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gh-154544: Fix TracebackException mutating attributes during formatting #154545

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gh-154544: Fix TracebackException mutating attributes during formatting #154545

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11 changes: 11 additions & 0 deletions Lib/test/test_traceback.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1864,6 +1864,17 @@ def test_no_keyword_suggestion_for_comma_errors(self):
self.assertIn("Perhaps you forgot a comma", stderr_text)
self.assertNotIn("Did you mean", stderr_text)

def test_format_is_idempotent(self):
code = "fr x in range(10):\n pass\n"
try:
compile(code, "<test>", "exec")
except SyntaxError as exc:
te = traceback.TracebackException(type(exc), exc, None)
r1 = list(te.format_exception_only())
r2 = list(te.format_exception_only())
self.assertEqual(r1, r2)
self.assertEqual(te.msg, exc.msg)

@requires_debug_ranges()
@force_not_colorized_test_class
class PurePythonTracebackErrorCaretTests(
Expand Down
49 changes: 29 additions & 20 deletions Lib/traceback.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1519,14 +1519,14 @@ def _find_keyword_typos(self):
except SyntaxError:
continue

# Keep token.line but handle offsets correctly
self.text = token.line
self.offset = token.start[1] + 1
self.end_offset = token.end[1] + 1
self.lineno = start[0]
self.end_lineno = end[0]
self.msg = f"invalid syntax. Did you mean '{suggestion}'?"
return
return (
token.line,
token.start[1] + 1,
token.end[1] + 1,
start[0],
end[0],
f"invalid syntax. Did you mean '{suggestion}'?",
)


def _format_syntax_error(self, stype, **kwargs):
Expand Down Expand Up @@ -1555,31 +1555,38 @@ def _format_syntax_error(self, stype, **kwargs):
# text = " foo\n"
# rtext = " foo"
# ltext = "foo"
typo = None
with suppress(Exception):
self._find_keyword_typos()
text = self.text
typo = self._find_keyword_typos()
if typo is not None:
text, offset, end_offset, lineno, end_lineno, msg = typo
else:
offset = self.offset
end_offset = self.end_offset
lineno = self.lineno
end_lineno = self.end_lineno
msg = self.msg
rtext = text.rstrip('\n')
ltext = rtext.lstrip(' \n\f')
spaces = len(rtext) - len(ltext)
if self.offset is None:
if offset is None:
yield ' {}\n'.format(ltext)
elif isinstance(self.offset, int):
offset = self.offset
if self.lineno == self.end_lineno:
elif isinstance(offset, int):
if lineno == end_lineno:
end_offset = (
self.end_offset
end_offset
if (
isinstance(self.end_offset, int)
and self.end_offset != 0
isinstance(end_offset, int)
and end_offset != 0
)
else offset
)
else:
end_offset = len(rtext) + 1

if self.text and offset > len(self.text):
if text and offset > len(text):
offset = len(rtext) + 1
if self.text and end_offset > len(self.text):
if text and end_offset > len(text):
end_offset = len(rtext) + 1
if offset >= end_offset or end_offset < 0:
end_offset = offset + 1
Expand Down Expand Up @@ -1611,7 +1618,9 @@ def _format_syntax_error(self, stype, **kwargs):
)
else:
yield ' {}\n'.format(ltext)
msg = self.msg or "<no detail available>"
if not isinstance(text, str):
msg = self.msg
msg = msg or "<no detail available>"
yield "{}{}{}: {}{}{}{}\n".format(
theme.type,
stype,
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-18-10-00-00.gh-issue-154544.TbKwTp.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :func:`traceback.TracebackException.format_exception_only` mutating
its own attributes when suggesting a keyword typo fix, making repeated
calls produce different output. Patch by tonghuaroot.
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