Code Sample

```python import xarray as xr import pandas as pd

What I'm doing with comparison, I'd like to do with actual

comparison = xr.Dataset({'data': (['time'], [100, 30, 10, 3, 1]), 'altitude': (['time'], [5, 10, 15, 20, 25])}, coords={'time': pd.date_range('2014-09-06', periods=5, freq='1s')})

With altitude as a data var, I can do the following:

comparison.swap_dims({'time': 'altitude'}).interp(altitude=12.0).data

And

for (time, g) in comparison.groupby('time'): print(time) print(g.altitude.values)

@xr.register_dataset_accessor('acc') class Accessor(object): def init(self, xarray_ds): self._ds = xarray_ds self._altitude = None

@property
def altitude(self):
    """ An expensive calculation that results in data that not everyone needs. """
    if self._altitude is None:
        self._altitude = xr.DataArray([5, 10, 15, 20, 25],
                                      coords=[('time', self._ds.time)])
    return self._altitude

actual = xr.Dataset({'data': (['time'], [100, 30, 10, 3, 1])}, coords={'time': pd.date_range('2014-09-06', periods=5, freq='1s')})

This doesn't work:

actual.swap_dims({'time': 'altitude'}).interp(altitude=12.0).data

Neither does this:

for (time, g) in actual.groupby('time'): print(time) print(g.acc.altitude.values)

```

Problem description

I've been using accessors to extend xarray with some custom computation. The altitude in the above dataset is not used every time the data is loaded, but when it is, it is an expensive computation to make (which is why I put it in as an accessor; if it isn't needed, it isn't computed).

Problem is, once it has been computed, I'd like to be able to use it as if it is a regular data_var of the dataset. For example, to interp on the newly computed column, or use it in a groupby.

Please advise if I'm going about this in the wrong way and how I should think about this problem instead.

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.6.5.final.0 python-bits: 64 OS: Linux OS-release: 4.18.16-arch1-1-ARCH machine: x86_64 processor: byteorder: little LC_ALL: None LANG: en_CA.UTF-8 LOCALE: en_CA.UTF-8 xarray: 0.10.8 pandas: 0.23.1 numpy: 1.14.5 scipy: 1.1.0 netCDF4: 1.4.0 h5netcdf: 0.6.1 h5py: 2.8.0 Nio: None zarr: None bottleneck: 1.2.1 cyordereddict: None dask: 0.17.5 distributed: 1.21.8 matplotlib: 2.2.2 cartopy: None seaborn: None setuptools: 39.2.0 pip: 9.0.3 conda: None pytest: 3.6.1 IPython: None sphinx: None