From 24dd23705897f75889073c364941e08f9b6e00d5 Mon Sep 17 00:00:00 2001
From: tristannew <tristannew@ymail.com>
Date: Sun, 12 May 2024 20:43:21 +0100
Subject: [PATCH] Detailed Error Message for `get_dimensionality()` (#1874)

---
 CHANGES                       |  2 ++
 pint/facets/plain/registry.py |  7 ++++++-
 pint/testsuite/test_errors.py | 10 ++++++++++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/CHANGES b/CHANGES
index 66281cc6d..9ec58b72c 100644
--- a/CHANGES
+++ b/CHANGES
@@ -49,6 +49,8 @@ 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)
 -----------------
diff --git a/pint/facets/plain/registry.py b/pint/facets/plain/registry.py
index 277a6f7a2..09fd220ee 100644
--- a/pint/facets/plain/registry.py
+++ b/pint/facets/plain/registry.py
@@ -736,7 +736,12 @@ 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:
diff --git a/pint/testsuite/test_errors.py b/pint/testsuite/test_errors.py
index 370ccfc9d..e0c4ec3f4 100644
--- a/pint/testsuite/test_errors.py
+++ b/pint/testsuite/test_errors.py
@@ -144,3 +144,13 @@ 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"
+        )