gh-155059: Make the delimiters parameter keyword-only

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/csv.rst

@@ -299,7 +299,7 @@ The :mod:`!csv` module defines the following classes:
    .. versionadded:: 3.2
 
 
-.. class:: Sniffer(delimiters=None)
+.. class:: Sniffer(*, delimiters=None)
 
    The :class:`Sniffer` class is used to deduce the format of a CSV file.
 

Lib/csv.py

@@ -246,7 +246,7 @@ class Sniffer:
     # argument is not specified.
     delimiters = [c for c in map(chr, range(128)) if not c.isalnum()]
 
-    def __init__(self, delimiters=None):
+    def __init__(self, *, delimiters=None):
         if delimiters is None:
             # in case there is more than one possible delimiter
             self.preferred = [',', '\t', ';', ' ', ':']

Lib/test/test_csv.py

@@ -1505,13 +1505,14 @@ def test_delimiters(self):
         self.assertEqual(dialect.quotechar, '"')
 
     def test_delimiters_in_constructor(self):
-        dialect = csv.Sniffer("?,").sniff(self.sample3)
+        dialect = csv.Sniffer(delimiters="?,").sniff(self.sample3)
         self.assertEqual(dialect.delimiter, "?")
         dialect = csv.Sniffer(delimiters="/,").sniff(self.sample3)
         self.assertEqual(dialect.delimiter, "/")
         # The argument of sniff() takes precedence.
-        dialect = csv.Sniffer("/,").sniff(self.sample3, "?,")
+        dialect = csv.Sniffer(delimiters="/,").sniff(self.sample3, "?,")
         self.assertEqual(dialect.delimiter, "?")
+        self.assertRaises(TypeError, csv.Sniffer, "?,")
         # has_header() uses them as well.
         sniffer = csv.Sniffer(delimiters=",")
         self.assertIs(sniffer.has_header(self.header1 + self.sample1), True)
@@ -1524,10 +1525,12 @@ def test_preferred_in_constructor(self):
         self.assertEqual(csv.Sniffer().sniff(sample).delimiter, ',')
         # The delimiters given to the constructor are preferred in the
         # given order.
-        self.assertEqual(csv.Sniffer(";,").sniff(sample).delimiter, ';')
-        self.assertEqual(csv.Sniffer(",;").sniff(sample).delimiter, ',')
+        sniffer = csv.Sniffer(delimiters=";,")
+        self.assertEqual(sniffer.sniff(sample).delimiter, ';')
+        self.assertEqual(csv.Sniffer(delimiters=",;").sniff(sample).delimiter,
+                         ',')
         # The argument of sniff() does not affect the preference.
-        self.assertEqual(csv.Sniffer(";,").sniff(sample, ",;").delimiter, ';')
+        self.assertEqual(sniffer.sniff(sample, ",;").delimiter, ';')
 
     def test_sniff_escapechar(self):
         # gh-83273: escaped delimiters make the delimiter frequencies