Address review feedback on validation and fallback coverage

Signed-off-by: Ali Zulfiqar <codewithfourtix@gmail.com>

Author: codewithfourtix

src/commoncode/urn.py

@@ -125,12 +125,12 @@ def decode(urn):
     """
     segments = [unquote_plus(p) for p in urn.split(":")]
 
-    if len(segments) < 3:
-        raise URNValidationError("Invalid URN: missing namespace or object type.")
-
     if not segments[0] == ("urn"):
         raise URNValidationError("Invalid URN prefix. Expected 'urn'.")
 
+    if len(segments) < 3:
+        raise URNValidationError("Invalid URN: missing namespace or object type.")
+
     if not segments[1] == ("dje"):
         raise URNValidationError("Invalid URN namespace. Expected 'dje'.")
 

tests/test_urn.py

@@ -148,3 +148,9 @@ def test_truncated_urns_raise_validation_error(self):
             with self.subTest(value=value):
                 with self.assertRaises(urn.URNValidationError):
                     urn.decode(value)
+
+
+class InvalidPrefixTestCase(unittest.TestCase):
+    def test_short_non_urn_keeps_prefix_error(self):
+        with self.assertRaisesRegex(urn.URNValidationError, "Invalid URN prefix"):
+            urn.decode("x")