https://github.com/python/cpython/commit/31577863cb902717b735e5876020732ba52520b4
commit: 31577863cb902717b735e5876020732ba52520b4
branch: main
author: Serhiy Storchaka <[email protected]>
committer: serhiy-storchaka <[email protected]>
date: 2026-08-13T10:21:03Z
summary:
gh-75008: Detect the lineterminator in csv.Sniffer.sniff() (GH-155061)
It is guessed by a majority vote among the line endings of the sample,
instead of always being '\r\n'.
files:
A Misc/NEWS.d/next/Library/2026-08-01-23-50-47.gh-issue-75008.0Boa3r.rst
M Doc/library/csv.rst
M Doc/whatsnew/3.16.rst
M Lib/csv.py
M Lib/test/test_csv.py
diff --git a/Doc/library/csv.rst b/Doc/library/csv.rst
index 631b82ab9a335a8..53288e810bffcf6 100644
--- a/Doc/library/csv.rst
+++ b/Doc/library/csv.rst
@@ -328,10 +328,15 @@ The :mod:`!csv` module defines the following classes:
are preferred, in that order,
no matter how many times each of them occurs.
+ The *lineterminator* parameter is deduced separately,
+ by a majority vote among the line endings of the sample.
+ A tie is broken in the order ``'\r\n'``, ``'\n'``, ``'\r'``,
+ so a sample without a complete line gives ``'\r\n'``.
+
.. versionchanged:: next
The dialect is now deduced by trial parsing
and the results may differ from those of earlier Python versions.
- The *escapechar* parameter can now be detected,
+ The *escapechar* and *lineterminator* parameters can now be detected,
and the requested *delimiters* are not restricted to ASCII.
diff --git a/Doc/whatsnew/3.16.rst b/Doc/whatsnew/3.16.rst
index b017535b96979d9..fc09afc96b16763 100644
--- a/Doc/whatsnew/3.16.rst
+++ b/Doc/whatsnew/3.16.rst
@@ -135,6 +135,11 @@ csv
The results may differ from those of earlier Python versions.
(Contributed by Serhiy Storchaka in :gh:`83273`.)
+* :meth:`csv.Sniffer.sniff` now detects the *lineterminator* parameter
+ by a majority vote among the line endings of the sample,
+ instead of always returning ``'\r\n'``.
+ (Contributed by Serhiy Storchaka in :gh:`75008`.)
+
curses
------
diff --git a/Lib/csv.py b/Lib/csv.py
index 0c5c40c759240ca..b406d82efffd9f3 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -389,9 +389,9 @@ def sniff(self, sample, delimiters=None):
class dialect(Dialect):
_name = "sniffed"
- lineterminator = '\r\n'
quoting = QUOTE_MINIMAL
+ dialect.lineterminator = self._detect_lineterminator(lines)
dialect.delimiter = delimiter
# _csv.reader won't accept a quotechar of ''
dialect.quotechar = quotechar or '"'
@@ -614,6 +614,24 @@ def _detect_skipinitialspace(self, lines, delimiter,
quotechar,
for kept_row, skipped_row in zip(*results)]
return all(first) or not any(first)
+ def _detect_lineterminator(self, lines):
+ """
+ Detect the line terminator by majority vote among the line
+ endings. A line break inside a quoted field is counted too,
+ but it takes more of them than of the real ones to win the
+ vote. A tie is broken in the order '\\r\\n', '\\n', '\\r',
+ so a sample without a complete line gives '\\r\\n'.
+ """
+ counts = dict.fromkeys(('\r\n', '\n', '\r'), 0)
+ for line in lines:
+ for lineterminator in counts:
+ if line.endswith(lineterminator):
+ counts[lineterminator] += 1
+ break
+ # max() returns the first of equal candidates, and dict
+ # preserves the insertion order.
+ return max(counts, key=counts.get)
+
def has_header(self, sample):
# Creates a dictionary of types of data in each column. If any
# column is of a single type (say, integers), *except* for the first
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 7a679d627ff8c08..73e282d1abf7177 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -1699,6 +1699,39 @@ def test_sniff_crlf_lineterminator(self):
dialect = sniffer.sniff(sample)
self.assertEqual(dialect.delimiter, ',')
self.assertEqual(dialect.quotechar, '"')
+ self.assertEqual(dialect.lineterminator, '\r\n')
+
+ def test_sniff_lineterminator(self):
+ sniffer = csv.Sniffer()
+ for lineterminator in '\r\n', '\n', '\r':
+ with self.subTest(lineterminator=lineterminator):
+ sample = lineterminator.join(['a,b,c', 'd,e,f', 'g,h,i', ''])
+ dialect = sniffer.sniff(sample)
+ self.assertEqual(dialect.lineterminator, lineterminator)
+ self.assertEqual(dialect.delimiter, ',')
+ # The majority wins.
+ sample = 'a,b,c\nd,e,f\r\ng,h,i\n'
+ self.assertEqual(sniffer.sniff(sample).lineterminator, '\n')
+ sample = 'a,b,c\r\nd,e,f\ng,h,i\r\n'
+ self.assertEqual(sniffer.sniff(sample).lineterminator, '\r\n')
+ # A line break inside a quoted field is counted too, but it is
+ # outvoted by the real ones.
+ sample = 'a,"x\ny",c\r\nd,e,f\r\ng,h,i\r\n'
+ self.assertEqual(sniffer.sniff(sample).lineterminator, '\r\n')
+
+ def test_sniff_lineterminator_tie(self):
+ # A tie is broken in the order '\r\n', '\n', '\r'.
+ sniffer = csv.Sniffer()
+ for sample, lineterminator in (
+ ('a,b,c\nd,e,f\r\n', '\r\n'),
+ ('a,b,c\r\nd,e,f\ng,h,i\rj,k,l', '\r\n'),
+ ('a,b,c\nd,e,f\rg,h,i', '\n'),
+ # A sample without a complete line is a tie of zeros.
+ ('a,b,c', '\r\n'),
+ ):
+ with self.subTest(sample=sample):
+ self.assertEqual(sniffer.sniff(sample).lineterminator,
+ lineterminator)
def test_sniff_excel_tab_with_quotes(self):
# gh-62029: tab-delimited data with a quoted field containing
diff --git
a/Misc/NEWS.d/next/Library/2026-08-01-23-50-47.gh-issue-75008.0Boa3r.rst
b/Misc/NEWS.d/next/Library/2026-08-01-23-50-47.gh-issue-75008.0Boa3r.rst
new file mode 100644
index 000000000000000..19545f041093bd4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-08-01-23-50-47.gh-issue-75008.0Boa3r.rst
@@ -0,0 +1,2 @@
+:meth:`csv.Sniffer.sniff` now detects the *lineterminator* parameter by a
+majority vote among the line endings of the sample.
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