diff --git a/Tests/modules/system_related/test_os.py b/Tests/modules/system_related/test_os.py
index d27c6dece..1f77237d6 100644
--- a/Tests/modules/system_related/test_os.py
+++ b/Tests/modules/system_related/test_os.py
@@ -37,18 +37,20 @@ def test_strerror(self):
         elif is_osx:
             self.assertEqual(os.strerror(40), "Message too long")
 
+
     def test_open_abplus(self):
         # equivalent to open(self.temp_file, "ab+"), see also test_file.test_open_abplus
         fd = os.open(self.temp_file, os.O_APPEND | os.O_CREAT | os.O_RDWR)
         try:
             f = open(fd, mode="ab+", closefd=False)
-            f.write(b"abc")
-            f.seek(0)
-            self.assertEqual(f.read(2), b"ab")
-            f.write(b"def")
-            self.assertEqual(f.read(2), b"")
-            f.seek(0)
-            self.assertEqual(f.read(6), b"abcdef")
+            f.raw.write(b"abcxxxx")
+            f.raw.seek(0)
+            self.assertEqual(f.raw.read(2), b"ab")
+            f.raw.seek(0, 2)
+            f.raw.write(b"def")
+            self.assertEqual(f.raw.read(2), b"")
+            f.raw.seek(0)
+            self.assertEqual(f.raw.read(), b"abcxxxxdef")
             f.close()
         finally:
             os.close(fd)
diff --git a/Tests/test_dict_stdlib.py b/Tests/test_dict_stdlib.py
index d5bc81f13..46b1fdf94 100644
--- a/Tests/test_dict_stdlib.py
+++ b/Tests/test_dict_stdlib.py
@@ -23,12 +23,14 @@ def load_tests(loader, standard_tests, pattern):
             test.test_dict.DictTest('test_oob_indexing_dictiter_iternextitem'),
             test.test_dict.DictTest('test_setdefault_atomic'),
         ]
+
+        skip_tests = []
         if is_mono:
-            failing_tests += [
+            skip_tests += [
                 test.test_dict.DictTest('test_container_iterator'), # /~https://github.com/IronLanguages/ironpython3/issues/544
             ]
 
-        return generate_suite(tests, failing_tests)
+        return generate_suite(tests, failing_tests, skip_tests)
 
     else:
         return tests
diff --git a/Tests/test_file.py b/Tests/test_file.py
index 964cf5498..4c44994af 100644
--- a/Tests/test_file.py
+++ b/Tests/test_file.py
@@ -845,14 +845,15 @@ def test_open_wbplus(self):
             f.seek(0)
             self.assertEqual(f.read(), b"abdez")
 
+
     def test_open_abplus(self):
         with open(self.temp_file, "ab+") as f:
-            f.write(b"abc")
-            f.seek(0)
-            self.assertEqual(f.read(2), b"ab")
-            f.write(b"def")
-            self.assertEqual(f.read(2), b"")
-            f.seek(0)
-            self.assertEqual(f.read(6), b"abcdef")
+            f.raw.write(b"abc")
+            f.raw.seek(0)
+            self.assertEqual(f.raw.read(2), b"ab")
+            f.raw.write(b"def")
+            self.assertEqual(f.raw.read(2), b"")
+            f.raw.seek(0)
+            self.assertEqual(f.raw.read(6), b"abcdef")
 
 run_test(__name__)
diff --git a/Tests/test_strformat.py b/Tests/test_strformat.py
index e3f64f01e..1a7a4c5bd 100644
--- a/Tests/test_strformat.py
+++ b/Tests/test_strformat.py
@@ -226,13 +226,27 @@ class bad2(object):
             def __format__(self, *args):
                 return 42
 
