diff -r 1bf721567067 Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py	Fri Mar 01 21:28:23 2013 +0200
+++ b/Lib/importlib/_bootstrap.py	Fri Mar 01 16:57:55 2013 -0500
@@ -1344,13 +1344,12 @@
         self._loaders = loaders
         # Base (directory) path
         self.path = path or '.'
-        self._path_mtime = -1
-        self._path_cache = set()
-        self._relaxed_path_cache = set()
+        self.invalidate_caches()
 
     def invalidate_caches(self):
         """Invalidate the directory mtime."""
-        self._path_mtime = -1
+        self._path_cache = None
+        self._relaxed_path_cache = None
 
     find_module = _find_module_shim
 
@@ -1359,13 +1358,8 @@
         package portions. Returns (loader, list-of-portions)."""
         is_namespace = False
         tail_module = fullname.rpartition('.')[2]
-        try:
-            mtime = _os.stat(self.path).st_mtime
-        except OSError:
-            mtime = -1
-        if mtime != self._path_mtime:
+        if self._path_cache is None:
             self._fill_cache()
-            self._path_mtime = mtime
         # tail_module keeps the original casing, for __file__ and friends
         if _relax_case():
             cache = self._relaxed_path_cache