From 9d69f54464511bceaf4ed068b23f0196caeaebb6 Mon Sep 17 00:00:00 2001
From: sricursion <sricursion@gmail.com>
Date: Thu, 8 Oct 2026 01:29:28 +0530
Subject: [PATCH] gh-158984: Fix xmlns:prefix attributes set with
 setAttribute() in minidom

Take the prefix of an existing namespace declaration from the attribute
name instead of Attr.prefix, which is None for attributes created with
the DOM Level 1 API.
---
 Lib/test/test_minidom.py | 49 ++++++++++++++++++++++++++++++++++++++++
 Lib/xml/dom/minidom.py   |  2 +-
 2 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_minidom.py b/Lib/test/test_minidom.py
index 446bbe096bd19d..ee31fd24bdaec4 100644
--- a/Lib/test/test_minidom.py
+++ b/Lib/test/test_minidom.py
@@ -6,6 +6,7 @@
 from test import support
 import unittest
 
+import xml.dom.expatbuilder
 import xml.dom.minidom
 
 from xml.dom.minidom import parse, Attr, Node, Document, Element, parseString
@@ -683,6 +684,54 @@ def testWriteXMLExistingNamespaceDeclarations(self):
                 self.assertEqual(dom.documentElement.toxml(), str)
                 dom.unlink()
 
+    def testWriteXMLDOM1PrefixDeclaration(self):
+        # setAttribute() does not set the prefix of the attribute,
+        # but "xmlns:p" still declares the prefix "p".
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "p:root"))
+        root.setAttribute("xmlns:p", "http://xml.python.org/ns")
+        domstr = dom.documentElement.toxml()
+        dom.unlink()
+        self.assertEqual(domstr,
+                '<p:root xmlns:p="http://xml.python.org/ns"/>')
+        dom = parseString(domstr)
+        self.assertEqual(dom.documentElement.namespaceURI,
+                         "http://xml.python.org/ns")
+        dom.unlink()
+
+    def testWriteXMLDOM1PrefixDeclarationNoNamespace(self):
+        # A prefix declaration is not a declaration of the default namespace.
+        dom = Document()
+        root = dom.appendChild(dom.createElement("root"))
+        root.setAttribute("xmlns:xsi", "http://xml.python.org/ns")
+        self.assertEqual(dom.documentElement.toxml(),
+                '<root xmlns:xsi="http://xml.python.org/ns"/>')
+        dom.unlink()
+
+    def testWriteXMLDOM1PrefixDeclarationDefaultNamespace(self):
+        # The element still needs a declaration of its own default namespace.
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "root"))
+        root.setAttribute("xmlns:x", "http://xml.python.org/ns")
+        domstr = dom.documentElement.toxml()
+        dom.unlink()
+        self.assertEqual(domstr,
+                '<root xmlns="http://xml.python.org/ns" '
+                'xmlns:x="http://xml.python.org/ns"/>')
+        dom = parseString(domstr)
+        self.assertEqual(dom.documentElement.namespaceURI,
+                         "http://xml.python.org/ns")
+        dom.unlink()
+
+    def testWriteXMLParsedWithoutNamespaces(self):
+        src = ('<root xmlns:p="http://xml.python.org/ns">'
+               '<p:child/><other/></root>')
+        dom = xml.dom.expatbuilder.parseString(src, namespaces=False)
+        self.assertEqual(dom.documentElement.toxml(), src)
+        dom.unlink()
+
     def testWriteXMLNotANamespaceDeclaration(self):
         # an attribute whose name only starts with "xmlns" is not one
         dom = parseString('<root xmlns="http://xml.python.org/ns">'
diff --git a/Lib/xml/dom/minidom.py b/Lib/xml/dom/minidom.py
index 7cb652a323dcc2..253934eecfaaa5 100644
--- a/Lib/xml/dom/minidom.py
+++ b/Lib/xml/dom/minidom.py
@@ -433,7 +433,7 @@ def _fixup_namespaces(element, nsmap):
                 # Declarations already present in the document take precedence.
                 nsmap = _bind_namespace(
                     nsmap, inherited,
-                    attr.localName if attr.prefix else None, attr.value)
+                    attr.localName if ":" in name else None, attr.value)
                 attr_uri = None
             elif attr_uri == XML_NAMESPACE:
                 # The xml prefix is bound by definition.