Index: Lib/test/support.py
===================================================================
--- Lib/test/support.py	(revision 71863)
+++ Lib/test/support.py	(working copy)
@@ -516,31 +516,33 @@
     a context manager."""
 
     def __init__(self):
-        self._environ = os.environ
-        self._unset = set()
-        self._reset = dict()
+        self._changed = {}
 
     def set(self, envvar, value):
-        if envvar not in self._environ:
-            self._unset.add(envvar)
-        else:
-            self._reset[envvar] = self._environ[envvar]
-        self._environ[envvar] = value
+        if envvar not in self._changed:
+            self._changed[envvar] = os.environ.get(envvar)
+        os.environ[envvar] = value
 
     def unset(self, envvar):
-        if envvar in self._environ:
-            self._reset[envvar] = self._environ[envvar]
-            del self._environ[envvar]
+        if envvar not in self._changed:
+            self._changed[envvar] = os.environ.get(envvar)
+        del os.environ[envvar]
 
+    def clear(self):
+        for envvar in os.environ:
+           self.unset(envvar)
+
     def __enter__(self):
         return self
 
     def __exit__(self, *ignore_exc):
-        for envvar, value in self._reset.items():
-            self._environ[envvar] = value
-        for unset in self._unset:
-            del self._environ[unset]
+        for (k, v) in self._changed.items():
+            if v is None:
+                del os.environ[k]
+            else:
+                os.environ[k] = v
 
+
 class TransientResource(object):
 
     """Raise ResourceDenied if an exception is raised while the context manager