If you want to return your newly-calculated altitude and also have it be a full data_var in your dataset, one way would be to just alter the original dataset in-place. Something like

```python

import xarray as xr import pandas as pd import xarray.testing as xrt

@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)])

        # Here we add the calculated altitude to the dataset as a new data variable
        self._ds['altitude'] = self._altitude

    # Return just the altitude dataarray
    return self._altitude

expected = 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')})

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

Return newly-calculated altitude, but also store it in the actual dataset for later

altitude = actual.acc.altitude

Check that worked

xrt.assert_equal(actual, expected) xrt.assert_equal(actual['altitude'], actual.acc.altitude) ```