gh-161400: Fix csv.Sniffer dropping empty space-delimited fields

Lib/csv.py

@@ -599,7 +599,8 @@ def _detect_skipinitialspace(self, lines, delimiter, quotechar,
         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]))
+            return (self._modal_share(counts[1])[1] >=
+                    self._modal_share(counts[0])[1])
         # 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.

Lib/test/test_csv.py

@@ -1828,6 +1828,24 @@ def test_sniff_skipinitialspace_data(self):
         self.assertEqual(next(csv.reader(StringIO(sample), dialect)),
                          ['a', ' b'])
 
+    def test_sniff_skipinitialspace_empty_field(self):
+        # Skipping the spaces loses the empty field, which makes the row
+        # widths less consistent, also when the sample has another short
+        # row or is cut off in the middle of the last row.
+        sniffer = csv.Sniffer()
+        sample = ('name age city\n'
+                  'alice 30 paris\n'
+                  'bob  london\n'
+                  'carol 25 "new york"\n'
+                  'dave 40 berlin\n')
+        for last in ('erin 35\n', 'erin 35'):
+            with self.subTest(last=last):
+                dialect = sniffer.sniff(sample + last)
+                self.assertEqual(dialect.delimiter, ' ')
+                self.assertIs(dialect.skipinitialspace, False)
+                rows = list(csv.reader(StringIO(sample), dialect))
+                self.assertEqual(rows[2], ['bob', '', 'london'])
+
     def test_sniff_skipinitialspace_not_skipped(self):
         # A quoted or escaped space is not skipped, so it is not
         # an evidence of the padding.

Misc/NEWS.d/next/Library/2026-11-05-09-52-26.gh-issue-161400.VevsID.rst

@@ -0,0 +1,3 @@
+Fix :meth:`csv.Sniffer.sniff` returning ``skipinitialspace=True``
+for a space-delimited sample with an unquoted empty field,
+which made the sniffed dialect lose the field.