issues: 253107677

Reported on the mailing list:

Original datasets: ```

ds_xr <xarray.DataArray (time: 12775)> array([-0.01, -0.01, -0.01, ..., -0.27, -0.27, -0.27]) Coordinates: * time (time) datetime64[ns] 1979-01-01 1979-01-02 1979-01-03 ...

slope_itcp_ds <xarray.Dataset> Dimensions: (lat: 73, level: 2, lon: 144, time: 366) Coordinates: * lon (lon) float32 0.0 2.5 5.0 7.5 10.0 12.5 ... * lat (lat) float32 90.0 87.5 85.0 82.5 80.0 ... * level (level) float64 0.0 1.0 * time (time) datetime64[ns] 2010-01-01 ... Data variables: xarray_dataarray_variable (time, level, lat, lon) float64 -0.8795 ... Attributes: CDI: Climate Data Interface version 1.7.1 (http://mpimet.mpg.de/... Conventions: CF-1.4 history: Fri Aug 25 18:55:50 2017: cdo -inttime,2010-01-01,00:00:00,... CDO: Climate Data Operators version 1.7.1 (http://mpimet.mpg.de/... ```

Issue: Grouping by month works and outputs this: ```

ds_xr.groupby('time.month') - slope_itcp_ds.groupby('time.month').mean('time') <xarray.Dataset> Dimensions: (lat: 73, level: 2, lon: 144, time: 12775) Coordinates: * lon (lon) float32 0.0 2.5 5.0 7.5 10.0 12.5 ... * lat (lat) float32 90.0 87.5 85.0 82.5 80.0 ... * level (level) float64 0.0 1.0 month (time) int64 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ... * time (time) datetime64[ns] 1979-01-01 ... Data variables: xarray_dataarray_variable (time, level, lat, lon) float64 1.015 ... ```

Grouping by dayofyear doesn't work and gives this traceback: ```

ds_xr.groupby('time.dayofyear') - slope_itcp_ds.groupby('time.dayofyear').mean('time') KeyError Traceback (most recent call last) <ipython-input-10-01c0cf4c980a> in <module>() ----> 1 ds_xr.groupby('time.dayofyear') - slope_itcp_ds.groupby('time.dayofyear').mean('time')

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/groupby.py in func(self, other) 316 g = f if not reflexive else lambda x, y: f(y, x) 317 applied = self._yield_binary_applied(g, other) --> 318 combined = self._combine(applied) 319 return combined 320 return func

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/groupby.py in _combine(self, applied, shortcut) 532 combined = self._concat_shortcut(applied, dim, positions) 533 else: --> 534 combined = concat(applied, dim) 535 combined = _maybe_reorder(combined, dim, positions) 536

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in concat(objs, dim, data_vars, coords, compat, positions, indexers, mode, concat_over) 118 raise TypeError('can only concatenate xarray Dataset and DataArray ' 119 'objects, got %s' % type(first_obj)) --> 120 return f(objs, dim, data_vars, coords, compat, positions) 121 122

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in _dataset_concat(datasets, dim, data_vars, coords, compat, positions) 210 datasets = align(*datasets, join='outer', copy=False, exclude=[dim]) 211 --> 212 concat_over = _calc_concat_over(datasets, dim, data_vars, coords) 213 214 def insert_result_variable(k, v):

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in _calc_concat_over(datasets, dim, data_vars, coords) 190 if dim in v.dims) 191 concat_over.update(process_subset_opt(data_vars, 'data_vars')) --> 192 concat_over.update(process_subset_opt(coords, 'coords')) 193 if dim in datasets[0]: 194 concat_over.add(dim)

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in process_subset_opt(opt, subset) 165 for ds in datasets[1:]) 166 # all nonindexes that are not the same in each dataset --> 167 concat_new = set(k for k in getattr(datasets[0], subset) 168 if k not in concat_over and differs(k)) 169 elif opt == 'all':

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in <genexpr>(.0) 166 # all nonindexes that are not the same in each dataset 167 concat_new = set(k for k in getattr(datasets[0], subset) --> 168 if k not in concat_over and differs(k)) 169 elif opt == 'all': 170 concat_new = (set(getattr(datasets[0], subset)) -

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in differs(vname) 163 v = datasets[0].variables[vname] 164 return any(not ds.variables[vname].equals(v) --> 165 for ds in datasets[1:]) 166 # all nonindexes that are not the same in each dataset 167 concat_new = set(k for k in getattr(datasets[0], subset)

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/combine.py in <genexpr>(.0) 163 v = datasets[0].variables[vname] 164 return any(not ds.variables[vname].equals(v) --> 165 for ds in datasets[1:]) 166 # all nonindexes that are not the same in each dataset 167 concat_new = set(k for k in getattr(datasets[0], subset)

/data/keeling/a/ahuang11/anaconda3/lib/python3.6/site-packages/xarray/core/utils.py in getitem(self, key) 288 289 def getitem(self, key): --> 290 return self.mapping[key] 291 292 def iter(self):

KeyError: 'lon' ```