diff --git a/Lib/test/test_json/test_decode.py b/Lib/test/test_json/test_decode.py
index 124045b1318..79fb239b35d 100644
--- a/Lib/test/test_json/test_decode.py
+++ b/Lib/test/test_json/test_decode.py
@@ -8,14 +8,34 @@ from test import support
 class TestDecode:
     def test_decimal(self):
         rval = self.loads('1.1', parse_float=decimal.Decimal)
-        self.assertTrue(isinstance(rval, decimal.Decimal))
+        self.assertIsInstance(rval, decimal.Decimal)
         self.assertEqual(rval, decimal.Decimal('1.1'))
 
     def test_float(self):
         rval = self.loads('1', parse_int=float)
-        self.assertTrue(isinstance(rval, float))
+        self.assertIsInstance(rval, float)
         self.assertEqual(rval, 1.0)
 
+    def test_bytes(self):
+        self.assertEqual(self.loads(b"1"), 1)
+
+    def test_parse_constant(self):
+        for constant, expected in [
+            ("Infinity", "INFINITY"),
+            ("-Infinity", "-INFINITY"),
+            ("NaN", "NAN"),
+        ]:
+            self.assertEqual(
+                self.loads(constant, parse_constant=str.upper), expected
+            )
+
+    def test_constant_invalid_case(self):
+        for constant in [
+            "nan", "NAN", "naN", "infinity", "INFINITY", "inFiniTy"
+        ]:
+            with self.assertRaises(self.JSONDecodeError):
+                self.loads(constant)
+
     def test_empty_objects(self):
         self.assertEqual(self.loads('{}'), {})
         self.assertEqual(self.loads('[]'), [])
@@ -88,7 +108,8 @@ class TestDecode:
             self.json.load(StringIO(bom_json))
         self.assertIn('BOM', str(cm.exception))
         # make sure that the BOM is not detected in the middle of a string
-        bom_in_str = '"{}"'.format(''.encode('utf-8-sig').decode('utf-8'))
+        bom = ''.encode('utf-8-sig').decode('utf-8')
+        bom_in_str = f'"{bom}"'
         self.assertEqual(self.loads(bom_in_str), '\ufeff')
         self.assertEqual(self.json.load(StringIO(bom_in_str)), '\ufeff')
 
diff --git a/Lib/test/test_json/test_encode_basestring_ascii.py b/Lib/test/test_json/test_encode_basestring_ascii.py
index 4bbc6c71489..6a39b72a09d 100644
--- a/Lib/test/test_json/test_encode_basestring_ascii.py
+++ b/Lib/test/test_json/test_encode_basestring_ascii.py
@@ -23,8 +23,7 @@ class TestEncodeBasestringAscii:
         for input_string, expect in CASES:
             result = self.json.encoder.encode_basestring_ascii(input_string)
             self.assertEqual(result, expect,
-                '{0!r} != {1!r} for {2}({3!r})'.format(
-                    result, expect, fname, input_string))
+                f'{result!r} != {expect!r} for {fname}({input_string!r})')
 
     def test_ordered_dict(self):
         # See issue 6105
diff --git a/Lib/test/test_json/test_fail.py b/Lib/test/test_json/test_fail.py
index d6bce605e21..a74240f1107 100644
--- a/Lib/test/test_json/test_fail.py
+++ b/Lib/test/test_json/test_fail.py
@@ -89,7 +89,7 @@ class TestFail:
             except self.JSONDecodeError:
                 pass
             else:
-                self.fail("Expected failure for fail{0}.json: {1!r}".format(idx, doc))
+                self.fail(f"Expected failure for fail{idx}.json: {doc!r}")
 
     def test_non_string_keys_dict(self):
         data = {'a' : 1, (1, 2) : 2}
diff --git a/Lib/test/test_json/test_unicode.py b/Lib/test/test_json/test_unicode.py
index 2e8bba27752..68629cceeb9 100644
--- a/Lib/test/test_json/test_unicode.py
+++ b/Lib/test/test_json/test_unicode.py
@@ -20,12 +20,17 @@ class TestUnicode:
     def test_encoding5(self):
         u = '\N{GREEK SMALL LETTER ALPHA}\N{GREEK CAPITAL LETTER OMEGA}'
         j = self.dumps(u, ensure_ascii=False)
-        self.assertEqual(j, '"{0}"'.format(u))
+        self.assertEqual(j, f'"{u}"')
 
     def test_encoding6(self):
         u = '\N{GREEK SMALL LETTER ALPHA}\N{GREEK CAPITAL LETTER OMEGA}'
         j = self.dumps([u], ensure_ascii=False)
-        self.assertEqual(j, '["{0}"]'.format(u))
+        self.assertEqual(j, f'["{u}"]')
+
+    def test_encoding7(self):
+        u = '\N{GREEK SMALL LETTER ALPHA}\N{GREEK CAPITAL LETTER OMEGA}'
+        j = self.dumps(u + "\n", ensure_ascii=False)
+        self.assertEqual(j, f'"{u}\\n"')
 
     def test_big_unicode_encode(self):
         u = '\U0001d120'
@@ -34,13 +39,13 @@ class TestUnicode:
 
     def test_big_unicode_decode(self):
         u = 'z\U0001d120x'
-        self.assertEqual(self.loads('"' + u + '"'), u)
+        self.assertEqual(self.loads(f'"{u}"'), u)
         self.assertEqual(self.loads('"z\\ud834\\udd20x"'), u)
 
     def test_unicode_decode(self):
         for i in range(0, 0xd7ff):
             u = chr(i)
-            s = '"\\u{0:04x}"'.format(i)
+            s = f'"\\u{i:04x}"'
             self.assertEqual(self.loads(s), u)
 
     def test_unicode_preservation(self):