gh-118318: Remove outdated c14n references from ElementTree (GH-118319)

https://github.com/python/cpython/commit/87b120fdb58afd8f7cb79daa87122334e295b70c
commit: 87b120fdb58afd8f7cb79daa87122334e295b70c
branch: main
author: Daniel Hillier <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-08-09T21:06:42Z
summary:

gh-118318: Remove outdated c14n references from ElementTree (GH-118319)

Some references to a removed `c14n` serialization method remained in
ElementTree.py. Attempting to use `method='c14n'` in the `write()`
method would result in `ValueError: unknown method 'c14n'`.

Removes the `write_c14n()` method that uses `method='c14n'` and also
raises the same error.

files:
A Misc/NEWS.d/next/Library/2026-08-09-21-30-00.gh-issue-118318.Xr4Kd8.rst
M Doc/whatsnew/3.16.rst
M Lib/xml/etree/ElementTree.py

diff --git a/Doc/whatsnew/3.16.rst b/Doc/whatsnew/3.16.rst
index 210bcafe65f5e9c..9edbccf3adef9e5 100644
--- a/Doc/whatsnew/3.16.rst
+++ b/Doc/whatsnew/3.16.rst
@@ -727,6 +727,17 @@ tarfile
 * The undocumented and unused :attr:`!tarfile.TarFile.tarfile` attribute
   has been deprecated since Python 3.13.
 
+xml.etree.ElementTree
+---------------------
+
+* The undocumented :meth:`!xml.etree.ElementTree.ElementTree.write_c14n` method
+  and the support of ``method="c14n"`` in
+  :meth:`~xml.etree.ElementTree.ElementTree.write` and
+  :func:`~xml.etree.ElementTree.tostring`.
+  They never worked and always raised :exc:`ValueError`.
+  Use :func:`xml.etree.ElementTree.canonicalize` instead.
+  (Contributed by Daniel Hillier in :gh:`118318`.)
+
 .. Add removals above alphabetically, not here at the end.
 
 
diff --git a/Lib/xml/etree/ElementTree.py b/Lib/xml/etree/ElementTree.py
index 53727d7940b3f2a..7708d38b7759cb1 100644
--- a/Lib/xml/etree/ElementTree.py
+++ b/Lib/xml/etree/ElementTree.py
@@ -714,7 +714,7 @@ def write(self, file_or_filename,
           *default_namespace* -- sets the default XML namespace (for
                                  "xmlns")
 
-          *method* -- either "xml" (default), "html, "text", or "c14n"
+          *method* -- either "xml" (default), "html" or "text"
 
           *short_empty_elements* -- controls the formatting of elements
                                     that contain no content.  If True
@@ -730,10 +730,7 @@ def write(self, file_or_filename,
         elif method not in _serialize:
             raise ValueError("unknown method %r" % method)
         if not encoding:
-            if method == "c14n":
-                encoding = "utf-8"
-            else:
-                encoding = "us-ascii"
+            encoding = "us-ascii"
         with _get_writer(file_or_filename, encoding) as (write, declared_encoding):
             if method == "xml" and (xml_declaration or
                     (xml_declaration is None and
@@ -749,10 +746,6 @@ def write(self, file_or_filename,
                 serialize(write, self._root, qnames, namespaces,
                           short_empty_elements=short_empty_elements)
 
-    def write_c14n(self, file):
-        # lxml.etree compatibility.  use output method instead
-        return self.write(file, method="c14n")
-
 # --------------------------------------------------------------------
 # serialization support
 
@@ -987,8 +980,6 @@ def _serialize_text(write, elem):
     "xml": _serialize_xml,
     "html": _serialize_html,
     "text": _serialize_text,
-# this optional method is imported at the end of the module
-#   "c14n": _serialize_c14n,
 }
 
 
@@ -1100,7 +1091,7 @@ def tostring(element, encoding=None, method=None, *,
 
     *element* is an Element instance, *encoding* is an optional output
     encoding defaulting to US-ASCII, *method* is an optional output which
-    can be one of "xml" (default), "html", "text" or "c14n",
+    can be one of "xml" (default), "html" or "text",
     *default_namespace* sets the default XML namespace (for "xmlns").
 
     Returns an (optionally) encoded string containing the XML data.
diff --git a/Misc/NEWS.d/next/Library/2026-08-09-21-30-00.gh-issue-118318.Xr4Kd8.rst b/Misc/NEWS.d/next/Library/2026-08-09-21-30-00.gh-issue-118318.Xr4Kd8.rst
new file mode 100644
index 000000000000000..31a39e50c72dd5f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-08-09-21-30-00.gh-issue-118318.Xr4Kd8.rst
@@ -0,0 +1,5 @@
+Remove the ``write_c14n()`` method of :class:`xml.etree.ElementTree.ElementTree`
+and the support of ``method="c14n"`` in :meth:`~xml.etree.ElementTree.ElementTree.write`
+and :func:`~xml.etree.ElementTree.tostring`.  They never worked and always
+raised :exc:`ValueError`.  Use :func:`xml.etree.ElementTree.canonicalize`
+instead.

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