diff -r b6d6f3b4b100 Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py	Tue Dec 17 22:17:26 2013 +1000
+++ b/Lib/importlib/_bootstrap.py	Tue Dec 17 23:26:11 2013 +1000
@@ -718,13 +718,22 @@
     def __init__(self, module):
         self._module = module
         self._spec = module.__spec__
+        parent_name, __, submodule_name = module_name.rpartition('.')
+        if submodule:
+            self._parent_module = sys.modules[parent_name]
+            self._submodle_name = submodule_name
+        else:
+            self._parent_module = self._submodule_name = None
 
     def __enter__(self):
         # This must be done before putting the module in sys.modules
         # (otherwise an optimization shortcut in import.c becomes
         # wrong)
         self._spec._initializing = True
-        sys.modules[self._spec.name] = self._module
+        module_name = self._spec.name
+        sys.modules[module_name] = self._module
+        if self._parent_module:
+            setattr(self._parent_module, self._submodule_name, self._module)
 
     def __exit__(self, *args):
         try:
@@ -734,6 +743,11 @@
                     del sys.modules[spec.name]
                 except KeyError:
                     pass
+                if self._parent_module:
+                    try:
+                        delattr(self._parent_module, self._submodule_name)
+                    except AttributeError:
+                        pass
             else:
                 _verbose_message('import {!r} # {!r}', spec.name, spec.loader)
         finally:
@@ -2148,9 +2162,11 @@
     else:
         module = _SpecMethods(spec)._load_unlocked()
     if parent:
-        # Set the module as an attribute on its parent.
+        # Ensure the module is an attribute on its parent.
         parent_module = sys.modules[parent]
-        setattr(parent_module, name.rpartition('.')[2], module)
+        submodule_name = name.rpartition('.')[2]
+        if getattr(parent_module, submodule_name, None) is not module:
+            setattr(parent_module, submodule_name, module)
     return module