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
189481649,MDU6SXNzdWUxODk0ODE2NDk=,1123,Assigning several variables at once in a dataset,6333189,closed,0,,,3,2016-11-15T19:20:44Z,2021-05-02T20:29:34Z,2021-05-02T20:29:34Z,NONE,,,,"Hi,

I cannot tell if I'm doing something illegitimate. I have a hard time to understand the error I'm facing:

```python
import numpy as np
import xarray as xr

ds = xr.Dataset(
    {'varA': (['x', 'time'], np.random.rand(2, 2)),
     'varB': (['x', 'time'], np.random.rand(2, 2)),
     'varC': (['x', 'time'], np.random.rand(2, 2))},
    coords={'x': [0, 1.], 'time': [0, 1.]})

print(ds)

# I want to center 'varA' and 'varB' by substracting their mean values along the time dimension:
ds[['varA', 'varB']] -= ds[['varA', 'varB']].mean(dim='time')
```

Produces the output:
```
<xarray.Dataset>
Dimensions:  (time: 2, x: 2)
Coordinates:
  * x        (x) float64 0.0 1.0
  * time     (time) float64 0.0 1.0
Data variables:
    varA     (x, time) float64 0.7737 0.2608 0.009832 0.8498
    varC     (x, time) float64 0.9732 0.6492 0.5118 0.5792
    varB     (x, time) float64 0.04973 0.4012 0.01248 0.8716
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-36-aa87123fb25d> in <module>()
      7 print(ds)
      8 
----> 9 ds[['varA', 'varB']] -= ds[['varA', 'varB']].mean(dim='time')

C:\Users\poimat04\AppData\Local\Continuum\Miniconda2\envs\AxialLLRCA\lib\site-packages\xarray\core\dataset.pyc in __setitem__(self, key, value)
    536             raise NotImplementedError('cannot yet use a dictionary as a key '
    537                                       'to set Dataset values')
--> 538         self.update({key: value})
    539 
    540     def __delitem__(self, key):

TypeError: unhashable type: 'list'
```

Is it forbidden to assign several variables at once? I'm puzzled with this TypeError.

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