https://github.com/python/cpython/commit/b8862aeac3d7cc4ed661c232130cd51c8f691a95
commit: b8862aeac3d7cc4ed661c232130cd51c8f691a95
branch: main
author: Serhiy Storchaka <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-07-27T18:03:17+03:00
summary:
gh-154744: Improve detection of skipinitialspace in csv.Sniffer (GH-154745)
Detect the padding by parsing the sample both with and without
skipinitialspace and comparing the two readings, instead of testing
whether every field following a delimiter starts with a space.
files:
A Misc/NEWS.d/next/Library/2026-07-26-13-40-00.gh-issue-154744.Sk1Psp.rst
M Lib/csv.py
M Lib/test/test_csv.py
diff --git a/Lib/csv.py b/Lib/csv.py
index a84234f110dd70..c66717dc1ee59e 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -563,22 +563,40 @@ def _detect_doublequote(self, lines, delimiter,
quotechar, escapechar):
def _detect_skipinitialspace(self, lines, delimiter, quotechar,
escapechar, doublequote):
"""
- True only if every field following a delimiter starts with
- a space.
+ Detect whether the spaces following a delimiter are a part of
+ the format or of the data.
"""
- skipinitialspace = False
- try:
- for row in self._make_reader(lines, delimiter, quotechar,
- escapechar,
- doublequote=doublequote,
- skipinitialspace=False):
- for field in row[1:]:
- if not field.startswith(' '):
- return False
- skipinitialspace = True
- except Error:
- pass
- return skipinitialspace
+ results = []
+ for skipinitialspace in False, True:
+ rows = []
+ try:
+ rows.extend(self._make_reader(
+ lines, delimiter, quotechar, escapechar,
+ doublequote=doublequote,
+ skipinitialspace=skipinitialspace))
+ except Error:
+ # Keep the rows parsed before the error.
+ pass
+ results.append([row for row in rows if row])
+ if results[0] == results[1]:
+ return False # No evidence.
+ counts = [[len(row) for row in rows] for rows in results]
+ if counts[0] != counts[1]:
+ # Prefer the more consistent row widths.
+ return len(set(counts[1])) <= len(set(counts[0]))
+ # Only some spaces are stripped. A field differs only if
+ # a space was skipped at its start, which tells the padding
+ # apart from the spaces inside quoted or escaped fields.
+ if not all(kept_field != skipped_field
+ for kept_row, skipped_row in zip(*results)
+ for kept_field, skipped_field in zip(kept_row[1:],
+ skipped_row[1:])):
+ return False
+ # The first field of a row is commonly not padded ('a, b, c'),
+ # so the first fields need only agree with each other.
+ first = [kept_row[0] != skipped_row[0]
+ for kept_row, skipped_row in zip(*results)]
+ return all(first) or not any(first)
def has_header(self, sample):
# Creates a dictionary of types of data in each column. If any
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index ded53e936d1963..91170cc16b3ac9 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -1688,6 +1688,60 @@ def test_sniff_skipinitialspace_quoted(self):
self.assertEqual(dialect.quotechar, "'")
self.assertIs(dialect.skipinitialspace, True)
+ def test_sniff_skipinitialspace_quoted_fields(self):
+ # A quote is only a quote at the very start of a field, so not
+ # skipping the space splits the quoted field.
+ sniffer = csv.Sniffer()
+ sample = 'a, "b,c"\nd,e\nf,g\n'
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.delimiter, ',')
+ self.assertEqual(dialect.quotechar, '"')
+ self.assertIs(dialect.skipinitialspace, True)
+ self.assertEqual(next(csv.reader(StringIO(sample), dialect)),
+ ['a', 'b,c'])
+
+ # But without a delimiter inside the quotes nothing is split,
+ # so only the padding counts.
+ sample = 'a, "b"\nd,e\nf,g\n'
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.delimiter, ',')
+ self.assertEqual(dialect.quotechar, '"')
+ self.assertIs(dialect.skipinitialspace, False)
+ self.assertEqual(next(csv.reader(StringIO(sample), dialect)),
+ ['a', ' "b"'])
+
+ def test_sniff_skipinitialspace_data(self):
+ # The spaces are a part of the data if only some fields
+ # are padded.
+ sniffer = csv.Sniffer()
+ sample = 'a, b\nc,d\ne, f\n'
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.delimiter, ',')
+ self.assertIs(dialect.skipinitialspace, False)
+ self.assertEqual(next(csv.reader(StringIO(sample), dialect)),
+ ['a', ' b'])
+
+ def test_sniff_skipinitialspace_not_skipped(self):
+ # A quoted or escaped space is not skipped, so it is not
+ # an evidence of the padding.
+ sniffer = csv.Sniffer()
+ sample = 'a," b"\nc, d\n'
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.delimiter, ',')
+ self.assertEqual(dialect.quotechar, '"')
+ self.assertIs(dialect.skipinitialspace, False)
+ self.assertEqual(list(csv.reader(StringIO(sample), dialect)),
+ [['a', ' b'], ['c', ' d']])
+
+ # The escaped delimiter forces the escapechar detection.
+ sample = 'a,\\ b\\,c\nd, e\n'
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.delimiter, ',')
+ self.assertEqual(dialect.escapechar, '\\')
+ self.assertIs(dialect.skipinitialspace, False)
+ self.assertEqual(list(csv.reader(StringIO(sample), dialect)),
+ [['a', ' b,c'], ['d', ' e']])
+
def test_sniff_regex_backtracking(self):
# gh-109638: this artificial sample used to take minutes.
sniffer = csv.Sniffer()
diff --git
a/Misc/NEWS.d/next/Library/2026-07-26-13-40-00.gh-issue-154744.Sk1Psp.rst
b/Misc/NEWS.d/next/Library/2026-07-26-13-40-00.gh-issue-154744.Sk1Psp.rst
new file mode 100644
index 00000000000000..500826c195346f
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-26-13-40-00.gh-issue-154744.Sk1Psp.rst
@@ -0,0 +1 @@
+Improve detection of ``skipinitialspace`` in :meth:`csv.Sniffer.sniff`.
_______________________________________________
Python-checkins mailing list -- [email protected]
To unsubscribe send an email to [email protected]
https://mail.python.org/mailman3//lists/python-checkins.python.org
Member address: [email protected]
