gh-142188: Fix csv.Sniffer to handle CR and mixed line endings

Author: ChuheLin

Lib/csv.py

@@ -364,7 +364,7 @@ def _guess_delimiter(self, data, delimiters):
         """
         from collections import Counter, defaultdict
 
-        data = list(filter(None, data.split('\n')))
+        data = list(filter(None, data.splitlines()))
 
         # build frequency tables
         chunkLength = min(10, len(data))

Lib/test/test_csv.py

@@ -1517,6 +1517,12 @@ def test_zero_mode_tie_order_colon_first(self):
         with self.assertRaisesRegex(csv.Error, "Could not determine delimiter"):
             sniffer.sniff(sample)
 
+    def test_sniff_mixed_newlines(self):
+        # gh-142188: Sniffer should handle CR and mixed newlines
+        sample = "User,ID\rAlice,001\nBob,002"
+        sniffer = csv.Sniffer()
+        dialect = sniffer.sniff(sample)
+        self.assertEqual(dialect.delimiter, ',')
 
 class NUL:
     def write(s, *args):