Index: Lib/string.py
===================================================================
--- Lib/string.py	(revision 51012)
+++ Lib/string.py	(working copy)
@@ -81,23 +81,6 @@
 ####################################################################
 import re as _re
 
-class _multimap:
-    """Helper class for combining multiple mappings.
-
-    Used by .{safe_,}substitute() to combine the mapping and keyword
-    arguments.
-    """
-    def __init__(self, primary, secondary):
-        self._primary = primary
-        self._secondary = secondary
-
-    def __getitem__(self, key):
-        try:
-            return self._primary[key]
-        except KeyError:
-            return self._secondary[key]
-
-
 class _TemplateMetaclass(type):
     pattern = r"""
     %(delim)s(?:
@@ -150,7 +133,8 @@
         if not args:
             mapping = kws
         elif kws:
-            mapping = _multimap(kws, args[0])
+            mapping = args[0]
+            mapping.update(kws)
         else:
             mapping = args[0]
         # Helper function for .sub()
@@ -176,7 +160,8 @@
         if not args:
             mapping = kws
         elif kws:
-            mapping = _multimap(kws, args[0])
+            mapping = args[0]
+            mapping.update(kws)
         else:
             mapping = args[0]
         # Helper function for .sub()
Index: Lib/test/test_pep292.py
===================================================================
--- Lib/test/test_pep292.py	(revision 51012)
+++ Lib/test/test_pep292.py	(working copy)
@@ -2,6 +2,7 @@
 # Author: barry@python.org (Barry Warsaw)
 # License: http://www.opensource.org/licenses/PythonSoftFoundation.php
 
+import UserDict
 import unittest
 from string import Template
 
@@ -183,7 +184,27 @@
         raises(ValueError, s.substitute, dict(gift='bud', who='you'))
         eq(s.safe_substitute(), 'this &gift is for &{who} &')
 
+    def test_mapping_updated_with_kws(self):
 
+        class LowerCasedMapping(UserDict.UserDict):
+            """A mapping in which all keys are lower-cased.
+            """
+            def __setitem__(self, name, val):
+                self.data[name.lower()] = val
+            def __getitem__(self, name):
+                return self.data[name.lower()]
+            def update(self, dct):
+                for k, v in dct.items():
+                    self[k] = v
+
+        eq = self.assertEqual
+        raises = self.assertRaises
+        s = Template('Greetings, $program!')
+        d = LowerCasedMapping(program='perl')
+        eq(s.substitute(d, PROGRAM='python'), 'Greetings, python!')
+        eq(s.safe_substitute(d, PROGRAM='python'), 'Greetings, python!')
+
+
 def test_main():
     from test import test_support
     test_classes = [TestTemplate,]