gh-161110: Make minidom getElementById() stable for duplicate IDs

Visit child nodes in document order and keep the first cached element
for each ID, so the first element in document order is returned.

Author: gpshead

Doc/library/xml.dom.rst

@@ -756,6 +756,12 @@ inherits properties from :class:`Node`.
    Return the element with the given ID, or ``None``.
    Only attributes declared as being of type ID in the DTD
    or by :meth:`Element.setIdAttribute` are searched.
+   If several elements have the same ID, the first one
+   in document order is returned.
+
+   .. versionchanged:: next
+      Previously, which of the elements with the same ID was returned
+      was unspecified and could change between calls.
 
 
 .. method:: Document.getElementsByTagName(tagName)

Lib/test/test_minidom.py

@@ -2143,6 +2143,42 @@ def testSetIdAttributeNode(self):
         self.confirm(e.isSameNode(doc.getElementById("w"))
                 and a2.isId)
 
+    def check_duplicate_id(self, doc):
+        # The first element in document order is returned, and lookups
+        # of other IDs which continue the search do not change that.
+        for i in range(2):
+            self.assertEqual(doc.getElementById("x").getAttribute("n"), "1")
+            self.assertEqual(doc.getElementById("y").getAttribute("n"), "0")
+            self.assertEqual(doc.getElementById("x").getAttribute("n"), "1")
+            self.assertIsNone(doc.getElementById("missing"))
+            self.assertEqual(doc.getElementById("x").getAttribute("n"), "1")
+            self.assertEqual(doc.getElementById("z").getAttribute("n"), "4")
+            # clear the cache and start the search again
+            doc.documentElement.setAttribute("a", str(i))
+
+    def testGetElementByIdDuplicate(self):
+        body = ("<doc>"
+                "<e id='y' n='0'/>"
+                "<f><e id='x' n='1'/></f>"
+                "<e id='x' n='2'><e id='x' n='3'/></e>"
+                "<e id='z' n='4'/>"
+                "</doc>")
+        with self.subTest("ATTLIST"):
+            doc = parseString("<!DOCTYPE doc [<!ATTLIST e id ID #IMPLIED>]>" +
+                              body)
+            self.check_duplicate_id(doc)
+        with self.subTest("setIdAttribute"):
+            doc = parseString(body)
+            for e in doc.getElementsByTagName("e"):
+                e.setIdAttribute("id")
+            self.check_duplicate_id(doc)
+        with self.subTest("ATTLIST and setIdAttribute"):
+            doc = parseString("<!DOCTYPE doc [<!ATTLIST e n CDATA #IMPLIED>]>" +
+                              body)
+            for e in doc.getElementsByTagName("e"):
+                e.setIdAttribute("id")
+            self.check_duplicate_id(doc)
+
     def assert_recursive_equal(self, doc, doc2):
         stack = [(doc, doc2)]
         while stack:

Lib/xml/dom/minidom.py

@@ -2030,8 +2030,9 @@ def getElementById(self, id):
         result = None
         while stack:
             node = stack.pop()
-            # add child elements to stack for continued searching
-            stack.extend([child for child in node.childNodes
+            # add child elements to stack for continued searching,
+            # last child first so that they are visited in document order
+            stack.extend([child for child in reversed(node.childNodes)
                           if child.nodeType in _nodeTypes_with_children])
             # check this node
             info = self._get_elem_info(node)
@@ -2042,27 +2043,27 @@ def getElementById(self, id):
                 for attr in node.attributes.values():
                     if attr.namespaceURI:
                         if info.isIdNS(attr.namespaceURI, attr.localName):
-                            self._id_cache[attr.value] = node
+                            self._id_cache.setdefault(attr.value, node)
                             if attr.value == id:
                                 result = node
                             elif not node._magic_id_nodes:
                                 break
                     elif info.isId(attr.name):
-                        self._id_cache[attr.value] = node
+                        self._id_cache.setdefault(attr.value, node)
                         if attr.value == id:
                             result = node
                         elif not node._magic_id_nodes:
                             break
                     elif attr._is_id:
-                        self._id_cache[attr.value] = node
+                        self._id_cache.setdefault(attr.value, node)
                         if attr.value == id:
                             result = node
                         elif node._magic_id_nodes == 1:
                             break
             elif node._magic_id_nodes:
                 for attr in node.attributes.values():
                     if attr._is_id:
-                        self._id_cache[attr.value] = node
+                        self._id_cache.setdefault(attr.value, node)
                         if attr.value == id:
                             result = node
             if result is not None:

Misc/NEWS.d/next/Library/2026-10-22-05-20-11.gh-issue-161110.miKrSQ.rst

@@ -0,0 +1,4 @@
+Fix :meth:`~xml.dom.Document.getElementById` in :mod:`xml.dom.minidom` so
+that it returns the first element in document order when several elements
+have the same ID. Previously, the result could differ between calls on an
+unmodified document.