diff --git a/Lib/selectors.py b/Lib/selectors.py
--- a/Lib/selectors.py
+++ b/Lib/selectors.py
@@ -6,7 +6,7 @@ This module allows high-level and effici
 
 
 from abc import ABCMeta, abstractmethod
-from collections import namedtuple
+from collections import namedtuple, Mapping
 import functools
 import select
 import sys
@@ -44,6 +44,24 @@ SelectorKey = namedtuple('SelectorKey', 
 selected event mask and attached data."""
 
 
+class _SelectorMapping(Mapping):
+
+    def __init__(self, selector):
+        self._fd_to_key = selector._fd_to_key
+
+    def __len__(self):
+        return len(self._fd_to_key)
+
+    def __getitem__(self, fileobj):
+        try:
+            return self._fd_to_key[_fileobj_to_fd(fileobj)]
+        except KeyError:
+            raise KeyError("{!r} is not registered".format(fileobj)) from None
+
+    def __iter__(self):
+        return iter(self._fd_to_key)
+
+
 class BaseSelector(metaclass=ABCMeta):
     """Base selector class.
 
@@ -151,16 +169,9 @@ class BaseSelector(metaclass=ABCMeta):
         """
         self._fd_to_key.clear()
 
-    def get_key(self, fileobj):
-        """Return the key associated to a registered file object.
-
-        Returns:
-        SelectorKey for this file object
-        """
-        try:
-            return self._fd_to_key[_fileobj_to_fd(fileobj)]
-        except KeyError:
-            raise KeyError("{!r} is not registered".format(fileobj)) from None
+    def get_map(self):
+        """Get a mapping of file descriptors to selector keys."""
+        return _SelectorMapping(self)
 
     def __enter__(self):
         return self
diff --git a/Lib/test/test_selectors.py b/Lib/test/test_selectors.py
--- a/Lib/test/test_selectors.py
+++ b/Lib/test/test_selectors.py
@@ -106,7 +106,7 @@ class BaseSelectorTestCase(unittest.Test
         # modify events
         key2 = s.modify(rd, selectors.EVENT_WRITE)
         self.assertNotEqual(key.events, key2.events)
-        self.assertEqual(key2, s.get_key(rd))
+        self.assertEqual(key2, s.get_map()[rd])
 
         s.unregister(rd)
 
@@ -118,7 +118,7 @@ class BaseSelectorTestCase(unittest.Test
         key2 = s.modify(rd, selectors.EVENT_READ, d2)
         self.assertEqual(key.events, key2.events)
         self.assertNotEqual(key.data, key2.data)
-        self.assertEqual(key2, s.get_key(rd))
+        self.assertEqual(key2, s.get_map()[rd])
         self.assertEqual(key2.data, d2)
 
         # modify unknown file obj
@@ -136,10 +136,12 @@ class BaseSelectorTestCase(unittest.Test
         s.register(wr, selectors.EVENT_WRITE)
 
         s.close()
-        self.assertRaises(KeyError, s.get_key, rd)
-        self.assertRaises(KeyError, s.get_key, wr)
+        with self.assertRaises(KeyError):
+            s.get_map()[rd]
+        with self.assertRaises(KeyError):
+            s.get_map()[wr]
 
-    def test_get_key(self):
+    def test_get_map(self):
         s = self.SELECTOR()
         self.addCleanup(s.close)
 
@@ -147,11 +149,23 @@ class BaseSelectorTestCase(unittest.Test
         self.addCleanup(rd.close)
         self.addCleanup(wr.close)
 
+        keys = s.get_map()
+        self.assertEqual(len(keys), 0)
+        self.assertEqual(list(keys), [])
         key = s.register(rd, selectors.EVENT_READ, "data")
-        self.assertEqual(key, s.get_key(rd))
+        self.assertIn(rd, keys)
+        self.assertEqual(key, keys[rd])
+        self.assertEqual(len(keys), 1)
+        self.assertEqual(list(keys), [rd.fileno()])
+        self.assertEqual(list(keys.values()), [key])
 
         # unknown file obj
-        self.assertRaises(KeyError, s.get_key, 999999)
+        with self.assertRaises(KeyError):
+            keys[999999]
+
+        # Read-only mapping
+        with self.assertRaises(TypeError):
+            del keys[rd]
 
     def test_select(self):
         s = self.SELECTOR()
@@ -185,8 +199,8 @@ class BaseSelectorTestCase(unittest.Test
             sel.register(rd, selectors.EVENT_READ)
             sel.register(wr, selectors.EVENT_WRITE)
 
-        self.assertRaises(KeyError, s.get_key, rd)
-        self.assertRaises(KeyError, s.get_key, wr)
+        self.assertNotIn(rd, s.get_map())
+        self.assertNotIn(wr, s.get_map())
 
     def test_fileno(self):
         s = self.SELECTOR()