diff -r eb1b93fd68f5 Lib/test/test_io.py
--- a/Lib/test/test_io.py	Thu May 12 02:07:00 2011 +0200
+++ b/Lib/test/test_io.py	Sat May 28 14:42:56 2011 -0400
@@ -801,6 +801,69 @@
         self.assertEqual(bufio.readinto(b), 1)
         self.assertEqual(b, b"cb")
 
+    def test_prefetch(self):
+        rawio = self.MockRawIO((b'abc', b'd', b'ef', None))
+        bufio = self.tp(rawio)
+        b = bytearray(2)
+        # Invalid args
+        self.assertRaises(TypeError, bufio.prefetch, None, 0, 0)
+        self.assertRaises(TypeError, bufio.prefetch, 0, None, 0)
+        self.assertRaises(TypeError, bufio.prefetch, 0, 0, None)
+        self.assertRaises(ValueError, bufio.prefetch, b, 0, 4)
+        self.assertRaises(ValueError, bufio.prefetch, b, -1, 0)
+        self.assertRaises(ValueError, bufio.prefetch, b, 0, -1)
+
+        self.assertEqual(bufio.prefetch(b), 2)
+        self.assertEqual(b, b'ab')
+        self.assertEqual(bufio.prefetch(b, 0, 0), 2)
+        self.assertEqual(b, b'ab')
+        self.assertEqual(bufio.prefetch(b, 0, 2), 2)
+        self.assertEqual(b, b'ab')
+        self.assertEqual(rawio._reads, 1)
+        self.assertEqual(bufio.prefetch(b, 1, 2), 2)
+        self.assertEqual(b, b'de')
+        self.assertEqual(rawio._reads, 3)
+        self.assertEqual(bufio.prefetch(b, 0, 2), 1)
+        self.assertEqual(b, b'fe')
+        self.assertEqual(rawio._reads, 4)
+        self.assertEqual(bufio.prefetch(b, 1, 1), 0)
+        self.assertEqual(b, b'fe')
+        self.assertEqual(rawio._reads, 5)
+        # EOF during skip
+        b = bytearray(2)
+        bufio.seek(0)
+        self.assertEqual(bufio.prefetch(b, 8, 2), 0)
+        self.assertEqual(b, b'\x00\x00')
+
+    def test_unseekable_prefetch(self):
+        rawio = self.MockUnseekableIO(b'abc' * 10)
+        bufio = self.tp(rawio)
+        b = bytearray(3)
+        self.assertEqual(bufio.prefetch(b, 0, 0), 3)
+        self.assertEqual(b, b'abc')
+        self.assertEqual(bufio.prefetch(b, 1, 2), 3)
+        self.assertEqual(b, b'bca')
+        self.assertEqual(bufio.prefetch(b, 24, 0), 3)
+        self.assertEqual(b, b'abc')
+        self.assertEqual(bufio.prefetch(b, 2, 3), 1)
+        self.assertEqual(b, b'cbc')
+        # large skips > bufsize
+        bufsize = 8 * 1024
+        data = b'abcxyz' * bufsize
+        bufio = self.tp(self.MockUnseekableIO(data))
+        b = bytearray(9)
+        self.assertEqual(bufio.prefetch(b, len(data) - 9, 2), 9)
+        self.assertEqual(b, b'xyzabcxyz')
+        self.assertEqual(bufio.prefetch(b, 1, 7), 6)
+        self.assertEqual(b, b'abcxyzxyz')
+        # EOF during skip
+        b = bytearray(2)
+        rawio = self.MockUnseekableIO(b'abc')
+        bufio = self.tp(rawio)
+        self.assertEqual(bufio.prefetch(b, 8, 2), 0)
+        self.assertEqual(b, b'\x00\x00')
+
+
     def test_readlines(self):
         def bufio():
             rawio = self.MockRawIO((b"abc\n", b"d\n", b"ef"))
@@ -1185,7 +1248,6 @@
                                      DeprecationWarning)):
             self.tp(self.MockRawIO(), 8, 12)
 
-
 class CBufferedWriterTest(BufferedWriterTest):
     tp = io.BufferedWriter
 
@@ -1510,6 +1572,7 @@
 
     # You can't construct a BufferedRandom over a non-seekable stream.
     test_unseekable = None
+    test_unseekable_prefetch = None
 
 class CBufferedRandomTest(BufferedRandomTest):
     tp = io.BufferedRandom