Index: Modules/gcmodule.c
===================================================================
--- Modules/gcmodule.c	(revision 86684)
+++ Modules/gcmodule.c	(working copy)
@@ -68,6 +68,9 @@
 /* Python string used to look for __del__ attribute. */
 static PyObject *delstr = NULL;
 
+/* a callback to be invoked when collection is done */
+static PyObject *callback = NULL;
+
 /* This is the number of objects who survived the last full collection. It
    approximates the number of long lived objects tracked by the GC.
 
@@ -111,7 +114,7 @@
    thusly: "each full garbage collection is more and more costly as the
    number of objects grows, but we do fewer and fewer of them").
 
-   This heuristic was suggested by Martin von Löwis on python-dev in
+   This heuristic was suggested by Martin von Löwis on python-dev in
    June 2008. His original analysis and proposal can be found at:
     http://mail.python.org/pipermail/python-dev/2008-June/080579.html
 */
@@ -948,6 +951,13 @@
     int i;
     Py_ssize_t n = 0;
 
+    /* Invoke callback */
+    if (callback) {
+        PyObject *r = PyObject_CallFunction(callback, "l", 1);
+        Py_XDECREF(r);
+        PyErr_Clear();
+    }
+
     /* Find the oldest generation (highest numbered) where the count
      * exceeds the threshold.  Objects in the that generation and
      * generations younger than it will be collected. */
@@ -964,6 +974,14 @@
             break;
         }
     }
+
+    /* Invoke callback */
+    if (callback) {
+        PyObject *r = PyObject_CallFunction(callback, "ll", 0, n);
+        Py_XDECREF(r);
+        PyErr_Clear();
+    }
+
     return n;
 }
 
@@ -1256,6 +1274,33 @@
 }
 
 
+static PyObject *
+gc_get_callback(PyObject *self, PyObject *noargs)
+{
+    PyObject *cb = callback;
+    if (!cb)
+        cb = Py_None;
+    Py_INCREF(cb);
+    return cb;
+}
+
+
+static PyObject *
+gc_set_callback(PyObject *self, PyObject *args)
+{
+    PyObject *cb;
+    if (!PyArg_ParseTuple(args, "O:set_callback", &cb))
+        return 0;
+    if (cb == Py_None)
+        cb = 0;
+    Py_XINCREF(cb);
+    Py_XDECREF(callback);
+    callback = cb;
+    Py_INCREF(Py_None);
+    return Py_None;
+}
+
+
 PyDoc_STRVAR(gc__doc__,
 "This module provides access to the garbage collector for reference cycles.\n"
 "\n"
@@ -1290,6 +1335,8 @@
         gc_get_referrers__doc__},
     {"get_referents",  gc_get_referents, METH_VARARGS,
         gc_get_referents__doc__},
+    {"get_callback", gc_get_callback, METH_NOARGS,  ""},
+    {"set_callback", gc_set_callback, METH_VARARGS,  ""},
     {NULL,      NULL}           /* Sentinel */
 };