Index: Lib/configparser.py
===================================================================
--- Lib/configparser.py	(revision 88301)
+++ Lib/configparser.py	(working copy)
@@ -119,7 +119,7 @@
         between keys and values are surrounded by spaces.
 """
 
-from collections import MutableMapping, OrderedDict as _default_dict
+from collections import Mapping, MutableMapping, OrderedDict as _default_dict
 import functools
 import io
 import itertools
@@ -550,7 +550,39 @@
         else:
             return "%%(%s)s" % parser.optionxform(s)
 
+class _Chainmap(Mapping):
+    """Combine multiple mappings for successive lookups.
 
+    For example, to emulate Python's normal lookup sequence:
+
+        import __builtin__
+        pylookup = _Chainmap(locals(), globals(), vars(__builtin__))
+    """
+
+    def __init__(self, *maps):
+        self.maps = maps
+
+    def __getitem__(self, key):
+        for mapping in self.maps:
+            try:
+                return mapping[key]
+            except KeyError:
+                pass
+        raise KeyError(key)
+
+    def __iter__(self):
+        seen = set()
+        for mapping in self.maps:
+            s = set(mapping) - seen
+            for elem in s:
+                yield elem
+            seen.update(s)
+
+    def __len__(self):
+        s = set()
+        s.update(*self.maps)
+        return len(s)
+
 class RawConfigParser(MutableMapping):
     """ConfigParser that does not do interpolation."""
 
@@ -1099,23 +1131,24 @@
         return exc
 
     def _unify_values(self, section, vars):
-        """Create a copy of the DEFAULTSECT with values from a specific
-        `section' and the `vars' dictionary. If provided, values in `vars'
-        take precendence.
+        """Create a sequence of lookups with 'vars' taking priority over
+        the 'section' which takes priority over the DEFAULTSECT.
+
         """
-        d = self._defaults.copy()
+        sectiondict = {}
         try:
-            d.update(self._sections[section])
+            sectiondict = self._sections[section]
         except KeyError:
             if section != self.default_section:
                 raise NoSectionError(section)
         # Update with the entry specific variables
+        vardict = {}
         if vars:
             for key, value in vars.items():
                 if value is not None:
                     value = str(value)
-                d[self.optionxform(key)] = value
-        return d
+                vardict[self.optionxform(key)] = value
+        return _Chainmap(vardict, sectiondict, self._defaults)
 
     def _convert_to_boolean(self, value):
         """Return a boolean value translating from other types if necessary.