Issue #25047: Respect case writing XML encoding declarations

This restores the ability to write encoding names in uppercase like "UTF-8",
which worked in Python 2.

Lib/test/test_xml_etree.py

@@ -2396,14 +2396,21 @@ class IOTest(unittest.TestCase):
         elem = ET.Element("tag")
         elem.text = "abc"
         self.assertEqual(serialize(elem), '<tag>abc</tag>')
-        self.assertEqual(serialize(elem, encoding="utf-8"),
-                b'<tag>abc</tag>')
-        self.assertEqual(serialize(elem, encoding="us-ascii"),
-                b'<tag>abc</tag>')
+        for enc in ("utf-8", "us-ascii"):
+            with self.subTest(enc):
+                self.assertEqual(serialize(elem, encoding=enc),
+                        b'<tag>abc</tag>')
+                self.assertEqual(serialize(elem, encoding=enc.upper()),
+                        b'<tag>abc</tag>')
         for enc in ("iso-8859-1", "utf-16", "utf-32"):
-            self.assertEqual(serialize(elem, encoding=enc),
-                    ("<?xml version='1.0' encoding='%s'?>\n"
-                     "<tag>abc</tag>" % enc).encode(enc))
+            with self.subTest(enc):
+                self.assertEqual(serialize(elem, encoding=enc),
+                        ("<?xml version='1.0' encoding='%s'?>\n"
+                         "<tag>abc</tag>" % enc).encode(enc))
+                upper = enc.upper()
+                self.assertEqual(serialize(elem, encoding=upper),
+                        ("<?xml version='1.0' encoding='%s'?>\n"
+                         "<tag>abc</tag>" % upper).encode(enc))
 
         elem = ET.Element("tag")
         elem.text = "<&\"\'>"

Lib/xml/etree/ElementTree.py

@@ -756,14 +756,13 @@ class ElementTree:
                 encoding = "utf-8"
             else:
                 encoding = "us-ascii"
-        else:
-            encoding = encoding.lower()
-        with _get_writer(file_or_filename, encoding) as write:
+        enc_lower = encoding.lower()
+        with _get_writer(file_or_filename, enc_lower) as write:
             if method == "xml" and (xml_declaration or
                     (xml_declaration is None and
-                     encoding not in ("utf-8", "us-ascii", "unicode"))):
+                     enc_lower not in ("utf-8", "us-ascii", "unicode"))):
                 declared_encoding = encoding
-                if encoding == "unicode":
+                if enc_lower == "unicode":
                     # Retrieve the default encoding for the xml declaration
                     import locale
                     declared_encoding = locale.getpreferredencoding()

Misc/NEWS

@@ -81,6 +81,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #25047: The XML encoding declaration written by Element Tree now
+  respects the letter case given by the user. This restores the ability to
+  write encoding names in uppercase like "UTF-8", which worked in Python 2.
+
 - Issue #19143: platform module now reads Windows version from kernel32.dll to
   avoid compatibility shims.