Index: Python/_warnings.c
===================================================================
--- Python/_warnings.c	(revision 66997)
+++ Python/_warnings.c	(working copy)
@@ -603,14 +603,33 @@
 static PyObject *
 do_warn(PyObject *message, PyObject *category, Py_ssize_t stack_level)
 {
-    PyObject *filename, *module, *registry, *res;
+    PyObject *filename, *module, *registry, *res, *func;
     int lineno;
 
     if (!setup_context(stack_level, &filename, &lineno, &module, &registry))
         return NULL;
 
-    res = warn_explicit(category, message, filename, lineno, module, registry,
-                        NULL);
+    func = get_warnings_attr("warn_explicit");
+    if (func == NULL) {
+        if (PyErr_Occurred())
+            return NULL;
+        res = warn_explicit(category, message, filename, lineno, module, registry,
+                            NULL);
+    }
+    else {
+        PyObject *lineno_obj;
+        lineno_obj = PyInt_FromLong(lineno);
+        if (lineno_obj == NULL) {
+            Py_DECREF(filename);
+            Py_DECREF(registry);
+            Py_DECREF(module);
+            return NULL;
+        }
+        res = PyObject_CallFunctionObjArgs(func, message, category, filename, lineno_obj,
+                                           module, registry, NULL);
+        Py_DECREF(lineno_obj);
+        Py_DECREF(func);
+    }
     Py_DECREF(filename);
     Py_DECREF(registry);
     Py_DECREF(module);
Index: Lib/test/test_warnings.py
===================================================================
--- Lib/test/test_warnings.py	(revision 66997)
+++ Lib/test/test_warnings.py	(working copy)
@@ -60,10 +60,12 @@
         # The 'warnings' module must be explicitly set so that the proper
         # interaction between _warnings and 'warnings' can be controlled.
         sys.modules['warnings'] = self.module
+        self.save_warn_explicit = self.module.warn_explicit
         super(BaseTest, self).setUp()
 
     def tearDown(self):
         sys.modules['warnings'] = original_warnings
+        self.module.warn_explicit = self.save_warn_explicit
         super(BaseTest, self).tearDown()
 
 
@@ -424,6 +426,14 @@
                 result = stream.getvalue()
         self.failUnless(text in result)
 
+    def test_replace_warn_explicit(self):
+        self.called = False
+        def my_warn_explicit(*args):
+            self.called = True
+        self.module.warn_explicit = my_warn_explicit
+        self.module.warn("foo")
+        self.assertTrue(self.called)
+
     def test_showwarning_not_callable(self):
         self.module.filterwarnings("always", category=UserWarning)
         old_showwarning = self.module.showwarning