Error out in case of alias inheritance.

anntzer · anntzer · commit 07a7311bf48e · 2018-01-01T20:52:03.000-08:00
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
@@ -2783,7 +2783,8 @@ class so far, an alias named ``get_alias`` will be defined; the same will
     exception will be raised.
 
     The alias map is stored as the ``_alias_map`` attribute on the class and
-    can be used by `~.normalize_kwargs`.
+    can be used by `~.normalize_kwargs` (which assumes that higher priority
+    aliases come last).
     """
     if cls is None:
         return functools.partial(_define_aliases, alias_d)
@@ -2804,7 +2805,10 @@ def method(self, *args, **kwargs):
                     method.__doc__ = "alias for `{}`".format(prefix + prop)
                     setattr(cls, prefix + alias, method)
         if not exists:
-            raise ValueError("property {} does not exist".format(prop))
+            raise ValueError(
+                "Neither getter nor setter exists for {!r}".format(prop))
 
+    if hasattr(cls, "_alias_map"):
+        raise NotImplementedError("Parent class already defines aliases")
     cls._alias_map = alias_d
     return cls
