Allow expanding single keys to multiple keys.

.isel(X=5, Y=5) → .isel(xi_rho=5, xi_u=5, xi_v=5, eta_rho=10, ...)

Closes xarray-contrib#13

cf_xarray/accessor.py

@@ -1,5 +1,6 @@
 import functools
 import inspect
+from collections import ChainMap
 from typing import Any, List, Optional, Set, Union
 
 import xarray as xr
@@ -95,7 +96,7 @@ def _get_axis_coord_single(var, key, *args):
     results = _get_axis_coord(var, key, *args)
     if len(results) > 1:
         raise ValueError(
-            "Multiple results for {key!r} found: {results!r}. Is this valid CF? Please open an issue."
+            f"Multiple results for {key!r} found: {results!r}. Is this valid CF? Please open an issue."
         )
     else:
         return results[0]
@@ -335,20 +336,34 @@ def _process_signature(self, func, args, kwargs, key_mappers):
     def _rewrite_values(self, kwargs, key_mappers: dict, var_kws):
         """ rewrites 'dim' for example using 'mapper' """
         updates: dict = {}
-        key_mappers.update(dict.fromkeys(var_kws, _get_axis_coord_single))
+
+        # allow multiple return values here.
+        # these are valid for .sel, .isel, .coarsen
+        key_mappers.update(dict.fromkeys(var_kws, _get_axis_coord))
+
         for key, mapper in key_mappers.items():
             value = kwargs.get(key, None)
+
             if value is not None:
                 if isinstance(value, str):
                     value = [value]
 
                 if isinstance(value, dict):
                     # this for things like isel where **kwargs captures things like T=5
-                    updates[key] = {
-                        mapper(self._obj, k, False, k): v for k, v in value.items()
-                    }
+                    # .sel, .isel, .rolling
+                    # Account for multiple names matching the key.
+                    # e.g. .isel(X=5) → .isel(xi_rho=5, xi_u=5, xi_v=5, xi_psi=5)
+                    # where xi_* have attrs["axis"] = "X"
+                    updates[key] = ChainMap(
+                        *[
+                            dict.fromkeys(mapper(self._obj, k, False, k), v)
+                            for k, v in value.items()
+                        ]
+                    )
+
                 elif value is Ellipsis:
                     pass
+
                 else:
                     # things like sum which have dim
                     updates[key] = [mapper(self._obj, v, False, v) for v in value]

cf_xarray/tests/test_accessor.py

@@ -90,6 +90,26 @@ def test_kwargs_methods(obj):
     assert_identical(expected, actual)
 
 
+def test_kwargs_expand_key_to_multiple_keys():
+
+    ds = xr.Dataset()
+    ds.coords["x1"] = ("x1", range(30), {"axis": "X"})
+    ds.coords["y1"] = ("y1", range(20), {"axis": "Y"})
+    ds.coords["x2"] = ("x2", range(10), {"axis": "X"})
+    ds.coords["y2"] = ("y2", range(5), {"axis": "Y"})
+
+    ds["v1"] = (("x1", "y1"), np.ones((30, 20)) * 15)
+    ds["v2"] = (("x2", "y2"), np.ones((10, 5)) * 15)
+
+    actual = ds.cf.isel(X=5, Y=10)
+    expected = ds.isel(x1=5, y1=10, x2=5, y2=10)
+    assert_identical(actual, expected)
+
+    actual = ds.cf.coarsen(X=10, Y=5)
+    expected = ds.coarsen(x1=10, y1=5, x2=10, y2=5)
+    assert_identical(actual.mean(), expected.mean())
+
+
 @pytest.mark.parametrize("obj", objects)
 def test_args_methods(obj):
     with raise_if_dask_computes():