Don't apply mappers to DataArrays

cf_xarray/accessor.py

@@ -26,7 +26,12 @@
 
 from .criteria import coordinate_criteria, regex
 from .helpers import bounds_to_vertices
-from .utils import _is_datetime_like, invert_mappings, parse_cell_methods_attr
+from .utils import (
+    _is_datetime_like,
+    always_iterable,
+    invert_mappings,
+    parse_cell_methods_attr,
+)
 
 #: Classes wrapped by cf_xarray.
 _WRAPPED_CLASSES = (
@@ -68,7 +73,7 @@
 def apply_mapper(
     mappers: Union[Mapper, Tuple[Mapper, ...]],
     obj: Union[DataArray, Dataset],
-    key: str,
+    key: Any,
     error: bool = True,
     default: Any = None,
 ) -> List[Any]:
@@ -79,8 +84,13 @@ def apply_mapper(
     It should return a list in all other cases including when there are no
     results for a good key.
     """
-    if default is None:
-        default = []
+
+    if not isinstance(key, str):
+        if default is None:
+            raise ValueError("`default` must be provided when `key` is not a string.")
+        return list(always_iterable(default))
+
+    default = [] if default is None else list(always_iterable(default))
 
     def _apply_single_mapper(mapper):
 
@@ -917,8 +927,7 @@ def _rewrite_values(
             value = kwargs[key]
             mappers = all_mappers[key]
 
-            if isinstance(value, str):
-                value = [value]
+            value = always_iterable(value)
 
             if isinstance(value, dict):
                 # this for things like isel where **kwargs captures things like T=5

cf_xarray/tests/test_accessor.py

@@ -317,8 +317,13 @@ def test_weighted(obj):
     with raise_if_dask_computes(max_computes=2):
         # weights are checked for nans
         expected = obj.weighted(obj["cell_area"]).sum("lat")
-        actual = obj.cf.weighted("area").sum("Y")
-    assert_identical(expected, actual)
+        actuals = [
+            obj.cf.weighted("area").sum("Y"),
+            obj.cf.weighted(obj["cell_area"]).sum("Y"),
+            obj.cf.weighted(weights=obj["cell_area"]).sum("Y"),
+        ]
+    for actual in actuals:
+        assert_identical(expected, actual)
 
 
 @pytest.mark.parametrize("obj", objects)

cf_xarray/utils.py

@@ -1,5 +1,5 @@
 from collections import defaultdict
-from typing import Dict
+from typing import Any, Dict, Iterable
 
 from xarray import DataArray
 
@@ -53,3 +53,7 @@ def invert_mappings(*mappings):
             for name in v:
                 merged[name] |= {k}
     return merged
+
+
+def always_iterable(obj: Any) -> Iterable:
+    return [obj] if not isinstance(obj, (tuple, list, set, dict)) else obj

doc/examples/introduction.ipynb

@@ -25,6 +25,22 @@
     "import xarray as xr"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "`cf_xarray` works best when `xarray` keeps attributes by default.\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "xr.set_options(keep_attrs=True)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -914,6 +930,7 @@
     "    * 110e3\n",
     ")\n",
     "# and set proper attributes\n",
+    "ds[\"cell_area\"].attrs = dict(standard_name=\"cell_area\", units=\"m2\")\n",
     "ds.air.attrs[\"cell_measures\"] = \"area: cell_area\""
    ]
   },
@@ -1000,7 +1017,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.1"
+   "version": "3.8.10"
   },
   "toc": {
    "base_numbering": 1,