gh-57336: Deprecate setting read-only attributes in xml.dom.minidom

In the DOM model most node attributes are read-only, but in minidom only
localName and length raised NoModificationAllowedErr.  Setting the other
ones silently left the node inconsistent with the document: for example,
setting Attr.name renamed the attribute node, but the owner element still
kept it under the old name.

Setting them now emits a DeprecationWarning.  They are also documented as
read-only.

Author: serhiy-storchaka

Doc/library/xml.dom.rst

@@ -235,6 +235,11 @@ Node Objects
 
 All of the components of an XML document are subclasses of :class:`Node`.
 
+.. versionchanged:: next
+   Setting a read-only attribute is deprecated.
+   It now emits a :exc:`DeprecationWarning`
+   and will raise :exc:`NoModificationAllowedErr` in a future version of Python.
+
 
 .. attribute:: Node.nodeType
 
@@ -304,8 +309,9 @@ All of the components of an XML document are subclasses of :class:`Node`.
 
 .. attribute:: Node.prefix
 
-   The part of the :attr:`tagName` preceding the colon if there is one, else the
-   empty string.  The value is a string, or ``None``.
+   The part of the :attr:`Element.tagName` preceding the colon if there is one,
+   else the empty string.  The value is a string, or ``None``.
+   This is a read-only attribute.
 
 
 .. attribute:: Node.namespaceURI
@@ -455,12 +461,14 @@ following attributes:
 
    The public identifier for the external subset of the document type definition.
    This will be a string or ``None``.
+   This is a read-only attribute.
 
 
 .. attribute:: DocumentType.systemId
 
    The system identifier for the external subset of the document type definition.
    This will be a URI as a string, or ``None``.
+   This is a read-only attribute.
 
 
 .. attribute:: DocumentType.internalSubset
@@ -474,6 +482,7 @@ following attributes:
 
    The name of the root element as given in the ``DOCTYPE`` declaration, if
    present.
+   This is a read-only attribute.
 
 
 .. attribute:: DocumentType.entities
@@ -587,6 +596,7 @@ of that class.
 
    The element type name.  In a namespace-using document it may have colons in it.
    The value is a string.
+   This is a read-only attribute.
 
 
 .. method:: Element.getElementsByTagName(tagName)
@@ -690,6 +700,7 @@ Attr Objects
 
    The attribute name.
    In a namespace-using document it may include a colon.
+   This is a read-only attribute.
 
 
 .. attribute:: Attr.localName
@@ -703,6 +714,7 @@ Attr Objects
 
    The part of the name preceding the colon if there is one, else the
    empty string.
+   This is a read-only attribute.
 
 
 .. attribute:: Attr.value

Doc/whatsnew/3.16.rst

@@ -799,6 +799,18 @@ New deprecations
     open them one by one instead.
     (Contributed by Serhiy Storchaka in :gh:`152638`.)
 
+* :mod:`xml.dom.minidom`:
+
+  * Setting a read-only attribute of a node is deprecated.
+    It now emits a :exc:`DeprecationWarning`
+    and will raise :exc:`xml.dom.NoModificationAllowedErr` in the future.
+    This affects the :attr:`!nodeType`, :attr:`!nodeName`, :attr:`!name`,
+    :attr:`!tagName`, :attr:`!target`, :attr:`!prefix`, :attr:`!namespaceURI`,
+    :attr:`!publicId` and :attr:`!systemId` attributes.
+    Use ``Document.renameNode()`` to rename an element
+    or an attribute.
+    (Contributed by Serhiy Storchaka in :gh:`57336`.)
+
 .. Add deprecations above alphabetically, not here at the end.
 
