gh-95950: Add a test for both `csv.Dialect` and `kwargs` (GH-95951)

(cherry picked from commit 1c01bd28)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Lib/test/test_csv.py

@@ -421,6 +421,34 @@ def test_register_kwargs(self):
         self.assertEqual(csv.get_dialect(name).delimiter, ';')
         self.assertEqual([['X', 'Y', 'Z']], list(csv.reader(['X;Y;Z'], name)))
 
+    def test_register_kwargs_override(self):
+        class mydialect(csv.Dialect):
+            delimiter = "\t"
+            quotechar = '"'
+            doublequote = True
+            skipinitialspace = False
+            lineterminator = '\r\n'
+            quoting = csv.QUOTE_MINIMAL
+
+        name = 'test_dialect'
+        csv.register_dialect(name, mydialect,
+                             delimiter=';',
+                             quotechar="'",
+                             doublequote=False,
+                             skipinitialspace=True,
+                             lineterminator='\n',
+                             quoting=csv.QUOTE_ALL)
+        self.addCleanup(csv.unregister_dialect, name)
+
+        # Ensure that kwargs do override attributes of a dialect class:
+        dialect = csv.get_dialect(name)
+        self.assertEqual(dialect.delimiter, ';')
+        self.assertEqual(dialect.quotechar, "'")
+        self.assertEqual(dialect.doublequote, False)
+        self.assertEqual(dialect.skipinitialspace, True)
+        self.assertEqual(dialect.lineterminator, '\n')
+        self.assertEqual(dialect.quoting, csv.QUOTE_ALL)
+
     def test_incomplete_dialect(self):
         class myexceltsv(csv.Dialect):
             delimiter = "\t"