diff -r c33596e6f723 Lib/test/test_warnings.py
--- a/Lib/test/test_warnings.py	Sat Apr 16 23:14:40 2011 +0300
+++ b/Lib/test/test_warnings.py	Sun Apr 17 13:10:13 2011 +0200
@@ -519,6 +519,19 @@
             finally:
                 self.module.showwarning = old_showwarning
 
+    def test_showwarning_from_c(self):
+        #Issue #10271: test we can override showarning() with a PyCFunction
+        from _testcapi import showwarning_from_c
+        with original_warnings.catch_warnings(module=self.module):
+            self.module.filterwarnings("always", category=UserWarning)
+            old_showwarning = self.module.showwarning
+            self.module.showwarning = showwarning_from_c
+            try:
+                self.assertRaisesRegex(Exception, "DON'T PANIC",
+                                       self.module.warn, "Warning!")
+            finally:
+                self.module.showwarning = old_showwarning
+
     def test_show_warning_output(self):
         # With showarning() missing, make sure that output is okay.
         text = 'test show_warning'
diff -r c33596e6f723 Modules/_testcapimodule.c
--- a/Modules/_testcapimodule.c	Sat Apr 16 23:14:40 2011 +0300
+++ b/Modules/_testcapimodule.c	Sun Apr 17 13:10:13 2011 +0200
@@ -2246,6 +2246,27 @@
     return NULL;
 }
 
+/* Issue #10271: test we can override warnings.showwarning() with a PyCFunction.
+   Run via Lib/test/test_warnings.py */
+static PyObject *
+showwarning_from_c(PyObject *self, PyObject *args)
+{
+    PyObject *message, *category, *filename, *lineno;
+    PyObject *tmp = NULL;
+
+    if (!PyArg_ParseTuple(args, "OOOO:showwarning_from_c",
+                          &message, &category, &filename, &lineno))
+        return NULL;
+    tmp = PyObject_Str(message);
+    if (tmp && !PyUnicode_CompareWithASCIIString(tmp, "Warning!"))
+        PyErr_SetString(PyExc_Exception, "DON'T PANIC");
+    Py_XDECREF(tmp);
+    if (PyErr_Occurred())
+        return NULL;
+    else
+        Py_RETURN_NONE;
+}
+
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
     {"raise_memoryerror",   (PyCFunction)raise_memoryerror,  METH_NOARGS},
@@ -2329,6 +2350,7 @@
     {"make_memoryview_from_NULL_pointer", (PyCFunction)make_memoryview_from_NULL_pointer,
      METH_NOARGS},
     {"crash_no_current_thread", (PyCFunction)crash_no_current_thread, METH_NOARGS},
+    {"showwarning_from_c", (PyCFunction)showwarning_from_c, METH_VARARGS},
     {NULL, NULL} /* sentinel */
 };