diff -r d60c00015f01 Include/import.h
--- a/Include/import.h	Wed Oct 05 19:53:43 2011 +0200
+++ b/Include/import.h	Wed Oct 05 22:38:58 2011 -0300
@@ -74,6 +74,8 @@
     );
 PyAPI_FUNC(int)_PyImport_FixupExtensionUnicode(PyObject*, char *, PyObject *);
 
+const int the_byte_compiled_file_is_older(PyObject*);
+
 struct _inittab {
     char *name;
     PyObject* (*initfunc)(void);
diff -r d60c00015f01 Python/import.c
--- a/Python/import.c	Wed Oct 05 19:53:43 2011 +0200
+++ b/Python/import.c	Wed Oct 05 22:38:58 2011 -0300
@@ -329,7 +329,7 @@
         /* Forked as a side effect of import */
         long me = PyThread_get_thread_ident();
         PyThread_acquire_lock(import_lock, 0);
-	/* XXX: can the previous line fail? */
+        /* XXX: can the previous line fail? */
         import_lock_thread = me;
         import_lock_level--;
     } else {
@@ -1277,6 +1277,51 @@
     return 1;
 }
 
+const int
+the_byte_compiled_file_is_older(PyObject *filename)
+{
+    struct stat st;
+    FILE *fpc;
+    FILE *fp;
+    char buf[MAXPATHLEN+1];
+    char *cpathname;
+    char *pathname;
+    PyObject *filebytes;
+
+    filebytes = PyUnicode_EncodeFSDefault(filename);
+    if (filebytes == NULL) {
+        PyErr_Clear();
+        return 0;
+    }
+    pathname = PyBytes_AS_STRING(filebytes);
+    fp = fopen(pathname, "r");
+
+    if (fstat(fileno(fp), &st) != 0) {
+        PyErr_Format(PyExc_RuntimeError,
+                     "unable to get file status from '%s'",
+                     pathname);
+        return 0;
+    }
+#if SIZEOF_TIME_T > 4
+    /* Python's .pyc timestamp handling presumes that the timestamp fits
+       in 4 bytes. This will be fine until sometime in the year 2038,
+       when a 4-byte signed time_t will overflow.
+     */
+    if (st.st_mtime >> 32) {
+        PyErr_SetString(PyExc_OverflowError,
+            "modification time overflows a 4 byte field");
+        return 0;
+    }
+#endif
+    cpathname = make_compiled_pathname(
+        pathname, buf, (size_t)MAXPATHLEN + 1, !Py_OptimizeFlag);
+
+    if (cpathname != NULL &&
+        (fpc = check_compiled_module(pathname, st.st_mtime, cpathname)))
+            return 0;
+    return 1;
+}
+
 /* Load a source module from a given file and return its module
    object WITH INCREMENTED REFERENCE COUNT.  If there's a matching
    byte-compiled file, use that instead. */
diff -r d60c00015f01 Python/traceback.c
--- a/Python/traceback.c	Wed Oct 05 19:53:43 2011 +0200
+++ b/Python/traceback.c	Wed Oct 05 22:38:58 2011 -0300
@@ -311,7 +311,12 @@
     if (err == 0)
         err = PyFile_WriteObject(lineobj, f, Py_PRINT_RAW);
     Py_DECREF(lineobj);
-    if  (err == 0)
+
+    //if the pyc is older than the source file
+    if (the_byte_compiled_file_is_older(filename) == 1)
+        err = PyFile_WriteString("      WARNING: Modified after import!", f);
+
+    if (err == 0)
         err = PyFile_WriteString("\n", f);
     return err;
 }