issues

Is there some way to do a groupby operation where some dimension is excluded from the operation, e.g., a vectorized version of something like this:

python vertlevels = [ds.vel.sel(nVertLevels=i).groupby('y').mean() for i in ds.nVertLevels] xavgvel = xr.concat(vertlevels, 'nVertLevels')

The application here is to average 3D data in the x coordinate to the unique y coordinates, but not the vertical coordinate.

Thus, we are basically looking for something that allows a coordinate to be excluded from the groupby operation, e.g., in this case the vertical coordinate. Ideally this would also be possible within the context of the groupby_bins operation.

Any ideas on how this should work or a pointer on how to implement this type of operation more cleanly with existing infrastructure is greatly appreciated. This appears to be related to #324 and especially (perhaps identically) #924.

cc @vanroekel, @shoyer, @jhamman, @rabernat, @MaximilianR

In the case of a DataArray without coordinates, should cumsum work without specifying an axis, e.g., da.cumsum() is valid? This is not currently the obtained behavior.

```python In [1]: import xarray as xr imp In [2]: import numpy as np

In [3]: da = xr.DataArray(np.arange(10))

In [4]: da Out[4]: <xarray.DataArray (dim_0: 10)> array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) Dimensions without coordinates: dim_0

In [5]: da.cumsum(axis=0) Out[5]: <xarray.DataArray (dim_0: 10)> array([ 0, 1, 3, 6, 10, 15, 21, 28, 36, 45]) Dimensions without coordinates: dim_0

In [6]: da.cumsum()

ValueError Traceback (most recent call last) <ipython-input-5-4ff0efc782ee> in <module>() ----> 1 da.cumsum()

/Users/pwolfram/src/xarray/xarray/core/common.pyc in wrapped_func(self, dim, axis, skipna, keep_attrs, kwargs) 17 keep_attrs=False, kwargs): 18 return self.reduce(func, dim, axis, keep_attrs=keep_attrs, ---> 19 skipna=skipna, allow_lazy=True, **kwargs) 20 else: 21 def wrapped_func(self, dim=None, axis=None, keep_attrs=False,

/Users/pwolfram/src/xarray/xarray/core/dataarray.pyc in reduce(self, func, dim, axis, keep_attrs, kwargs) 1146 summarized data and the indicated dimension(s) removed. 1147 """ -> 1148 var = self.variable.reduce(func, dim, axis, keep_attrs, kwargs) 1149 return self._replace_maybe_drop_dims(var) 1150

/Users/pwolfram/src/xarray/xarray/core/variable.pyc in reduce(self, func, dim, axis, keep_attrs, allow_lazy, **kwargs) 898 if dim is None and axis is None: 899 raise ValueError("must supply either single 'dim' or 'axis' " --> 900 "argument to %s" % (func.name)) 901 902 if dim is not None:

ValueError: must supply either single 'dim' or 'axis' argument to cumsum

```