gh-158208: Don't write a duplicate xmlns attribute in xml.dom.minidom

Lib/test/test_minidom.py

@@ -612,6 +612,21 @@ def testWriteXMLDefaultNamespace(self):
                 '<child/><nons xmlns=""/></root>')
         dom.unlink()
 
+    def testWriteXMLNoDuplicateXmlns(self):
+        # gh-158208: setting an explicit xmlns attribute must not
+        # result in a duplicate xmlns declaration in the output.
+        dom = Document()
+        svg = dom.appendChild(dom.createElement("svg"))
+        svg.setAttribute("xmlns", "https://www.w3.org/2000/svg")
+        xml = dom.toxml()
+        self.assertEqual(
+            xml,
+            '<?xml version="1.0" ?><svg xmlns="https://www.w3.org/2000/svg"/>'
+        )
+        # The result must be well-formed XML.
+        parseString(xml)
+        dom.unlink()
+
     def testWriteXMLAttributeNamespacePrefix(self):
         dom = Document()
         root = dom.appendChild(dom.createElement("root"))

Lib/xml/dom/minidom.py

@@ -424,6 +424,7 @@ def _fixup_namespaces(element, nsmap):
     declarations = []
     # (name, value, namespace URI, attribute) of the attributes to write.
     entries = []
+    has_own_xmlns = False
     if attrs:
         for attr in attrs.values():
             name = attr.name
@@ -435,6 +436,8 @@ def _fixup_namespaces(element, nsmap):
                     nsmap, inherited,
                     attr.localName if attr.prefix else None, attr.value)
                 attr_uri = None
+                if name == "xmlns":
+                    has_own_xmlns = True
             elif attr_uri == XML_NAMESPACE:
                 # The xml prefix is bound by definition.
                 attr_uri = None
@@ -446,8 +449,10 @@ def _fixup_namespaces(element, nsmap):
         if nsmap.get(prefix) != uri:
             nsmap = _bind_namespace(nsmap, inherited, prefix, uri)
             declarations.append(("xmlns:" + prefix if prefix else "xmlns", uri))
-    elif nsmap.get(None) and ':' not in element.tagName:
+    elif (nsmap.get(None) and ':' not in element.tagName
+            and not has_own_xmlns):
         # The element is in no namespace, undeclare the default one.
+        # Don't undeclare if the element itself declared xmlns.
         nsmap = _bind_namespace(nsmap, inherited, None, None)
         declarations.append(("xmlns", ""))
 

Misc/NEWS.d/next/Library/2026-09-26-11-53-32.gh-issue-158208.AbCdEf.rst

@@ -0,0 +1,3 @@
+Fix :mod:`xml.dom.minidom` to not write a duplicate ``xmlns`` attribute
+when an element has an explicit ``xmlns`` attribute set with
+:meth:`~xml.dom.Element.setAttribute`.