Detailed Error Message for get_dimensionality()

CHANGES

@@ -12,6 +12,8 @@ Pint Changelog
   (PR #1819)
 - Add numpy.linalg.norm implementation.
   (PR #1251)
+- Improved error message in `get_dimensionality()` when non existent units are passed.
+  (PR #1874, Issue #1716)
 
 0.22 (2023-05-25)
 -----------------

pint/facets/plain/registry.py

@@ -727,7 +727,12 @@ def _get_dimensionality_recurse(
         for key in ref:
             exp2 = exp * ref[key]
             if _is_dim(key):
-                reg = self._dimensions[key]
+                try:
+                    reg = self._dimensions[key]
+                except KeyError:
+                    raise ValueError(
+                        f"{key} is not defined as dimension in the pint UnitRegistry"
+                    )
                 if isinstance(reg, DerivedDimensionDefinition):
                     self._get_dimensionality_recurse(reg.reference, exp2, accumulator)
                 else:

pint/testsuite/test_errors.py

@@ -142,3 +142,13 @@ def test_pickle_definition_syntax_error(self, subtests):
 
                     with pytest.raises(PintError):
                         raise ex
+
+    def test_dimensionality_error_message(self):
+        ureg = UnitRegistry(system="SI")
+        with pytest.raises(ValueError) as error:
+            ureg.get_dimensionality("[bilbo]")
+
+        assert (
+            str(error.value)
+            == "[bilbo] is not defined as dimension in the pint UnitRegistry"
+        )