 .. include:: ../deprecations/pending-removal-in-3.17.rst

Lib/test/test_minidom.py

@@ -4,6 +4,7 @@
 import pickle
 import io
 from test import support
+from test.support import warnings_helper
 import unittest
 
 import xml.dom.minidom
@@ -777,7 +778,9 @@ def _setupCloneElement(self, deep):
         self._testCloneElementCopiesAttributes(
             root, clone, "testCloneElement" + (deep and "Deep" or "Shallow"))
         # mutilate the original so shared data is detected
-        root.tagName = root.nodeName = "MODIFIED"
+        with warnings_helper.check_warnings(
+                ('', DeprecationWarning), quiet=True):
+            root.tagName = root.nodeName = "MODIFIED"
         root.setAttribute("attr", "NEW VALUE")
         root.setAttribute("added", "VALUE")
         return dom, clone
@@ -1325,6 +1328,38 @@ def checkRenameNodeSharedConstraints(self, doc, node):
         self.assertRaises(xml.dom.WrongDocumentErr, doc2.renameNode, node,
                           xml.dom.EMPTY_NAMESPACE, "foo")
 
+    def test_readonly_attributes(self):
+        # These attributes are read-only in the DOM, and setting them
+        # is deprecated (gh-57336).
+        doc = parseString('<!DOCTYPE doc PUBLIC "p" "s">'
+                          '<doc a="v">text<!--c--><?pi d?><![CDATA[x]]></doc>')
+        elem = doc.documentElement
+        attr = elem.attributes["a"]
+        text, comment, pi, cdata = elem.childNodes
+        for node, name in [
+            (doc, "nodeType"), (doc, "nodeName"),
+            (doc.doctype, "name"), (doc.doctype, "nodeName"),
+            (doc.doctype, "publicId"), (doc.doctype, "systemId"),
+            (elem, "nodeType"),
+            (elem, "tagName"), (elem, "nodeName"),
+            (elem, "prefix"), (elem, "namespaceURI"),
+            (attr, "name"), (attr, "nodeName"),
+            (attr, "prefix"), (attr, "namespaceURI"),
+            (text, "nodeName"), (comment, "nodeName"), (cdata, "nodeName"),
+            (pi, "nodeName"), (pi, "target"),
+        ]:
+            with self.subTest(node=type(node).__name__, name=name):
+                value = getattr(node, name)
+                with self.assertWarns(DeprecationWarning):
+                    setattr(node, name, value)
+                self.assertEqual(getattr(node, name), value)
+
+        # These are writable.
+        attr.value = "other"
+        text.data = "other"
+        self.assertEqual(attr.value, "other")
+        self.assertEqual(text.data, "other")
+
     def testRenameAttribute(self):
         doc = parseString("<doc a='v'/>")
         elem = doc.documentElement

Lib/xml/dom/minicompat.py

@@ -40,7 +40,8 @@
 #                    defproperty() should be used for each version of
 #                    the relevant _get_<property>() function.
 
-__all__ = ["NodeList", "EmptyNodeList", "StringTypes", "defproperty"]
+__all__ = ["NodeList", "EmptyNodeList", "StringTypes", "defproperty",
+           "defdeprecatedproperty"]
 
 import xml.dom
 
@@ -107,3 +108,22 @@ def set(self, value, name=name):
            "expected not to find _set_" + name
     prop = property(get, set, doc=doc)
     setattr(klass, name, prop)
+
+
+def defdeprecatedproperty(klass, name, doc, private=None):
+    """Define a read-only attribute whose setter is deprecated.
+
+    The value is stored in the *private* attribute ("_" + name by default).
+    Setting the attribute still works, but emits a DeprecationWarning.
+    """
+    if private is None:
+        private = "_" + name
+    def get(self, private=private):
+        return getattr(self, private)
+    def set(self, value, name=name, private=private):
+        import warnings
+        warnings.warn(f"attempt to modify read-only attribute {name!r} "
+                      f"is deprecated", DeprecationWarning, stacklevel=2)
+        setattr(self, private, value)
+    prop = property(get, set, doc=doc)
+    setattr(klass, name, prop)