-        self.assertRaisesMessage(TypeError, "bad.__format__ must return a str, not NoneType" if is_cli else "__format__ must return a str, not NoneType", '{0}'.format, bad())
-        self.assertRaisesMessage(TypeError, "bad2.__format__ must return a str, not int" if is_cli else "__format__ must return a str, not int", '{0}'.format, bad2())
+        if is_cli:
+            self.assertRaisesMessage(TypeError, "bad.__format__ must return a str, not NoneType", '{0}'.format, bad())
+            self.assertRaisesMessage(TypeError, "bad2.__format__ must return a str, not int", '{0}'.format, bad2())
+        elif sys.version_info >= (3, 5):
+            self.assertRaisesMessage(TypeError, "__format__ must return a str, not NoneType", '{0}'.format, bad())
+            self.assertRaisesMessage(TypeError, "__format__ must return a str, not int", '{0}'.format, bad2())
+        else:
+            self.assertRaisesMessage(TypeError, "__format__ method did not return string", '{0}'.format, bad())
+            self.assertRaisesMessage(TypeError, "__format__ method did not return string", '{0}'.format, bad2())
 
         self.assertRaisesMessage(ValueError, "Unknown conversion specifier x", '{0!x}'.format, 'abc')
 
-        self.assertRaisesMessage(TypeError, "bad.__format__ must return a str, not NoneType" if is_cli else "__format__ must return a str, not NoneType", format, bad())
-        self.assertRaisesMessage(TypeError, "bad2.__format__ must return a str, not int" if is_cli else "__format__ must return a str, not int", format, bad2())
+        if is_cli:
+            self.assertRaisesMessage(TypeError, "bad.__format__ must return a str, not NoneType", format, bad())
+            self.assertRaisesMessage(TypeError, "bad2.__format__ must return a str, not int", format, bad2())
+        elif sys.version_info >= (3, 5):
+            self.assertRaisesMessage(TypeError, "__format__ must return a str, not NoneType", format, bad())
+            self.assertRaisesMessage(TypeError, "__format__ must return a str, not int", format, bad2())
+        else:
+            self.assertRaisesMessage(TypeError, "__format__ method did not return string", format, bad())
+            self.assertRaisesMessage(TypeError, "__format__ method did not return string", format, bad2())
 
     def test_object__format__(self):
         self.assertEqual(object.__format__("aaa", ""), "aaa")
@@ -281,7 +295,10 @@ def test_str___format___errors(self):
                 if char == ',':
                     errors.append(('10' + char, "Cannot specify ',' with 's'."))
                 elif char == '_':
-                    errors.append(('10' + char, "Cannot specify '_' with 's'."))
+                    if is_cli or sys.version_info >= (3, 6):
+                        errors.append(('10' + char, "Cannot specify '_' with 's'."))
+                    else:
+                        errors.append(('10' + char, "Unknown format code '_' for object of type 'str'"))
                 elif 0x20 < x < 0x80:
                     errors.append(('10' + char, "Unknown format code '%s' for object of type 'str'" % char))
                 else:
@@ -570,7 +587,10 @@ def test_float___format__(self):
     def test_float___format___errors(self):
         errors = []
 
-        okChars = set(['\0', '%', 'E', 'F', 'G', 'e', 'f', 'g', 'n', ',', '_'] + [chr(x) for x in range(ord('0'), ord('9') + 1)])
+        okChars = set(['\0', '%', 'E', 'F', 'G', 'e', 'f', 'g', 'n', ','] + [chr(x) for x in range(ord('0'), ord('9') + 1)])
+        if is_cli or sys.version_info >= (3, 6):
+            okChars.add('_')
+
         # verify the okChars are actually ok
         for char in okChars:
             2.0.__format__('10' + char)
@@ -911,6 +931,9 @@ def test_int___format__(self):
                 ]
 
         for value, spec, result in tests_thousands:
+            if not is_cli and sys.version_info < (3, 6) and "_" in spec:
+                continue
+
             assert "d" not in spec
             self.assertEqual(value.__format__(spec), result)
             self.assertEqual(big(value).__format__(spec), result)
@@ -965,7 +988,9 @@ def test_int___format___errors(self):
         else:
             errors.append((OverflowError, sys.maxsize + 1, 'c', "Python int too large to convert to C long"))
 
-        okChars = set(['%', 'E', 'F', 'G', 'X', 'x', 'b', 'c', 'd', 'o', 'e', 'f', 'g', 'n', ',', '_'] + [chr(x) for x in range(ord('0'), ord('9') + 1)])
+        okChars = set(['%', 'E', 'F', 'G', 'X', 'x', 'b', 'c', 'd', 'o', 'e', 'f', 'g', 'n', ','] + [chr(x) for x in range(ord('0'), ord('9') + 1)])
+        if is_cli or sys.version_info >= (3, 6):
+            okChars.add('_')
 
         # verify the okChars are actually ok
         for char in okChars: