diff -r bf23a6c215f6 -r 3f967e00e267 Doc/library/io.rst
--- a/Doc/library/io.rst	Wed Apr 18 08:30:51 2012 -0500
+++ b/Doc/library/io.rst	Wed Apr 18 16:59:25 2012 +0200
@@ -291,6 +291,11 @@
       .. versionadded:: 3.1
          The ``SEEK_*`` constants.
 
+      .. versionadded:: 3.3
+         Some Operating Systems can support aditional values, like
+         :const:`os.SEEK_HOLE` or :const:`os.SEEK_DATA`. The valid values
+         for a file could depend of it being open in text or binary mode.
+
    .. method:: seekable()
 
       Return ``True`` if the stream supports random access.  If ``False``,
diff -r bf23a6c215f6 -r 3f967e00e267 Doc/library/os.rst
--- a/Doc/library/os.rst	Wed Apr 18 08:30:51 2012 -0500
+++ b/Doc/library/os.rst	Wed Apr 18 16:59:25 2012 +0200
@@ -992,6 +992,10 @@
    Parameters to the :func:`lseek` function. Their values are 0, 1, and 2,
    respectively. Availability: Windows, Unix.
 
+   .. versionadded:: 3.3
+      Some Operating Systems can support aditional values, like
+      :const:`os.SEEK_HOLE` or :const:`os.SEEK_DATA`.
+
 
 .. function:: mkdirat(dirfd, path, mode=0o777)
 
diff -r bf23a6c215f6 -r 3f967e00e267 Lib/_pyio.py
--- a/Lib/_pyio.py	Wed Apr 18 08:30:51 2012 -0500
+++ b/Lib/_pyio.py	Wed Apr 18 16:59:25 2012 +0200
@@ -306,6 +306,7 @@
         * 0 -- start of stream (the default); offset should be zero or positive
         * 1 -- current stream position; offset may be negative
         * 2 -- end of stream; offset is usually negative
+        Some Operating Systems / File Systems can provide additional values.
 
         Return an int indicating the new absolute position.
         """
@@ -866,7 +867,7 @@
         elif whence == 2:
             self._pos = max(0, len(self._buffer) + pos)
         else:
-            raise ValueError("invalid whence value")
+            raise ValueError("unsupported whence value")
         return self._pos
 
     def tell(self):
@@ -1041,8 +1042,6 @@
         return _BufferedIOMixin.tell(self) - 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
@@ -1138,8 +1137,6 @@
         return _BufferedIOMixin.tell(self) + 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 _BufferedIOMixin.seek(self, pos, whence)
@@ -1235,8 +1232,6 @@
         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.
@@ -1852,7 +1847,7 @@
                 self._decoder.reset()
             return position
         if whence != 0:
-            raise ValueError("invalid whence (%r, should be 0, 1 or 2)" %
+            raise ValueError("unsupported whence (%r, should be 0, 1 or 2)" %
                              (whence,))
         if cookie < 0:
             raise ValueError("negative seek position %r" % (cookie,))
diff -r bf23a6c215f6 -r 3f967e00e267 Lib/os.py
--- a/Lib/os.py	Wed Apr 18 08:30:51 2012 -0500
+++ b/Lib/os.py	Wed Apr 18 16:59:25 2012 +0200
@@ -116,6 +116,7 @@
 
 # Python uses fixed values for the SEEK_ constants; they are mapped
 # to native constants if necessary in posixmodule.c
+# Other possible SEEK values are directly imported from posixmodule.c
 SEEK_SET = 0
 SEEK_CUR = 1
 SEEK_END = 2
diff -r bf23a6c215f6 -r 3f967e00e267 Lib/test/test_posix.py
--- a/Lib/test/test_posix.py	Wed Apr 18 08:30:51 2012 -0500
+++ b/Lib/test/test_posix.py	Wed Apr 18 16:59:25 2012 +0200
@@ -1009,6 +1009,27 @@
         posix.RTLD_GLOBAL
         posix.RTLD_LOCAL
 
