issue_comments

Oh, I should have been a little clearer!

For now I've held off on making these changes dask-compatible (I could do it, but I'm not sure it is worth the extra complexity)

This comment only applies to the changes regarding duck_array_ops.mean, which is used by default on the coordinates involved in coarsen. Since indexes are always loaded into memory, i.e. backed by NumPy arrays, we don't really need to worry about dask-compatibility there. In other words with this PR a DataArray can hold dask array data indexed by a cftime time coordinate, and coarsen will work just fine: ``` In [1]: import xarray as xr

In [2]: import numpy as np

In [3]: data = np.random.random((10, 5))

In [4]: times = xr.cftime_range('2000', periods=10)

In [5]: da = xr.DataArray(data, coords={'time': times}, dims=['time', 'x'])

In [6]: da = da.chunk({'time': 1, 'x': 1})

In [7]: da Out[7]: <xarray.DataArray (time: 10, x: 5)> dask.array<shape=(10, 5), dtype=float64, chunksize=(1, 1)> Coordinates: * time (time) object 2000-01-01 00:00:00 ... 2000-01-10 00:00:00 Dimensions without coordinates: x

In [8]: da.coarsen(time=2).mean() Out[8]: <xarray.DataArray (time: 5, x: 5)> dask.array<shape=(5, 5), dtype=float64, chunksize=(1, 1)> Coordinates: * time (time) object 2000-01-01 12:00:00 ... 2000-01-09 12:00:00 Dimensions without coordinates: x ```

This would only come up as a possible issue if you tried to lazily take the mean of a DataArray of cftime objects, e.g.: ``` In [9]: da = xr.DataArray(times, dims=['t']).chunk()

In [10]: da Out[10]: <xarray.DataArray (t: 10)> dask.array<shape=(10,), dtype=object, chunksize=(10,)> Coordinates: * t (t) object 2000-01-01 00:00:00 ... 2000-01-10 00:00:00

In [11]: da.mean()

NotImplementedError Traceback (most recent call last) <ipython-input-19-c02402258881> in <module> ----> 1 da.mean()

~/xarray-dev/xarray/xarray/core/common.py in wrapped_func(self, dim, axis, skipna, kwargs) 23 kwargs): 24 return self.reduce(func, dim, axis, ---> 25 skipna=skipna, allow_lazy=True, **kwargs) 26 else: 27 def wrapped_func(self, dim=None, axis=None, # type: ignore

~/xarray-dev/xarray/xarray/core/dataarray.py in reduce(self, func, dim, axis, keep_attrs, kwargs) 1603 """ 1604 -> 1605 var = self.variable.reduce(func, dim, axis, keep_attrs, kwargs) 1606 return self._replace_maybe_drop_dims(var) 1607

~/xarray-dev/xarray/xarray/core/variable.py in reduce(self, func, dim, axis, keep_attrs, allow_lazy, kwargs) 1366 data = func(input_data, axis=axis, kwargs) 1367 else: -> 1368 data = func(input_data, **kwargs) 1369 1370 if getattr(data, 'shape', ()) == self.shape:

~/xarray-dev/xarray/xarray/core/duck_array_ops.py in mean(array, axis, skipna, **kwargs) 348 if isinstance(array, dask_array_type): 349 raise NotImplementedError( --> 350 'Computing the mean of an array containing ' 351 'cftime.datetime objects is not yet implemented on ' 352 'dask arrays.')

NotImplementedError: Computing the mean of an array containing cftime.datetime objects is not yet implemented on dask arrays. ``` but I think that's a pretty rare use case, hence why I've held off on adding that support for now.