[3.15] gh-110128: Ignore a truncated final row in csv.Sniffer

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))

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):

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.