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gh-157678: Allow csv.Sniffer to inspect large fields #157689

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gh-157678: Allow csv.Sniffer to inspect large fields #157689

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12 changes: 12 additions & 0 deletions Lib/csv.py
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Original file line number Diff line number Diff line change
Expand Up @@ -260,6 +260,18 @@ def __init__(self):


def sniff(self, sample, delimiters=None):
"""Analyze the sample and return a dialect reflecting its format."""
limit = field_size_limit()
try:
# Sniffer only needs to count fields, so it should not reject a
# sample merely because a field exceeds the reader's limit.
field_size_limit(max(limit, len(sample)))
return self._sniff(sample, delimiters)
finally:
field_size_limit(limit)


def _sniff(self, sample, delimiters=None):
"""
Analyze the sample and return a Dialect subclass reflecting the
parameters found. If the optional delimiters parameter is
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11 changes: 11 additions & 0 deletions Lib/test/test_csv.py
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Expand Up @@ -1543,6 +1543,17 @@ def test_sniff(self):
self.assertEqual(dialect.quotechar, "'")
self.assertIs(dialect.skipinitialspace, False)

def test_sniff_field_larger_than_field_size_limit(self):
sniffer = csv.Sniffer()
sample = 'a,"' + 'x' * 200000 + '",b\n' + 'c,d,e\n' * 50
limit = csv.field_size_limit(100)
try:
dialect = sniffer.sniff(sample)
self.assertEqual(dialect.delimiter, ',')
self.assertEqual(csv.field_size_limit(), 100)
finally:
csv.field_size_limit(limit)

def test_delimiters(self):
sniffer = csv.Sniffer()
dialect = sniffer.sniff(self.sample3)
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-09-17-15-30-00.gh-issue-157678.SnifQ7.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Allow :meth:`csv.Sniffer.sniff` to process fields larger than the configured
:func:`csv.field_size_limit` without changing the configured limit.
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