LP#1869455: C14N 2.0 serialisation failed for unprefixed attributes when a default namespace was defined.

2 files changed, 14 insertions, 1 deletions

diff --git a/src/lxml/serializer.pxi b/src/lxml/serializer.pxi
index 3a26f752..d66f59a7 100644
--- a/src/lxml/serializer.pxi
+++ b/src/lxml/serializer.pxi
@@ -1078,7 +1078,12 @@ cdef class C14NWriterTarget:
                 self._declared_ns_stack[-1].append((uri, prefix))
                 return f'{prefix}:{tag}' if prefix else tag, tag, uri
 
-        raise ValueError(f'Namespace "{uri}" is not declared in scope')
+        if not uri:
+            # As soon as a default namespace is defined,
+            # anything that has no namespace (and thus, no prefix) goes there.
+            return tag, tag, uri
+
+        raise ValueError(f'Namespace "{uri}" of name "{tag}" is not declared in scope')
 
     def data(self, data):
         if not self._ignored_depth:
diff --git a/src/lxml/tests/test_elementtree.py b/src/lxml/tests/test_elementtree.py
index 48509ace..45c26cc0 100644
--- a/src/lxml/tests/test_elementtree.py
+++ b/src/lxml/tests/test_elementtree.py
@@ -4640,6 +4640,14 @@ class _C14NTest(unittest.TestCase):
         #self.assertEqual(c14n_roundtrip("<doc xmlns:x='http://example.com/x' xmlns='http://example.com/default'><b y:a1='1' xmlns='http://example.com/default' a3='3' xmlns:y='http://example.com/y' y:a2='2'/></doc>"),
         #'<doc xmlns:x="http://example.com/x"><b xmlns:y="http://example.com/y" a3="3" y:a1="1" y:a2="2"></b></doc>')
 
+        # Namespace issues
+        xml = '<X xmlns="http://nps/a"><Y targets="abc,xyz"></Y></X>'
+        self.assertEqual(c14n_roundtrip(xml), xml)
+        xml = '<X xmlns="http://nps/a"><Y xmlns="http://nsp/b" targets="abc,xyz"></Y></X>'
+        self.assertEqual(c14n_roundtrip(xml), xml)
+        xml = '<X xmlns="http://nps/a"><Y xmlns:b="http://nsp/b" b:targets="abc,xyz"></Y></X>'
+        self.assertEqual(c14n_roundtrip(xml), xml)
+
     def test_c14n_exclusion(self):
         c14n_roundtrip = self.c14n_roundtrip
         xml = textwrap.dedent("""\