gh-138621: Increase test coverage for csv.DictReader and csv.Sniffer

Lib/test/test_csv.py

@@ -918,6 +918,13 @@ 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)
+        reader.fieldnames = fieldnames
+        self.assertEqual(reader.fieldnames, fieldnames)
+
     def test_dict_writer_fieldnames_rejects_iter(self):
         fieldnames = ["a", "b", "c"]
         f = StringIO()
@@ -933,6 +940,7 @@ 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)
 
     def test_read_dict_fields(self):
         with TemporaryFile("w+", encoding="utf-8") as fileobj:
@@ -1351,6 +1359,59 @@ class TestSniffer(unittest.TestCase):
 abc\0def
 ghijkl\0mno
 ghi\0jkl
+"""
+
+    sample15 = "\n\n\n"
+    sample16 = "abc\ndef\nghi"
+
+    sample17 = """\
+name,age
+Alice,30
+Bob,40
+Carol,50
+Dave,60
+Eve,70
+Frank,80
+Grace,90
+Heidi,100
+Ivan,110
+Judy,120
+Karl,130
+Liam,140
+Mallory,150
+Niaj,160
+Olivia,170
+Peggy,180
+Quinn,190
+Rupert,200
+Sybil,210
+Trent,220
+Victor,not_a_number
+"""
+    sample18 = """\
+name,age
+Alice,30
+Bob,40
+Carol,50
+Dave,60
+Eve,70
+Frank,80
+Grace,90
+Heidi,100
+Ivan,110
+Judy,120
+Karl,130
+Liam,140
+Mallory,150
+Niaj,160
+Olivia,170
+Peggy,180
+Quinn,190
+Rupert,200
+Sybil,210
+Trent,220
+Uma,230
+Victor,not_a_number
 """
 
     def test_issue43625(self):
@@ -1374,6 +1435,11 @@ 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.assertIs(sniffer.has_header(self.sample17), False)
+        self.assertIs(sniffer.has_header(self.sample18), True)
+
     def test_guess_quote_and_delimiter(self):
         sniffer = csv.Sniffer()
         for header in (";'123;4';", "'123;4';", ";'123;4'", "'123;4'"):
@@ -1423,6 +1489,11 @@ 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()