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gh-90756: Fix ElementTree.write XML prolog for encoding 'utf-8-sig' #31043

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Merged
serhiy-storchaka merged 3 commits into from
Aug 13, 2026
+8 −0
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gh-90756: Fix ElementTree.write XML prolog for encoding 'utf-8-sig' #31043

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c52518f
Fix ElementTree.write XML prolog for encoding 'utf-8-sig'
pepr Feb 1, 2022
2916e19
Merge remote-tracking branch 'upstream/main' into pepr/main
serhiy-storchaka Aug 11, 2026
34fae67
Omit the XML declaration for utf-8-sig, and add tests and NEWS
serhiy-storchaka Aug 12, 2026
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4 changes: 4 additions & 0 deletions Lib/test/test_xml_etree.py
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Original file line number Diff line number Diff line change
Expand Up @@ -933,13 +933,15 @@ def test_tostring_xml_declaration_cases(self):
(b"<?xml version='1.0' encoding='ISO-8859-1'?>\n"
b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', None),
('<body><tag>ø</tag></body>', 'unicode', None),
(b"\xef\xbb\xbf<body><tag>\xc3\xb8</tag></body>", 'utf-8-sig', None),

# ... xml_declaration = False
(b"<body><tag>&#248;</tag></body>", None, False),
(b"<body><tag>\xc3\xb8</tag></body>", 'UTF-8', False),
(b"<body><tag>&#248;</tag></body>", 'US-ASCII', False),
(b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', False),
("<body><tag>ø</tag></body>", 'unicode', False),
(b"\xef\xbb\xbf<body><tag>\xc3\xb8</tag></body>", 'utf-8-sig', False),

# ... xml_declaration = True
(b"<?xml version='1.0' encoding='us-ascii'?>\n"
Expand All @@ -952,6 +954,8 @@ def test_tostring_xml_declaration_cases(self):
b"<body><tag>\xf8</tag></body>", 'ISO-8859-1', True),
("<?xml version='1.0' encoding='utf-8'?>\n"
"<body><tag>ø</tag></body>", 'unicode', True),
(b"\xef\xbb\xbf<?xml version='1.0' encoding='utf-8'?>\n"
b"<body><tag>\xc3\xb8</tag></body>", 'utf-8-sig', True),

]
for expected_retval, encoding, xml_declaration in TESTCASES:
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2 changes: 2 additions & 0 deletions Lib/xml/etree/ElementTree.py
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Expand Up @@ -732,6 +732,8 @@ def write(self, file_or_filename,
if not encoding:
encoding = "us-ascii"
with _get_writer(file_or_filename, encoding) as (write, declared_encoding):
if declared_encoding.lower() == "utf-8-sig":
declared_encoding = "utf-8"
if method == "xml" and (xml_declaration or
(xml_declaration is None and
encoding.lower() != "unicode" and
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-12-22-15-00.gh-issue-90756.Qn7dLm.rst
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@@ -0,0 +1,2 @@
:meth:`xml.etree.ElementTree.ElementTree.write` now treats the ``utf-8-sig``
encoding as ``utf-8`` in the XML declaration.
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