[3.15] gh-111487: Fix csv.Sniffer never testing the minimum delimiter consistency

Lib/csv.py

@@ -404,18 +404,19 @@ def _guess_delimiter(self, data, delimiters):
 
             # build a list of possible delimiters
             modeList = modes.items()
-            total = float(min(chunkLength * iteration, len(data)))
-            # (rows of consistent data) / (number of rows) = 100%
-            consistency = 1.0
-            # minimum consistency threshold
-            threshold = 0.9
-            while len(delims) == 0 and consistency >= threshold:
+            total = min(chunkLength * iteration, len(data))
+            # (rows of consistent data) / (number of rows) = 100%, counted down
+            # in whole percent to a minimum consistency threshold of 90%.
+            # Integer arithmetic: subtracting 0.01 from a float stopped at
+            # 0.9099999999999999, so the pass at the threshold never ran.
+            for consistency in range(100, 89, -1):
+                if delims:
+                    break
                 for k, v in modeList:
                     if v[0] > 0 and v[1] > 0:
-                        if ((v[1]/total) >= consistency and
+                        if (v[1] * 100 >= consistency * total and
                             (delimiters is None or k in delimiters)):
                             delims[k] = v
-                consistency -= 0.01
 
             if len(delims) == 1:
                 delim = list(delims.keys())[0]

Lib/test/test_csv.py

@@ -1521,6 +1521,21 @@ def test_guess_delimiter_crlf_not_chosen(self):
         self.assertEqual(sniffer.sniff(sample).delimiter, "|")
         self.assertNotEqual(sniffer.sniff(sample).delimiter, "\r")
 
+    def test_guess_delimiter_at_minimum_consistency(self):
+        # gh-111487: the consistency counter was stepped down by repeatedly
+        # subtracting 0.01 from a float, so it stopped at 0.9099999999999999
+        # and the pass at the documented 0.9 threshold never ran.  Here ";" is
+        # modal on 19 of the 20 rows, which scores (19 - 1) / 20 == 90% once
+        # the non-matching row is subtracted -- exactly the threshold.
+        # Every row uses a different letter so nothing else is consistent, and
+        # the odd row sits in the first chunk, which keeps that chunk below the
+        # threshold and defers the decision to the full 20-row chunk.
+        rows = [(chr(ord('a') + i) + ';') * 3 + chr(ord('a') + i)
+                for i in range(20)]
+        rows[4] = 'e;e;e'
+        sniffer = csv.Sniffer()
+        self.assertEqual(sniffer.sniff('\n'.join(rows)).delimiter, ';')
+
     def test_zero_mode_tie_order_independence(self):
         sniffer = csv.Sniffer()
         # ":" appears in half the rows (1, 0, 1, 0) - a tie between

Misc/NEWS.d/next/Library/2026-07-29-13-21-44.gh-issue-111487.Kw9pTz.rst

@@ -0,0 +1,5 @@
+Fix :meth:`csv.Sniffer.sniff` never testing the documented minimum delimiter
+consistency of 90%.  The consistency counter was stepped down by repeatedly
+subtracting ``0.01`` from a float, so it stopped at ``0.9099999999999999`` and
+the final pass at the threshold was skipped; it now counts down in whole
+percent using exact integer arithmetic.