+    @unittest.skipUnless('PC_MIN_HOLE_SIZE' in os.pathconf_names,
+                         "test needs an OS that reports file holes")
+    def test_fs_holes(self) :
+        # Even if the filesystem doesn't report holes,
+        # if the OS supports it the SEEK_* constants
+        # will be defined and will have a consistent
+        # behaviour:
+        # os.SEEK_DATA = current position
+        # os.SEEK_HOLE = end of file position
+        fp = open(support.TESTFN, 'r+b')
+        fp.write(b"hello")
+        fp.flush()
+        size = fp.tell()
+        fno = fp.fileno()
+        for i in range(size) :
+            self.assertEqual(i, os.lseek(fno, i, os.SEEK_DATA))
+            self.assertLessEqual(size, os.lseek(fno, i, os.SEEK_HOLE))
+        self.assertRaises(OSError, os.lseek, fno, size, os.SEEK_DATA)
+        self.assertRaises(OSError, os.lseek, fno, size, os.SEEK_HOLE)
+        fp.close()
+
 class PosixGroupsTester(unittest.TestCase):
 
     def setUp(self):
diff -r bf23a6c215f6 -r 3f967e00e267 Misc/NEWS
--- a/Misc/NEWS	Wed Apr 18 08:30:51 2012 -0500
+++ b/Misc/NEWS	Wed Apr 18 16:59:25 2012 +0200
@@ -342,6 +342,8 @@
 - Issue #14212: The re module didn't retain a reference to buffers it was
   scanning, resulting in segfaults.
 
+- Issue #10142: Support for SEEK_HOLE/SEEK_DATA (for example, under ZFS).
+
 - Issue #14259: The finditer() method of re objects did not take any
   keyword arguments, contrary to the documentation.
 
diff -r bf23a6c215f6 -r 3f967e00e267 Modules/_io/bufferedio.c
--- a/Modules/_io/bufferedio.c	Wed Apr 18 08:30:51 2012 -0500
+++ b/Modules/_io/bufferedio.c	Wed Apr 18 16:59:25 2012 +0200
@@ -1157,11 +1157,6 @@
     if (!PyArg_ParseTuple(args, "O|i:seek", &targetobj, &whence)) {
         return NULL;
     }
-    if (whence < 0 || whence > 2) {
-        PyErr_Format(PyExc_ValueError,
-                     "whence must be between 0 and 2, not %d", whence);
-        return NULL;
-    }
 
     CHECK_CLOSED(self, "seek of closed file")
 
@@ -1172,7 +1167,11 @@
     if (target == -1 && PyErr_Occurred())
         return NULL;
 
-    if (whence != 2 && self->readable) {
+    /* SEEK_SET and SEEK_CUR are special because we could seek inside the
+       buffer. Other whence values must be managed without this optimization.
+       Some Operating Systems can provide additional values, like
+       SEEK_HOLE/SEEK_DATA. */
+    if (((whence == 0) || (whence == 1)) && self->readable) {
         Py_off_t current, avail;
         /* Check if seeking leaves us inside the current buffer,
            so as to return quickly if possible. Also, we needn't take the
diff -r bf23a6c215f6 -r 3f967e00e267 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Wed Apr 18 08:30:51 2012 -0500
+++ b/Modules/posixmodule.c	Wed Apr 18 16:59:25 2012 +0200
@@ -11184,6 +11184,13 @@
 #endif
 
 
+#ifdef SEEK_HOLE /* ZFS */
+    if (ins(d, "SEEK_HOLE", (long)SEEK_HOLE)) return -1;
+#endif
+#ifdef SEEK_DATA /* ZFS */
+    if (ins(d, "SEEK_DATA", (long)SEEK_DATA)) return -1;
+#endif
+
 /* MS Windows */
 #ifdef O_NOINHERIT
     /* Don't inherit in child processes. */