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

Since gh-44376, _fixup_namespaces() adds xmlns="" to an element in no
namespace under a default namespace.  When the element declares the
default namespace itself, as one built with createElement() and
setAttribute("xmlns", ...) does, its own xmlns attribute was written
too, so the output was not well-formed.

Do not add the undeclaration if the element already has an xmlns
attribute.  The default namespace is still treated as undeclared for
its children, so they are written as before.

Author: fedonman

Lib/test/test_minidom.py

@@ -702,6 +702,28 @@ def testWriteXMLNotANamespaceDeclaration(self):
                 '<child xmlns="" xmlnsabc="v"/></root>')
         dom.unlink()
 
+    def testWriteXMLOwnDefaultNamespaceDeclaration(self):
+        # An element in no namespace which declares the default namespace
+        # itself is written with that declaration only.
+        dom = Document()
+        svg = dom.appendChild(dom.createElement("svg"))
+        svg.setAttribute("xmlns", "http://www.w3.org/2000/svg")
+        svg.appendChild(dom.createElement("g"))
+        self.assertEqual(dom.documentElement.toxml(),
+                '<svg xmlns="http://www.w3.org/2000/svg"><g/></svg>')
+        dom.unlink()
+
+        dom = Document()
+        root = dom.appendChild(
+            dom.createElementNS("http://xml.python.org/ns", "root"))
+        svg = root.appendChild(dom.createElement("svg"))
+        svg.setAttribute("xmlns", "http://www.w3.org/2000/svg")
+        svg.appendChild(dom.createElement("g"))
+        self.assertEqual(dom.documentElement.toxml(),
+                '<root xmlns="http://xml.python.org/ns">'
+                '<svg xmlns="http://www.w3.org/2000/svg"><g/></svg></root>')
+        dom.unlink()
+
     def testWriteXMLDoesNotModifyDocument(self):
         dom = Document()
         root = dom.appendChild(

Lib/xml/dom/minidom.py

@@ -449,7 +449,8 @@ def _fixup_namespaces(element, nsmap):
     elif nsmap.get(None) and ':' not in element.tagName:
         # The element is in no namespace, undeclare the default one.
         nsmap = _bind_namespace(nsmap, inherited, None, None)
-        declarations.append(("xmlns", ""))
+        if not element.hasAttribute("xmlns"):
+            declarations.append(("xmlns", ""))
 
     items = []
     prefixes = None  # namespace URI -> prefix, built only when needed