Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 77320)
+++ Lib/io.py	(working copy)
@@ -1019,6 +1019,8 @@
         return self.raw.tell() - len(self._read_buf) + self._read_pos
 
     def seek(self, pos, whence=0):
+        if not (0 <= whence <= 2):
+            raise ValueError("invalid whence value")
         with self._read_lock:
             if whence == 1:
                 pos -= len(self._read_buf) - self._read_pos
@@ -1097,6 +1099,8 @@
         try:
             while self._write_buf:
                 n = self.raw.write(self._write_buf)
+                if n > len(self._write_buf) or n < 0:
+                    raise IOError("write() returned incorrect number of bytes : %d vs %d for buf %s"%(n, len(self._write_buf), self._write_buf))
                 del self._write_buf[:n]
                 written += n
         except BlockingIOError as e:
@@ -1109,6 +1113,8 @@
         return self.raw.tell() + len(self._write_buf)
 
     def seek(self, pos, whence=0):
+        if not (0 <= whence <= 2):
+            raise ValueError("invalid whence")
         with self._write_lock:
             self._flush_unlocked()
             return self.raw.seek(pos, whence)
@@ -1197,7 +1203,13 @@
         BufferedWriter.__init__(self, raw, buffer_size, max_buffer_size)
 
     def seek(self, pos, whence=0):
+        if not (0 <= whence <= 2):
+            raise ValueError("invalid whence")
         self.flush()
+        if self._read_buf:
+            # Undo read ahead.
+            with self._read_lock:
+                self.raw.seek(self._read_pos - len(self._read_buf), 1)
         # First do the raw seek, then empty the read buffer, so that
         # if the raw seek fails, we don't lose buffered data forever.
         pos = self.raw.seek(pos, whence)
Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 77320)
+++ Lib/test/test_io.py	(working copy)
@@ -100,6 +100,10 @@
         self.assertEqual(f.tell(), 13)
         self.assertEqual(f.truncate(12), 12)
         self.assertEqual(f.tell(), 12)
+        self.assertEqual(f.write(b"hij"), 3)
+        self.assertEqual(f.seek(0,1), 15)
+        self.assertEqual(f.tell(), 15)
+        self.assertEqual(f.truncate(12), 12)
         self.assertRaises(TypeError, f.seek, 0.0)
 
     def read_ops(self, f, buffered=False):
@@ -122,6 +126,10 @@
         self.assertEqual(f.seek(-6, 1), 5)
         self.assertEqual(f.read(5), b" worl")
         self.assertEqual(f.tell(), 10)
+        f.seek(0)
+        f.read(2)
+        f.seek(0, 1)
+        self.assertEqual(f.tell(), 2)
         self.assertRaises(TypeError, f.seek, 0.0)
         if buffered:
             f.seek(0)
@@ -175,6 +183,13 @@
         self.assertEqual(f.writable(), False)
         self.assertEqual(f.seekable(), True)
         self.read_ops(f, True)
+        f = io.open(test_support.TESTFN, "r+b")
+        self.assertEqual(f.readable(), True)
+        self.assertEqual(f.writable(), True)
+        self.assertEqual(f.seekable(), True)
+        self.write_ops(f)
+        f.seek(0)
+        self.read_ops(f, True)
         f.close()
 
     def test_readline(self):
@@ -492,7 +507,7 @@
         self.assertEquals(b"abcdefghijkl", writer._write_stack[0])
 
     def testWriteNonBlocking(self):
-        raw = MockNonBlockWriterIO((9, 2, 22, -6, 10, 12, 12))
+        raw = MockNonBlockWriterIO((9, 2, 10, -6, 10, 8, 12))
         bufio = io.BufferedWriter(raw, 8, 16)
 
         bufio.write(b"asdf")