Merge pull request #1780 from fonttools/varlib-add_avar-legible-error-message

Legible error message for axis mapping duplicate check

Lib/fontTools/varLib/__init__.py

@@ -141,8 +141,14 @@ def _add_avar(font, axes):
 		assert axis.maximum == max(keys)
 		assert axis.default in keys
 		# No duplicates
-		assert len(set(keys)) == len(keys)
-		assert len(set(vals)) == len(vals)
+		assert len(set(keys)) == len(keys), (
+			f"{axis.tag} axis: All axis mapping input='...' "
+			"values must be unique, but we found duplicates."
+		)
+		assert len(set(vals)) == len(vals), (
+			f"{axis.tag} axis: All axis mapping output='...' "
+			"values must be unique, but we found duplicates."
+		)
 		# Ascending values
 		assert sorted(vals) == vals
 

NEWS.rst

@@ -1,3 +1,6 @@
+- [varLib] Added legible error messages to the ``_add_avar()`` axis mapping duplicate
+  check.
+
 4.2.1 (released 2019-12-06)
 ---------------------------