From 697558791bc45c04048c487c045d782b7ff9ece3 Mon Sep 17 00:00:00 2001
From: JS <44579963+Punisheroot@users.noreply.github.com>
Date: Sat, 1 Aug 2026 17:11:17 +0200
Subject: [PATCH] [3.15] gh-110128: Ignore a truncated final row in csv.Sniffer

---
 Lib/csv.py                                       |  6 +++++-
 Lib/test/test_csv.py                             | 16 ++++++++++++++++
 ...026-07-31-20-00-00.gh-issue-110128.tRuNc8.rst |  2 ++
 3 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-31-20-00-00.gh-issue-110128.tRuNc8.rst

diff --git a/Lib/csv.py b/Lib/csv.py
index d666ae20bc7edf..166aa01d0e25b7 100644
--- a/Lib/csv.py
+++ b/Lib/csv.py
@@ -368,7 +368,11 @@ def _guess_delimiter(self, data, delimiters):
         """
         from collections import Counter, defaultdict
 
-        data = list(filter(None, data.split('\n')))
+        lines = data.split('\n')
+        if len(lines) > 1 and not data.endswith(('\r', '\n')):
+            # The sample may have been cut off in the middle of the last row.
+            lines.pop()
+        data = list(filter(None, lines))
 
         # build frequency tables
         chunkLength = min(10, len(data))
diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 2ab529b51c207d..58147e22ee0e04 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -1563,6 +1563,22 @@ def test_zero_mode_tie_order_colon_first(self):
         with self.assertRaisesRegex(csv.Error, "Could not determine delimiter"):
             sniffer.sniff(sample)
 
+    def test_sniff_truncated_sample(self):
+        sniffer = csv.Sniffer()
+        # A single row without a line terminator is complete enough to sniff.
+        self.assertEqual(sniffer.sniff("a,b").delimiter, ",")
+
+        # The last row may have been cut off in the middle of the sample.
+        for lineterminator in ("\n", "\r\n"):
+            with self.subTest(lineterminator=lineterminator):
+                sample = lineterminator.join((
+                    "a,b,c",
+                    "d,e,f",
+                    "g,h,i",
+                    "j,k",
+                ))
+                self.assertEqual(sniffer.sniff(sample).delimiter, ",")
+
 
 class NUL:
     def write(s, *args):
diff --git a/Misc/NEWS.d/next/Library/2026-07-31-20-00-00.gh-issue-110128.tRuNc8.rst b/Misc/NEWS.d/next/Library/2026-07-31-20-00-00.gh-issue-110128.tRuNc8.rst
new file mode 100644
index 00000000000000..b41e428331cd3a
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-31-20-00-00.gh-issue-110128.tRuNc8.rst
@@ -0,0 +1,2 @@
+Fix :meth:`csv.Sniffer.sniff` failing to detect the delimiter when a sample
+is truncated in the middle of its final row.