issues: 497427114

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497427114 MDU6SXNzdWU0OTc0MjcxMTQ= 3337 Dataset.groupby reductions give "Dataset does not contain dimensions error" in v0.13 1386642 closed 0     1 2019-09-24T03:01:00Z 2019-10-10T18:23:22Z 2019-10-10T18:23:22Z CONTRIBUTOR      

MCVE Code Sample

```python

ds = xr.DataArray(np.ones((4,5)), dims=['z', 'x']).to_dataset(name='a') ds.a.groupby('z').mean() <xarray.DataArray 'a' (z: 4)> array([1., 1., 1., 1.]) Dimensions without coordinates: z ds.groupby('z').mean() Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/common.py", line 91, in wrapped_func kwargs File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/groupby.py", line 848, in reduce return self.apply(reduce_dataset) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/groupby.py", line 796, in apply return self._combine(applied) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/groupby.py", line 800, in _combine applied_example, applied = peek_at(applied) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/utils.py", line 181, in peek_at peek = next(gen) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/groupby.py", line 795, in <genexpr> applied = (func(ds, *args, kwargs) for ds in self._iter_grouped()) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/groupby.py", line 846, in reduce_dataset return ds.reduce(func, dim, keep_attrs, **kwargs) File "/Users/noah/miniconda3/envs/broken/lib/python3.7/site-packages/xarray/core/dataset.py", line 3888, in reduce "Dataset does not contain the dimensions: %s" % missing_dimensions ValueError: Dataset does not contain the dimensions: ['z'] ds.dims Frozen(SortedKeysDict({'z': 4, 'x': 5})) ```

Problem Description

Groupby reduction operations on Dataset objects no longer seem to work in xarray v0.13. In the example, above I create an xarray dataset with one dataarray called "a". The same groupby operations fails on this Dataset, but succeeds when called directly on "a". Is this a bug or an intended change?

In addition the error message is confusing since z is one of the Dataset dimensions.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.3 | packaged by conda-forge | (default, Jul 1 2019, 14:38:56) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.6.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: None libnetcdf: None xarray: 0.13.0 pandas: 0.25.1 numpy: 1.17.2 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None 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: None cartopy: None seaborn: None numbagg: None setuptools: 41.2.0 pip: 19.2.3 conda: None pytest: None IPython: None sphinx: None 
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