diff -r 37905786b34b -r 1ecba3b8cf88 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 12:10:07 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_module_matches",
     # sys
     "is_jython", "check_impl_detail",
     # network
@@ -2188,6 +2188,19 @@
             return False
 
 
+def detect_module_mismatch(module1, module2, ignore=None, ignore_private=True):
+    """ Returns set of items in module1 not in module2, except for a
+    defined list of items to be ignored in this check.
+    """
+    missing_items = set(dir(module1)) - set(dir(module2))
+    if ignore:
+        missing_items = missing_items - set(ignore)
+    if ignore_private:
+        missing_items = [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 -r 1ecba3b8cf88 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 12:10:07 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_module_mismatch(pyio.RawIOBase, io.RawIOBase)
+        self.assertFalse(mismatch)
+
+    def test_RawIOBase_pyio_in_io_match(self):
+        """ Test that c RawIOBase class has all pyio RawIOBase methods """
+        mismatch = support.detect_module_mismatch(io.RawIOBase, pyio.RawIOBase)
+        self.assertFalse(mismatch)
+
+
 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,