diff -r 37905786b34b Lib/test/support/__init__.py
--- a/Lib/test/support/__init__.py	Sun Apr 12 23:24:17 2015 -0500
+++ b/Lib/test/support/__init__.py	Tue Apr 14 15:18:59 2015 -0400
@@ -88,7 +88,7 @@
     "skip_unless_symlink", "requires_gzip", "requires_bz2", "requires_lzma",
     "bigmemtest", "bigaddrspacetest", "cpython_only", "get_attribute",
     "requires_IEEE_754", "skip_unless_xattr", "requires_zlib",
-    "anticipate_failure", "load_package_tests",
+    "anticipate_failure", "load_package_tests", "detect_api_mismatch",
     # sys
     "is_jython", "check_impl_detail",
     # network
@@ -2188,6 +2188,21 @@
             return False
 
 
+def detect_api_mismatch(ref_api, other_api, *, ignore=None):
+    """Return set of items in ref_api not in other_api, except for a
+    defined list of items to be ignored in this check.
+    
+    By default this skips private attributes beginning with '_' but
+    includes all public magic methods, i.e. those ending in '__'.
+    """
+    missing_items = set(dir(ref_api)) - set(dir(other_api))
+    if ignore:
+        missing_items -= set(ignore)
+    missing_items = set(m for m in missing_items
+                        if not m.startswith('_') or m.endswith('__'))
+    return missing_items
+
+
 class SuppressCrashReport:
     """Try to prevent a crash report from popping up.
 
diff -r 37905786b34b Lib/test/test_io.py
--- a/Lib/test/test_io.py	Sun Apr 12 23:24:17 2015 -0500
+++ b/Lib/test/test_io.py	Tue Apr 14 15:18:59 2015 -0400
@@ -719,6 +719,20 @@
     pass
 
 
+class CPyMatchTest(unittest.TestCase):
+
+    @unittest.skip('test to be fixed by issue 9858')
+    def test_RawIOBase_io_in_pyio_match(self):
+        """Test that pyio RawIOBase class has all c RawIOBase methods"""
+        mismatch = support.detect_api_mismatch(pyio.RawIOBase, io.RawIOBase)
+        self.assertEqual(mismatch, set(), msg='Python RawIOBase does not have all C RawIOBase methods')
+
+    def test_RawIOBase_pyio_in_io_match(self):
+        """Test that c RawIOBase class has all pyio RawIOBase methods"""
+        mismatch = support.detect_api_mismatch(io.RawIOBase, pyio.RawIOBase)
+        self.assertEqual(mismatch, set(), msg='C RawIOBase does not have all Python RawIOBase methods')
+
+
 class CommonBufferedTests:
     # Tests common to BufferedReader, BufferedWriter and BufferedRandom
 
@@ -3663,7 +3677,7 @@
 
 
 def load_tests(*args):
-    tests = (CIOTest, PyIOTest,
+    tests = (CIOTest, PyIOTest, CPyMatchTest,
              CBufferedReaderTest, PyBufferedReaderTest,
              CBufferedWriterTest, PyBufferedWriterTest,
              CBufferedRWPairTest, PyBufferedRWPairTest,
diff -r 37905786b34b Lib/test/test_support.py
--- a/Lib/test/test_support.py	Sun Apr 12 23:24:17 2015 -0500
+++ b/Lib/test/test_support.py	Tue Apr 14 15:18:59 2015 -0400
@@ -280,6 +280,46 @@
             self.assertEqual(D["item"], 5)
         self.assertEqual(D["item"], 1)
 
+    def test_detect_api_mismatch(self):
+        class RefClass:
+            attribute1 = None
+            attribute2 = None
+            _hidden_attribute1 = None
+            __magic_1__ = None
+
+        class OtherClass:
+            attribute2 = None
+            attribute3 = None
+            __magic_1__ = None
+            __magic_2__ = None
+
+        missing_items = support.detect_api_mismatch(RefClass, OtherClass)
+        self.assertEqual({'attribute1'}, missing_items)
+
+        missing_items = support.detect_api_mismatch(OtherClass, RefClass)
+        self.assertEqual({'attribute3', '__magic_2__'}, missing_items)
+
+    def test_detect_api_mismatch__ignore(self):
+        class RefClass:
+            attribute1 = None
+            attribute2 = None
+            _hidden_attribute1 = None
+            __magic_1__ = None
+
+        class OtherClass:
+            attribute2 = None
+            attribute3 = None
+            __magic_1__ = None
+            __magic_2__ = None
+
+        ignore = ['attribute1', 'attribute3', '__magic_2__', 'not_in_either']
+
+        missing_items = support.detect_api_mismatch(RefClass, OtherClass, ignore=ignore)
+        self.assertEqual(set(), missing_items)
+
+        missing_items = support.detect_api_mismatch(OtherClass, RefClass, ignore=ignore)
+        self.assertEqual(set(), missing_items)
+
     # XXX -follows a list of untested API
     # make_legacy_pyc
     # is_resource_enabled