796b1/xarray/core/duck_array_ops.py#L284-L288

A possible workaround is to use `da.fillna(value={""A"": 0, ""C"": True})`


","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,528060435
https://github.com/pydata/xarray/issues/3570#issuecomment-692609976,https://api.github.com/repos/pydata/xarray/issues/3570,692609976,MDEyOklzc3VlQ29tbWVudDY5MjYwOTk3Ng==,10194086,2020-09-15T09:57:47Z,2020-09-15T09:57:47Z,MEMBER,"The straightforward idea unfortunately leads to some test failures. I.e. the following does not work:

```python
def fillna(data, other):
    # we need to pass data first so pint has a chance of returning the
    # correct unit
    # TODO: revert after https://github.com/hgrecco/pint/issues/1019 is fixed
    mask = isnull(data)

    if mask.any():
        return where(~mask, data, other)
    else:
        return data
```

","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,528060435