python
diff --git a/‎Lib/test/test_xml_etree.py‎
Lines changed: 18 additions & 0 deletions b/‎Lib/test/test_xml_etree.py‎
Lines changed: 18 additions & 0 deletions
diff --git a/‎Lib/xml/etree/ElementTree.py‎
Lines changed: 4 additions & 2 deletions b/‎Lib/xml/etree/ElementTree.py‎
Lines changed: 4 additions & 2 deletions
@@ -947,6 +947,24 @@ def test_tostring_default_namespace_attributes_round_trip(self):
             [sorted(e.attrib.items()) for e in elem.iter()]
         )
 
+    def test_tostring_default_namespace_registered_empty_prefix(self):
+        # gh-118416: the empty prefix is registered for other namespace,
+        # so it cannot be used for the default namespace
+        nsmap = ET.register_namespace._namespace_map
+        self.addCleanup(nsmap.pop, 'default', None)
+        ET.register_namespace('', 'default')
+        elem = ET.Element('{default}elem')
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode',
+                        default_namespace='otherdefault'),
+            '<ns1:elem xmlns="otherdefault" xmlns:ns1="default" />'
+        )
+        # without the option the registered prefix is used
+        self.assertEqual(
+            ET.tostring(elem, encoding='unicode'),
+            '<elem xmlns="default" />'
+        )
+
     def test_tostring_no_xml_declaration(self):
         elem = ET.XML('<body><tag/></body>')
         self.assertEqual(
 
@@ -827,14 +827,16 @@ def get_prefix(uri, isattr):
         if isattr:
             prefix = attr_prefixes.get(uri)
             if prefix is None:
-                # the default namespace is of no use for an attribute name
+                # the empty prefix is of no use for an attribute name
                 prefix = prefixes.get(uri) or None
         else:
             prefix = prefixes.get(uri)
         if prefix is not None:
             return prefix
         prefix = _namespace_map.get(uri)
-        if not prefix:
+        if prefix is None or not prefix and (isattr or default_namespace):
+            # the empty prefix is of no use for an attribute name,
+            # and the default namespace is used for other uri
             prefix = "ns%d" % len(namespaces)
         if prefix != "xml":
             namespaces[prefix] = uri