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[3.15] gh-109638: Fix exponential time in csv.Sniffer for doubled quotes #154868

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[3.15] gh-109638: Fix exponential time in csv.Sniffer for doubled quotes #154868

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10 changes: 6 additions & 4 deletions Lib/csv.py
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Original file line number Diff line number Diff line change
Expand Up @@ -331,10 +331,12 @@ def _guess_quote_and_delimiter(self, data, delimiters):
# if we see an extra quote between delimiters, we've got a
# double quoted format
dq_regexp = re.compile(
r"((%(delim)s)|^)\W*%(quote)s[^%(delim)s\n]*%(quote)s[^%(delim)s\n]*%(quote)s\W*((%(delim)s)|$)" % \
{'delim':re.escape(delim), 'quote':quotechar}, re.MULTILINE)


r"(?:%(delim)s|^) *+%(quote)s" # ,"
r"[^%(quote)s]*+%(quote)s%(quote)s" # the doubled quote
r"(?:%(quote)s%(quote)s|[^%(quote)s]++)*+" # the rest of the field
r"%(quote)s(?:%(delim)s|$)" # ",
% {'delim': re.escape(delim), 'quote': quotechar},
re.MULTILINE)

if dq_regexp.search(data):
doublequote = True
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7 changes: 7 additions & 0 deletions Lib/test/test_csv.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1564,6 +1564,13 @@ def test_zero_mode_tie_order_colon_first(self):
sniffer.sniff(sample)


def test_sniff_regex_backtracking(self):
# gh-109638: this artificial sample used to take minutes.
sniffer = csv.Sniffer()
sample = '"",' * 100 + '"' * 100 + '0' + '"' * 100 + '0'
self.assertEqual(sniffer.sniff(sample).delimiter, ',')


class NUL:
def write(s, *args):
pass
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-07-29-11-25-00.gh-issue-109638.Vt2Rn9.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix exponential time in :meth:`csv.Sniffer.sniff` for a sample which contains
many quote characters. A doubled quote character is now also detected in
a field which contains the delimiter or a line break.
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