Address review comment

Author: JanEricNitschke

Lib/test/test_csv.py

@@ -922,6 +922,7 @@ def test_dict_reader_set_fieldnames(self):
         fieldnames = ["a", "b", "c"]
         f = StringIO()
         reader = csv.DictReader(f)
+        self.assertIsNone(reader.fieldnames)
         reader.fieldnames = fieldnames
         self.assertEqual(reader.fieldnames, fieldnames)
 
@@ -940,7 +941,7 @@ def test_dict_writer_fieldnames_accepts_list(self):
     def test_dict_reader_fieldnames_is_optional(self):
         f = StringIO()
         reader = csv.DictReader(f, fieldnames=None)
-        self.assertEqual(reader.fieldnames, None)
+        self.assertIsNone(reader.fieldnames)
 
     def test_read_dict_fields(self):
         with TemporaryFile("w+", encoding="utf-8") as fileobj:
@@ -1366,11 +1367,12 @@ class TestSniffer(unittest.TestCase):
 
     sample17 = ["letter,offset"]
     sample17.extend(f"{chr(ord('a') + i)},{i}" for i in range(20))
-    sample17.append("v,twenty_one")
+    sample17.append("v,twenty_one")  # 'u' was skipped
     sample17 = '\n'.join(sample17)
+
     sample18 = ["letter,offset"]
     sample18.extend(f"{chr(ord('a') + i)},{i}" for i in range(21))
-    sample18.append("v,twenty_one")
+    sample18.append("v,twenty_one")  # 'u' was not skipped
     sample18 = '\n'.join(sample18)
 
     def test_issue43625(self):
@@ -1396,8 +1398,8 @@ def test_has_header_regex_special_delimiter(self):
 
     def test_has_header_checks_20_rows(self):
         sniffer = csv.Sniffer()
-        self.assertIs(sniffer.has_header(self.sample17), False)
-        self.assertIs(sniffer.has_header(self.sample18), True)
+        self.assertFalse(sniffer.has_header(self.sample17))
+        self.assertTrue(sniffer.has_header(self.sample18))
 
     def test_guess_quote_and_delimiter(self):
         sniffer = csv.Sniffer()
@@ -1453,7 +1455,6 @@ def test_delimiters(self):
         self.assertRaisesRegex(csv.Error, "Could not determine delimiter",
                                sniffer.sniff, self.sample16)
 
-
     def test_doublequote(self):
         sniffer = csv.Sniffer()
         dialect = sniffer.sniff(self.header1)