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
976790237,MDExOlB1bGxSZXF1ZXN0NzE3NjMxNjU0,5731,Make xr.corr and xr.map_blocks work without dask,9466648,closed,0,,,16,2021-08-23T08:53:04Z,2021-11-24T17:48:52Z,2021-11-24T17:25:24Z,CONTRIBUTOR,,0,pydata/xarray/pulls/5731,"- [X] Closes #3391
- [X] Closes #5715
- [X] Tests added
- [X] Passes `pre-commit run --all-files`
- [x] User visible changes (including notable bug fixes) are documented in `whats-new.rst`

","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/5731/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull
974488736,MDU6SXNzdWU5NzQ0ODg3MzY=,5715,Dask error on xarray.corr,9466648,closed,0,,,5,2021-08-19T09:59:28Z,2021-11-24T17:25:24Z,2021-11-24T17:25:24Z,CONTRIBUTOR,,,,"**What happened**:
When I use xarray.corr on two Dataarrays I get a `NameError: name 'dask' is not defined` error. Notice that dask is not installed in my environement.

**What you expected to happen**:
Obtain the correlation values without dask interfering (as it should be optional in my understanding)

**Minimal Complete Verifiable Example**:
```python
N = 100
ds = xr.Dataset(
    data_vars={
        'x': ('t', np.random.randn(N)),
        'y': ('t', np.random.randn(N))
    },
    coords={
        't': range(N)
    }
)
xr.corr(ds['y'], ds['x'])
```
Results in:
```
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
/tmp/ipykernel_732567/1992585666.py in <module>
----> 1 xr.corr(ds['y'], ds['x'])

~/.local/share/virtualenvs/e-sport-ml-IJ_mJ64l/lib/python3.8/site-packages/xarray/core/computation.py in corr(da_a, da_b, dim)
   1343         )
   1344 
-> 1345     return _cov_corr(da_a, da_b, dim=dim, method=""corr"")
   1346 
   1347 

~/.local/share/virtualenvs/e-sport-ml-IJ_mJ64l/lib/python3.8/site-packages/xarray/core/computation.py in _cov_corr(da_a, da_b, dim, ddof, method)
   1371             return da
   1372 
-> 1373     da_a = da_a.map_blocks(_get_valid_values, args=[da_b])
   1374     da_b = da_b.map_blocks(_get_valid_values, args=[da_a])
   1375 

~/.local/share/virtualenvs/e-sport-ml-IJ_mJ64l/lib/python3.8/site-packages/xarray/core/dataarray.py in map_blocks(self, func, args, kwargs, template)
   3811         from .parallel import map_blocks
   3812 
-> 3813         return map_blocks(func, self, args, kwargs, template)
   3814 
   3815     def polyfit(

~/.local/share/virtualenvs/e-sport-ml-IJ_mJ64l/lib/python3.8/site-packages/xarray/core/parallel.py in map_blocks(func, obj, args, kwargs, template)
    332             )
    333 
--> 334     if not dask.is_dask_collection(obj):
    335         return func(obj, *args, **kwargs)
    336 

NameError: name 'dask' is not defined
```

**Environment**:

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.6 (default, Dec 16 2020, 11:33:05) 
[GCC 10.2.0]
python-bits: 64
OS: Linux
OS-release: 5.13.6-arch1-1
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: 1.12.0
libnetcdf: None

xarray: 0.19.0
pandas: 1.3.1
numpy: 1.21.1
scipy: 1.7.1
netCDF4: None
pydap: None
h5netcdf: None
h5py: 3.3.0
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: 3.4.2
cartopy: None
seaborn: 0.11.1
numbagg: None
pint: None
setuptools: 51.0.0
pip: 20.3.1
conda: None
pytest: None
IPython: 7.26.0
sphinx: None
```
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