Add ancillary variables to getitem

Author: dcherian

cf_xarray/accessor.py

@@ -642,7 +642,10 @@ def __getitem__(self, key: Union[str, List[str]]):
                     ]
                     coords.extend(_strip_none_list(measures))
 
-            varnames.extend(coords)
+                if isinstance(self._obj, xr.Dataset) and "ancillary_variables" in attrs:
+                    anames = attrs["ancillary_variables"].split(" ")
+                    coords.extend(anames)
+
             if isinstance(self._obj, xr.DataArray):
                 ds = self._obj._to_temp_dataset()
             else:

cf_xarray/tests/datasets.py

@@ -47,3 +47,26 @@
     np.ones((20, 30)) * 15,
     {"coordinates": "TLONG TLAT", "standard_name": "sea_water_potential_temperature"},
 )
+
+
+# This dataset has ancillary variables
+
+anc = xr.Dataset()
+anc["q"] = (
+    ("x", "y"),
+    np.random.randn(10, 20),
+    dict(
+        standard_name="specific_humidity",
+        units="g/g",
+        ancillary_variables="q_error_limit q_detection_limit",
+    ),
+)
+anc["q_error_limit"] = (
+    ("x", "y"),
+    np.random.randn(10, 20),
+    dict(standard_name="specific_humidity standard_error", units="g/g"),
+)
+anc["q_detection_limit"] = xr.DataArray(
+    1e-3, attrs=dict(standard_name="specific_humidity detection_minimum", units="g/g"),
+)
+anc

cf_xarray/tests/test_accessor.py

@@ -8,7 +8,7 @@
 import cf_xarray  # noqa
 
 from . import raise_if_dask_computes
-from .datasets import airds, ds_no_attrs, popds
+from .datasets import airds, anc, ds_no_attrs, popds
 
 mpl.use("Agg")
 
@@ -45,6 +45,12 @@ def test_getitem_standard_name():
         ds.air.cf["air_temperature"]
 
 
+def test_getitem_ancillary_variables():
+    expected = anc.set_coords(["q_error_limit", "q_detection_limit"])["q"]
+    assert_identical(anc.cf["q"], expected)
+    assert_identical(anc.cf["specific_humidity"], expected)
+
+
 @pytest.mark.parametrize("obj", objects)
 @pytest.mark.parametrize(
     "attr, xrkwargs, cfkwargs",