gh-135640: Adds type checking to ElementTree.ElementTree constructor

Lib/test/test_xml_etree.py

@@ -218,6 +218,34 @@ 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())
+
+        # Test _setroot as well, since it also sets the _root object.
+
+        tree = ET.ElementTree()
+        with self.assertRaises(TypeError):
+            tree._setroot("")
+        with self.assertRaises(TypeError):
+            tree._setroot(ET.ElementTree())
+
+        # Make sure it accepts an Element-like object.
+
+        class ElementLike:
+            def __init__(self):
+                self.tag = "tag"
+
+        element_like = ElementLike()
+        try:
+            tree = ET.ElementTree(element_like)
+        except Exception as err:
+            self.fail(err)
+
     def test_interface(self):
         # Test element tree interface.
 

Lib/xml/etree/ElementTree.py

@@ -527,7 +527,10 @@ 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(f"element must be xml.etree.Element or "
+                            f"Element-like object, not "
+                            f"{type(element).__name__}")
         self._root = element # first node
         if file:
             self.parse(file)
@@ -543,7 +546,10 @@ def _setroot(self, element):
         with the given element.  Use with care!
 
         """
-        # assert iselement(element)
+        if not iselement(element):
+            raise TypeError(f"element must be xml.etree.Element or "
+                            f"Element-like object, not "
+                            f"{type(element).__name__}")
         self._root = element
 
     def parse(self, source, parser=None):
@@ -709,6 +715,9 @@ def write(self, file_or_filename,
                                     of start/end tags
 
         """
+        if not iselement(self._root):
+            raise TypeError(f"Root element must be of type xml.etree.Element "
+                            f"or Element-like object")
         if not method:
             method = "xml"
         elif method not in _serialize:

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.