Set default plotting kwargs (#38)

* Add get_valid_keys

* Decorate plotting functions.

This allows us to set default plotting kwargs.
1. xincrease, yincrease deepending on .attrs["positive"]

* Add test

* fix test

Author: dcherian

cf_xarray/accessor.py

@@ -2,6 +2,7 @@
 import inspect
 import itertools
 from collections import ChainMap
+from contextlib import suppress
 from typing import Callable, List, Mapping, MutableMapping, Optional, Set, Tuple, Union
 
 import xarray as xr
@@ -226,6 +227,13 @@ def _get_measure(
         else:
             return default
     measures = dict(zip(strings[slice(0, None, 2)], strings[slice(1, None, 2)]))
+    if key not in measures:
+        if error:
+            raise KeyError(
+                f"Cell measure {key!r} not found. Please use .cf.describe() to see a list of key names that can be interpreted."
+            )
+        else:
+            return default
     return measures[key]
 
 
@@ -244,6 +252,7 @@ def _getattr(
     accessor: "CFAccessor",
     key_mappers: Mapping[str, Mapper],
     wrap_classes: bool = False,
+    extra_decorator: Callable = None,
 ):
     """
     Common getattr functionality.
@@ -261,13 +270,17 @@ def _getattr(
         Only True for the high level CFAccessor.
         Facilitates code reuse for _CFWrappedClass and _CFWrapppedPlotMethods
         For both of those, wrap_classes is False.
+    extra_decorator: Callable (optional)
+        An extra decorator, if necessary. This is used by _CFPlotMethods to set default
+        kwargs based on CF attributes.
     """
-    func = getattr(obj, attr)
+    func: Callable = getattr(obj, attr)
 
     @functools.wraps(func)
     def wrapper(*args, **kwargs):
         arguments = accessor._process_signature(func, args, kwargs, key_mappers)
-        result = func(**arguments)
+        final_func = extra_decorator(func) if extra_decorator else func
+        result = final_func(**arguments)
         if wrap_classes and isinstance(result, _WRAPPED_CLASSES):
             result = _CFWrappedClass(result, accessor)
 
@@ -312,21 +325,58 @@ def __init__(self, obj, accessor):
         self.accessor = accessor
         self._keys = ("x", "y", "hue", "col", "row")
 
+    def _plot_decorator(self, func):
+        """
+        This decorator is used to set kwargs on plotting functions.
+        """
+        valid_keys = self.accessor.get_valid_keys()
+
+        @functools.wraps(func)
+        def _plot_wrapper(*args, **kwargs):
+            if "x" in kwargs:
+                if kwargs["x"] in valid_keys:
+                    xvar = self.accessor[kwargs["x"]]
+                else:
+                    xvar = self._obj[kwargs["x"]]
+                if "positive" in xvar.attrs:
+                    if xvar.attrs["positive"] == "down":
+                        kwargs.setdefault("xincrease", False)
+                    else:
+                        kwargs.setdefault("xincrease", True)
+
+            if "y" in kwargs:
+                if kwargs["y"] in valid_keys:
+                    yvar = self.accessor[kwargs["y"]]
+                else:
+                    yvar = self._obj[kwargs["y"]]
+                if "positive" in yvar.attrs:
+                    if yvar.attrs["positive"] == "down":
+                        kwargs.setdefault("yincrease", False)
+                    else:
+                        kwargs.setdefault("yincrease", True)
+
+            return func(*args, **kwargs)
+
+        return _plot_wrapper
+
     def __call__(self, *args, **kwargs):
         plot = _getattr(
             obj=self._obj,
             attr="plot",
             accessor=self.accessor,
             key_mappers=dict.fromkeys(self._keys, _get_axis_coord_single),
         )
-        return plot(*args, **kwargs)
+        return self._plot_decorator(plot)(*args, **kwargs)
 
     def __getattr__(self, attr):
         return _getattr(
             obj=self._obj.plot,
             attr=attr,
             accessor=self.accessor,
             key_mappers=dict.fromkeys(self._keys, _get_axis_coord_single),
+            # TODO: "extra_decorator" is more complex than I would like it to be.
+            # Not sure if there is a better way though
+            extra_decorator=self._plot_decorator,
         )
 
 
@@ -458,6 +508,29 @@ def _describe(self):
     def describe(self):
         print(self._describe())
 
+    def get_valid_keys(self) -> Set[str]:
+        """
+        Returns valid keys for .cf[]
+
+        Returns
+        -------
+        Set of valid key names that can be used with __getitem__ or .cf[key].
+        """
+        varnames = [
+            key
+            for key in _AXIS_NAMES + _COORD_NAMES
+            if _get_axis_coord(self._obj, key, error=False, default=None) != [None]
+        ]
+        with suppress(NotImplementedError):
+            measures = [
+                key
+                for key in _CELL_MEASURES
+                if _get_measure(self._obj, key, error=False) is not None
+            ]
+            if measures:
+                varnames.append(*measures)
+        return set(varnames)
+
 
 @xr.register_dataset_accessor("cf")
 class CFDatasetAccessor(CFAccessor):

cf_xarray/tests/test_accessor.py

@@ -15,6 +15,10 @@
 ds.coords["cell_area"] = (
     xr.DataArray(np.cos(ds.lat * np.pi / 180)) * xr.ones_like(ds.lon) * 105e3 * 110e3
 )
+ds_no_attrs = ds.copy(deep=True)
+for variable in ds_no_attrs.variables:
+    ds_no_attrs[variable].attrs = {}
+
 datasets = [ds, ds.chunk({"lat": 5})]
 dataarrays = [ds.air, ds.air.chunk({"lat": 5})]
 objects = datasets + dataarrays
@@ -121,6 +125,19 @@ def test_kwargs_expand_key_to_multiple_keys():
     assert_identical(actual.mean(), expected.mean())
 
 
+@pytest.mark.parametrize(
+    "obj, expected",
+    [
+        (ds, set(("latitude", "longitude", "time", "X", "Y", "T"))),
+        (ds.air, set(("latitude", "longitude", "time", "X", "Y", "T", "area"))),
+        (ds_no_attrs.air, set()),
+    ],
+)
+def test_get_valid_keys(obj, expected):
+    actual = obj.cf.get_valid_keys()
+    assert actual == expected
+
+
 @pytest.mark.parametrize("obj", objects)
 def test_args_methods(obj):
     with raise_if_dask_computes():
@@ -234,3 +251,15 @@ def test_getitem_uses_coordinates():
     )
     assert_identical(ds.UVEL.cf["X"], ds["ULONG"].reset_coords(drop=True))
     assert_identical(ds.TEMP.cf["X"], ds["TLONG"].reset_coords(drop=True))
+
+
+def test_plot_xincrease_yincrease():
+    ds = xr.tutorial.open_dataset("air_temperature").isel(time=slice(4), lon=slice(50))
+    ds.lon.attrs["positive"] = "down"
+    ds.lat.attrs["positive"] = "down"
+
+    f, ax = plt.subplots(1, 1)
+    ds.air.isel(time=1).cf.plot(ax=ax, x="X", y="Y")
+
+    for lim in [ax.get_xlim(), ax.get_ylim()]:
+        assert lim[0] > lim[1]