gh-136231: Document xml.etree.ElementTree.iselement behavior when used with types

Doc/library/xml.etree.elementtree.rst

@@ -611,8 +611,11 @@ Functions
 
 .. function:: iselement(element)
 
-   Check if an object appears to be a valid element object.  *element* is an
-   element instance.  Return ``True`` if this is an element object.
+   Check if *element* appears to be a valid element object or type. Return
+   ``True`` if this is an element object or type.
+
+   Because ``iselement`` behaves identically for both objects and types, code
+   requiring an object should check for this, see :func:`iselement`.
 
 
 .. function:: iterparse(source, events=None, parser=None)

Lib/test/test_xml_etree.py

@@ -218,6 +218,33 @@ class ElementTreeTest(unittest.TestCase):
     def serialize_check(self, elem, expected):
         self.assertEqual(serialize(elem), expected)
 
+    def test_constructor(self):
+        # Test constructor behavior.
+
+        with self.assertRaises(TypeError):
+            tree = ET.ElementTree("")
+        with self.assertRaises(TypeError):
+            tree = ET.ElementTree(ET.ElementTree())
+
+    def test_setroot(self):
+        # Test _setroot behavior.
+
+        tree = ET.ElementTree()
+        element = ET.Element("tag")
+        tree._setroot(element)
+        self.assertEqual(tree.getroot().tag, "tag")
+        self.assertEqual(tree.getroot(), element)
+
+        # Test behavior with an invalid root element
+
+        tree = ET.ElementTree()
+        with self.assertRaises(TypeError):
+            tree._setroot("")
+        with self.assertRaises(TypeError):
+            tree._setroot(ET.ElementTree())
+        with self.assertRaises(TypeError):
+            tree._setroot(None)
+
     def test_interface(self):
         # Test element tree interface.
 

Lib/xml/etree/ElementTree.py

@@ -527,7 +527,9 @@ class ElementTree:
 
     """
     def __init__(self, element=None, file=None):
-        # assert element is None or iselement(element)
+        if element is not None and not iselement(element):
+            raise TypeError('expected an Element, not %s' %
+                            type(element).__name__)
         self._root = element # first node
         if file:
             self.parse(file)
@@ -543,7 +545,9 @@ def _setroot(self, element):
         with the given element.  Use with care!
 
         """
-        # assert iselement(element)
+        if not iselement(element):
+            raise TypeError('expected an Element, not %s'
+                            % type(element).__name__)
         self._root = element
 
     def parse(self, source, parser=None):
@@ -709,6 +713,8 @@ def write(self, file_or_filename,
                                     of start/end tags
 
         """
+        if self._root is None:
+            raise TypeError('ElementTree not initialized')
         if not method:
             method = "xml"
         elif method not in _serialize:

Misc/NEWS.d/next/Documentation/2025-07-09-15-06-27.gh-issue-136231.Wx8W-w.rst

@@ -0,0 +1,2 @@
+Document that :func:`xml.etree.ElementTree.iselement` works identically on
+both object instances and types.

Misc/NEWS.d/next/Library/2025-06-22-02-16-17.gh-issue-135640.FXyFL6.rst

@@ -0,0 +1,4 @@
+Address bug where it was possible to call
+:func:`xml.etree.ElementTree.ElementTree.write` on an ElementTree object with
+an invalid root element. This behavior blanked the file passed to ``write``
+if it already existed.