Improve categorical converter error message

dstansby · dstansby · commit ffdc34c1fbbb · 2020-07-03T10:52:57.000+01:00
diff --git a/lib/matplotlib/category.py b/lib/matplotlib/category.py
@@ -50,6 +50,7 @@ def convert(value, unit, axis):
                 'Missing category information for StrCategoryConverter; '
                 'this might be caused by unintendedly mixing categorical and '
                 'numeric data')
+        StrCategoryConverter._validate_unit(unit)
         # dtype = object preserves numerical pass throughs
         values = np.atleast_1d(np.array(value, dtype=object))
         # pass through sequence of non binary numbers
@@ -80,6 +81,7 @@ def axisinfo(unit, axis):
 
         .. note: axis is not used
         """
+        StrCategoryConverter._validate_unit(unit)
         # locator and formatter take mapping dict because
         # args need to be pass by reference for updates
         majloc = StrCategoryLocator(unit._mapping)
@@ -109,6 +111,13 @@ def default_units(data, axis):
             axis.units.update(data)
         return axis.units
 
+    @staticmethod
+    def _validate_unit(unit):
+        if not hasattr(unit, '_mapping'):
+            raise ValueError(
+                f'Provided unit "{unit}" is not a valid for a categorical '
+                'converter, as it does not have a _mapping attribute.')
+
 
 class StrCategoryLocator(ticker.Locator):
     """Tick at every integer mapping of the string data."""
