fix(containers): could not make selections along inherited axes

This fixes an issue making selections along inherited axes and
straightens out some issues with `_make_selections` being an instance
method instead of a class method.

draco/core/containers.py

@@ -312,30 +312,42 @@ def dataset_spec(self):
         # Ensure that the dataset_spec is the same order on all ranks
         return {k: ddict[k] for k in sorted(ddict)}
 
-    @property
-    def axes(self):
-        """Return the set of axes for this container..
+    @classmethod
+    def _class_axes(cls):
+        """Return the set of axes for this container defined by this class and the base classes.
         """
         axes = set()
 
         # Iterate over the reversed MRO and look for _table_spec attributes
         # which get added to a temporary dict. We go over the reversed MRO so
         # that the `tdict.update` overrides tables in base classes.
-        for cls in inspect.getmro(self.__class__)[::-1]:
+        for c in inspect.getmro(cls)[::-1]:
 
             try:
-                axes |= set(cls._axes)
+                axes |= set(c._axes)
             except AttributeError:
                 pass
 
-        # Add in any axes found on the instance
+        # This must be the same order on all ranks, so we need to explicitly sort to get around the
+        # hash randomization
+        return tuple(sorted(axes))
+
+    @property
+    def axes(self):
+        """The set of axes for this container including any defined on the instance.
+        """
+        axes = set(self._class_axes())
+
+        # Add in any axes found on the instance (this is needed to support the table classes where
+        # the axes get added at run time)
         axes |= set(self.__dict__.get("_axes", []))
 
         # This must be the same order on all ranks, so we need to explicitly sort to get around the
         # hash randomization
         return tuple(sorted(axes))
 
-    def _make_selections(self, sel_args):
+    @classmethod
+    def _make_selections(cls, sel_args):
         """
         Match down-selection arguments to axes of datasets.
 
@@ -355,12 +367,12 @@ def _make_selections(self, sel_args):
         """
         # Check if all those axes exist
         for axis in sel_args.keys():
-            if axis not in self._axes:
+            if axis not in cls._class_axes():
                 raise RuntimeError("No '{}' axis found to select from.".format(axis))
 
         # Build selections dict
         selections = {}
-        for name, dataset in self._dataset_spec.items():
+        for name, dataset in cls._dataset_spec.items():
             ds_axes = dataset["axes"]
             sel = []
             ds_relevant = False