diff --git a/Lib/test/test_csv.py b/Lib/test/test_csv.py
index 60feab225a107c..98ee0c3cdd7a06 100644
--- a/Lib/test/test_csv.py
+++ b/Lib/test/test_csv.py
@@ -918,6 +918,14 @@ def test_dict_reader_fieldnames_accepts_list(self):
         reader = csv.DictReader(f, fieldnames)
         self.assertEqual(reader.fieldnames, fieldnames)
 
+    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)
+
     def test_dict_writer_fieldnames_rejects_iter(self):
         fieldnames = ["a", "b", "c"]
         f = StringIO()
@@ -933,6 +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.assertIsNone(reader.fieldnames)
 
     def test_read_dict_fields(self):
         with TemporaryFile("w+", encoding="utf-8") as fileobj:
@@ -1353,6 +1362,19 @@ class TestSniffer(unittest.TestCase):
 ghi\0jkl
 """
 
+    sample15 = "\n\n\n"
+    sample16 = "abc\ndef\nghi"
+
+    sample17 = ["letter,offset"]
+    sample17.extend(f"{chr(ord('a') + i)},{i}" for i in range(20))
+    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")  # 'u' was not skipped
+    sample18 = '\n'.join(sample18)
+
     def test_issue43625(self):
         sniffer = csv.Sniffer()
         self.assertTrue(sniffer.has_header(self.sample12))
@@ -1374,6 +1396,11 @@ def test_has_header_regex_special_delimiter(self):
         self.assertIs(sniffer.has_header(self.sample8), False)
         self.assertIs(sniffer.has_header(self.header2 + self.sample8), True)
 
+    def test_has_header_checks_20_rows(self):
+        sniffer = csv.Sniffer()
+        self.assertFalse(sniffer.has_header(self.sample17))
+        self.assertTrue(sniffer.has_header(self.sample18))
+
     def test_guess_quote_and_delimiter(self):
         sniffer = csv.Sniffer()
         for header in (";'123;4';", "'123;4';", ";'123;4'", "'123;4'"):
@@ -1423,6 +1450,10 @@ def test_delimiters(self):
         self.assertEqual(dialect.quotechar, "'")
         dialect = sniffer.sniff(self.sample14)
         self.assertEqual(dialect.delimiter, '\0')
+        self.assertRaisesRegex(csv.Error, "Could not determine delimiter",
+                               sniffer.sniff, self.sample15)
+        self.assertRaisesRegex(csv.Error, "Could not determine delimiter",
+                               sniffer.sniff, self.sample16)
 
     def test_doublequote(self):
         sniffer = csv.Sniffer()