diff -r 268259370a01 Lib/selectors.py
--- a/Lib/selectors.py	Mon Nov 04 13:18:19 2013 -0800
+++ b/Lib/selectors.py	Mon Nov 04 23:26:26 2013 +0100
@@ -138,11 +138,14 @@
             key = self._fd_to_key[_fileobj_to_fd(fileobj)]
         except KeyError:
             raise KeyError("{!r} is not registered".format(fileobj)) from None
-        if events != key.events or data != key.data:
-            # TODO: If only the data changed, use a shortcut that only
-            # updates the data.
+        if events != key.events:
             self.unregister(fileobj)
             return self.register(fileobj, events, data)
+        elif data != key.data:
+            # Use a shortcut to update the data.
+            key = key._replace(data=data)
+            self._fd_to_key[key.fd] = key
+            return key
         else:
             return key
 
diff -r 268259370a01 Lib/test/test_selectors.py
--- a/Lib/test/test_selectors.py	Mon Nov 04 13:18:19 2013 -0800
+++ b/Lib/test/test_selectors.py	Mon Nov 04 23:26:26 2013 +0100
@@ -6,6 +6,8 @@
 from test import support
 from time import sleep
 import unittest
+from unittest.mock import Mock
+
 try:
     from time import monotonic as time
 except ImportError:
@@ -46,6 +48,70 @@
     return match
 
 
+class FakeSelector(selectors.BaseSelector):
+    """Trivial non-abstract subclass of BaseSelector."""
+
+    def select(self, timeout=None):
+        raise NotImplementedError
+
+
+class _SelectorMappingTests(unittest.TestCase):
+
+    def test_len(self):
+        s = self.SELECTOR()
+        self.addCleanup(s.close)
+
+        map = selectors._SelectorMapping(s)
+        self.assertTrue(map.__len__() == 0)
+
+        f = Mock()
+        f.fileno.return_value = 10
+        s.register(f, selectors.EVENT_READ, None)
+        self.assertTrue(len(map) == 1)
+
+    def test_getitem(self):
+        s = self.SELECTOR()
+        self.addCleanup(s.close)
+
+        map = selectors._SelectorMapping(s)
+        f = Mock()
+        f.fileno.return_value = 10
+        s.register(f, selectors.EVENT_READ, None)
+        attended = selectors.SelectorKey(f, 10, selectors.EVENT_READ, None)
+        self.assertEqual(attended, map.__getitem__(f))
+
+    def test_getitem_key_error(self):
+        s = self.SELECTOR()
+        self.addCleanup(s.close)
+
+        map = selectors._SelectorMapping(s)
+        self.assertTrue(len(map) == 0)
+        f = Mock()
+        f.fileno.return_value = 10
+        s.register(f, selectors.EVENT_READ, None)
+        self.assertRaises(KeyError, map.__getitem__, 5)
+
+    def test_iter(self):
+        s = self.SELECTOR()
+        self.addCleanup(s.close)
+
+        map = selectors._SelectorMapping(s)
+        self.assertTrue(len(map) == 0)
+        f = Mock()
+        f.fileno.return_value = 5
+        s.register(f, selectors.EVENT_READ, None)
+
+        counter = 0
+        for fileno in map.__iter__():
+            self.assertEqual(5, fileno)
+            counter += 1
+        self.assertEqual(1, counter)
+
+        for idx in map:
+            self.assertEqual(f, map[idx].fileobj)
+
+
+
 class BaseSelectorTestCase(unittest.TestCase):
 
     def test_register(self):
@@ -113,6 +179,7 @@
         # modify data
         d1 = object()
         d2 = object()
+        d3 = object()
 
         key = s.register(rd, selectors.EVENT_READ, d1)
         key2 = s.modify(rd, selectors.EVENT_READ, d2)
@@ -121,6 +188,15 @@
         self.assertEqual(key2, s.get_key(rd))
         self.assertEqual(key2.data, d2)
 
+        # modify data uses shortcut
+
+        s.unregister = Mock()
+        s.register = Mock()
+        s.modify(rd, selectors.EVENT_READ, d3)
+        self.assertFalse(s.unregister.called)
+        self.assertFalse(s.register.called)
+
+
         # modify unknown file obj
         self.assertRaises(KeyError, s.modify, 999999, selectors.EVENT_READ)