issue_comments: 393345152

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https://github.com/pydata/xarray/issues/1569#issuecomment-393345152 https://api.github.com/repos/pydata/xarray/issues/1569 393345152 MDEyOklzc3VlQ29tbWVudDM5MzM0NTE1Mg== 5635139 2018-05-30T22:52:38Z 2018-05-30T22:52:38Z MEMBER

Are we making silent mistakes at the moment?

```python

In [1]: import xarray as xr In [2]: import pandas as pd

In [4]: import numpy as np

In [5]: ds=xr.Dataset(dict(a=(('z',), np.ones(10))), coords=dict(b=(('z'),np.arange(2).repeat(5)), c=(('z'),np.arange(5).repeat(2))))

In [6]: ds Out[6]: <xarray.Dataset> Dimensions: (z: 10) Coordinates: b (z) int64 0 0 0 0 0 1 1 1 1 1 c (z) int64 0 0 1 1 2 2 3 3 4 4 Dimensions without coordinates: z Data variables: a (z) float64 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

In [7]: ds.set_index(bc=['b','c']).groupby('bc').sum() Out[7]: <xarray.Dataset> Dimensions: (bc: 6) Coordinates: * bc (bc) MultiIndex - bc_level_0 (bc) int64 0 0 0 1 1 1 - bc_level_1 (bc) int64 0 1 2 2 3 4 Data variables: a (bc) float64 10.0 10.0 10.0 10.0 10.0 10.0 # <--- this is wrong? ```

With a DataArray, we do raise:

```python In [19]: ds['a'].set_index(bc=['b','c']) Out[19]: <xarray.DataArray 'a' (z: 10)> array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.]) Coordinates: * bc (bc) MultiIndex - b (bc) int64 0 0 0 0 0 1 1 1 1 1 - c (bc) int64 0 0 1 1 2 2 3 3 4 4 Dimensions without coordinates: z

In [20]: ds['a'].set_index(bc=['b','c']).groupby('bc')

KeyError Traceback (most recent call last) <ipython-input-20-82bf6782525c> in <module>() ----> 1 ds['a'].set_index(bc=['b','c']).groupby('bc')

/usr/local/lib/python3.6/site-packages/xarray/core/common.py in groupby(self, group, squeeze) 472 core.groupby.DatasetGroupBy 473 """ --> 474 return self._groupby_cls(self, group, squeeze=squeeze) 475 476 def groupby_bins(self, group, bins, right=True, labels=None, precision=3,

/usr/local/lib/python3.6/site-packages/xarray/core/groupby.py in init(self, obj, group, squeeze, grouper, bins, cut_kwargs) 213 group_dim, = group.dims 214 --> 215 expected_size = obj.sizes[group_dim] 216 if group.size != expected_size: 217 raise ValueError('the group variable\'s length does not '

/usr/local/lib/python3.6/site-packages/xarray/core/utils.py in getitem(self, key) 306 307 def getitem(self, key): --> 308 return self.mapping[key] 309 310 def iter(self):

KeyError: 'bc' ```

 
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