Rewrite dicts like .chunks and .sizes (#42)

* Rewrite dicts like .chunks and .sizes

* Fix tests and add fallback behaviour.

key-value pairs that are not "special" are copied over.

* Add docs.

* typo

cf_xarray/accessor.py

@@ -348,7 +348,38 @@ def _getattr(
         An extra decorator, if necessary. This is used by _CFPlotMethods to set default
         kwargs based on CF attributes.
     """
-    func: Callable = getattr(obj, attr)
+    attribute: Union[Mapping, Callable] = getattr(obj, attr)
+
+    if isinstance(attribute, Mapping):
+        if not attribute:
+            return dict(attribute)
+        # attributes like chunks / sizes
+        newmap = dict()
+        unused_keys = set(attribute.keys())
+        for key in _AXIS_NAMES + _COORD_NAMES:
+            value = _get_axis_coord(obj, key, error=False, default=None)
+            unused_keys -= set(value)
+            if value != [None]:
+                good_values = set(value) & set(obj.dims)
+                if not good_values:
+                    continue
+                if len(good_values) > 1:
+                    raise AttributeError(
+                        f"cf_xarray can't wrap attribute {attr!r} because there are multiple values for {key!r} viz. {good_values!r}. "
+                        f"There is no unique mapping from {key!r} to a value in {attr!r}."
+                    )
+                newmap.update({key: attribute[good_values.pop()]})
+        newmap.update({key: attribute[key] for key in unused_keys})
+        return newmap
+
+    elif isinstance(attribute, Callable):  # type: ignore
+        func: Callable = attribute
+
+    else:
+        raise AttributeError(
+            f"cf_xarray does not know how to wrap attribute '{type(obj).__name__}.{attr}'. "
+            "Please file an issue if you have a solution."
+        )
 
     @functools.wraps(func)
     def wrapper(*args, **kwargs):

cf_xarray/tests/datasets.py

@@ -71,6 +71,7 @@
 )
 anc
 
+
 multiple = xr.Dataset()
 multiple.coords["x1"] = ("x1", range(30), {"axis": "X"})
 multiple.coords["y1"] = ("y1", range(20), {"axis": "Y"})

cf_xarray/tests/test_accessor.py

@@ -17,6 +17,40 @@
 objects = datasets + dataarrays
 
 
+def test_dicts():
+    from .datasets import airds
+
+    actual = airds.cf.sizes
+    expected = {"X": 50, "Y": 25, "T": 4, "longitude": 50, "latitude": 25, "time": 4}
+    assert actual == expected
+
+    assert popds.cf.sizes == popds.sizes
+
+    with pytest.raises(AttributeError):
+        multiple.cf.sizes
+
+    assert airds.cf.chunks == {}
+
+    expected = {
+        "X": (50,),
+        "Y": (5, 5, 5, 5, 5),
+        "T": (4,),
+        "longitude": (50,),
+        "latitude": (5, 5, 5, 5, 5),
+        "time": (4,),
+    }
+    assert airds.chunk({"lat": 5}).cf.chunks == expected
+
+    with pytest.raises(AttributeError):
+        airds.da.cf.chunks
+
+    airds = airds.copy(deep=True)
+    airds.lon.attrs = {}
+    actual = airds.cf.sizes
+    expected = {"lon": 50, "Y": 25, "T": 4, "latitude": 25, "time": 4}
+    assert actual == expected
+
+
 def test_describe():
     actual = airds.cf._describe()
     expected = (

doc/examples/introduction.ipynb

@@ -411,6 +411,58 @@
     "pop.cf.get_valid_keys()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Feature: Rewriting property dictionaries\n",
+    "\n",
+    "`cf_xarray` will rewrite the `.sizes` and `.chunks` dictionaries so that one can index by a special CF axis or coordinate name"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "ds.cf.sizes"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Note the duplicate entries above:\n",
+    "\n",
+    "1. One for `X`, `Y`, `T`\n",
+    "2. and one for `longitude`, `latitude` and `time`.\n",
+    "\n",
+    "An error is raised if there are multiple `'X'` variables (for example)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "tags": [
+     "raises-exception"
+    ]
+   },
+   "outputs": [],
+   "source": [
+    "multiple.cf.sizes"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "multiple.v1.cf.sizes"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
@@ -637,9 +689,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "#### miscellaneous features\n",
-    "\n",
-    "You can mix \"special names\" and variable names"
+    "## Feature: mix \"special names\" and variable names"
    ]
   },
   {