gh-144270: make SubElement parent and tag positional-only

The C accelerator in _elementtree.c uses PyArg_ParseTuple, which
makes parent and tag positional-only. The Python fallback in
ElementTree.py allowed them as keyword arguments, creating an
inconsistency. Most users never noticed because the C version
overrides the Python version via "from _elementtree import *".

Add "/" after tag in the Python signature to match the C behavior.
The attrib parameter remains keyword-capable since the C version
accepts it via get_attrib_from_keywords().

Author: NekoAsakura

Doc/library/xml.etree.elementtree.rst

@@ -693,7 +693,7 @@ Functions
    .. versionadded:: 3.2
 
 
-.. function:: SubElement(parent, tag, attrib={}, **extra)
+.. function:: SubElement(parent, tag, /, attrib={}, **extra)
 
    Subelement factory.  This function creates an element instance, and appends
    it to an existing element.
@@ -704,6 +704,9 @@ Functions
    attributes.  *extra* contains additional attributes, given as keyword
    arguments.  Returns an element instance.
 
+   .. versionchanged:: 3.15
+      *parent* and *tag* are now positional-only parameters.
+
 
 .. function:: tostring(element, encoding="us-ascii", method="xml", *, \
                        xml_declaration=None, default_namespace=None, \

Lib/test/test_xml_etree.py

@@ -484,6 +484,26 @@ def test_attrib(self):
         self.assertEqual(ET.tostring(elem),
                 b'<test a="&#13;" b="&#13;&#10;" c="&#09;&#10;&#13; " d="&#10;&#10;&#13;&#13;&#09;&#09;  " />')
 
+    def test_subelement(self):
+        # Test SubElement positional-only parameters (gh-144270).
+        parent = ET.Element('parent')
+
+        # 'parent' and 'tag' are positional-only
+        with self.assertRaises(TypeError):
+            ET.SubElement(parent=parent, tag='fail')
+        with self.assertRaises(TypeError):
+            ET.SubElement(parent, tag='fail')
+
+        # 'attrib' can be passed as keyword
+        sub1 = ET.SubElement(parent, 'sub1', attrib={'key': 'value'})
+        self.assertEqual(sub1.get('key'), 'value')
+
+        # 'tag' and 'parent' as kwargs become XML attributes, not func params
+        sub2 = ET.SubElement(parent, 'sub2', tag='foo', parent='bar')
+        self.assertEqual(sub2.tag, 'sub2')
+        self.assertEqual(sub2.get('tag'), 'foo')
+        self.assertEqual(sub2.get('parent'), 'bar')
+
     def test_makeelement(self):
         # Test makeelement handling.
 

Lib/xml/etree/ElementTree.py

@@ -416,7 +416,7 @@ def itertext(self):
                 yield t
 
 
-def SubElement(parent, tag, attrib={}, **extra):
+def SubElement(parent, tag, /, attrib={}, **extra):
     """Subelement factory which creates an element instance, and appends it
     to an existing parent.