Index: Doc/library/zlib.rst
===================================================================
--- Doc/library/zlib.rst	(revision 71045)
+++ Doc/library/zlib.rst	(working copy)
@@ -111,7 +111,7 @@
    regardless of platform.
 
 
-.. function:: decompress(string[, wbits[, bufsize]])
+.. function:: decompress(string[, wbits[, bufsize[, offset]]])
 
    Decompresses the data in *string*, returning a string containing the
    uncompressed data.  The *wbits* parameter controls the size of the window
@@ -132,7 +132,16 @@
    don't have to get this value exactly right; tuning it will only save a few calls
    to :cfunc:`malloc`.  The default size is 16384.
 
+   *offset* is the initial position in *string* to start decompression.
+   When specified, it will cause the function's return value to be a (uncompressed, offset)
+   tuple, with the second part giving the position in the input string just after
+   any compressed data.  This is useful if *string* contains additional data after
+   any initial compressed part.
 
+.. versionchanged:: 2.7
+   Added the *offset* argument
+
+
 .. function:: decompressobj([wbits])
 
    Returns a decompression object, to be used for decompressing data streams that
Index: Lib/test/test_zlib.py
===================================================================
--- Lib/test/test_zlib.py	(revision 71045)
+++ Lib/test/test_zlib.py	(working copy)
@@ -103,9 +103,19 @@
         x = zlib.compress(data)
         self.assertEqual(zlib.decompress(x), data)
 
+    def test_offset(self):
+        # test the "tail" feature of decompression
+        a = HAMLET_SCENE
+        b = a.swapcase()*2
+        c = zlib.compress(a)+zlib.compress(b)
+        
+        aa, offset = zlib.decompress(c, offset=0)
+        bb, offset = zlib.decompress(c, offset=offset)
+        self.assertEqual(a, aa)
+        self.assertEqual(b, bb)
+        self.assertEqual(offset, len(c))
 
 
-
 class CompressObjectTestCase(unittest.TestCase):
     # Test compression object
     def test_pair(self):
@@ -380,6 +390,16 @@
             d.flush()
             self.assertRaises(ValueError, d.copy)
 
+    def test_unused_data(self):
+        #test any unused data
+        data = HAMLET_SCENE
+        unused = data.swapcase()*2
+        x = zlib.compress(data) + unused
+        dco = zlib.decompressobj()
+        y = dco.decompress(x) + dco.flush()
+        self.assertEqual(y, data)
+        self.assertEqual(dco.unused_data, unused)
+
 def genblock(seed, length, step=1024, generator=random):
     """length-byte stream of random data from a seed (in step-byte blocks)."""
     if seed is not None:
Index: Modules/zlibmodule.c
===================================================================
--- Modules/zlibmodule.c	(revision 71045)
+++ Modules/zlibmodule.c	(working copy)
@@ -186,28 +186,40 @@
 }
 
 PyDoc_STRVAR(decompress__doc__,
-"decompress(string[, wbits[, bufsize]]) -- Return decompressed string.\n"
+"decompress(string[, wbits[, bufsize[, offset]]]) -- Return decompressed string.\n"
 "\n"
 "Optional arg wbits is the window buffer size.  Optional arg bufsize is\n"
-"the initial output buffer size.");
+"the initial output buffer size.  Optional arg offset specifies\n"
+"the start position in the string to start decompresion and, if spceified,\n"
+"causes the return value to be a tuple, with the second element being\n"
+"the position in the string just afer the last piece of compressed data.");
 
 static PyObject *
-PyZlib_decompress(PyObject *self, PyObject *args)
+PyZlib_decompress(PyObject *self, PyObject *args, PyObject *kw)
 {
     PyObject *result_str;
     Byte *input;
     int length, err;
     int wsize=DEF_WBITS;
     Py_ssize_t r_strlen=DEFAULTALLOC;
+    Py_ssize_t offset = -1;
     z_stream zst;
+    char *keywords[] = {"string", "wbits", "bufsize", "offset", 0};
 
-    if (!PyArg_ParseTuple(args, "s#|in:decompress",
-			  &input, &length, &wsize, &r_strlen))
+    if (!PyArg_ParseTupleAndKeywords(args, kw, "s#|inn:decompress", keywords,
+			  &input, &length, &wsize, &r_strlen, &offset))
 	return NULL;
-
     if (r_strlen <= 0)
 	r_strlen = 1;
 
+    /* if specified, don't start at the beginning of the input string */
+    if (offset > 0) {
+	if (offset > length)
+	    offset = length;
+	length -= offset;
+	input += offset;
+    }
+
     zst.avail_in = length;
     zst.avail_out = r_strlen;
 
@@ -279,6 +291,11 @@
     }
 
     _PyString_Resize(&result_str, zst.total_out);
+    if (result_str != NULL && offset >= 0) {
+	/* compute new offset to current pos */
+	offset = length-zst.avail_in+offset;
+	return Py_BuildValue("Nn", result_str, offset);
+    }
     return result_str;
 
  error:
@@ -941,7 +958,7 @@
                     compressobj__doc__},
     {"crc32", (PyCFunction)PyZlib_crc32, METH_VARARGS,
               crc32__doc__},
-    {"decompress", (PyCFunction)PyZlib_decompress, METH_VARARGS,
+    {"decompress", (PyCFunction)PyZlib_decompress, METH_KEYWORDS,
                    decompress__doc__},
     {"decompressobj", (PyCFunction)PyZlib_decompressobj, METH_VARARGS,
                    decompressobj__doc__},