Add proper warnings on use of the backwards compatibility shim.

IPython/frontend.py

@@ -1,11 +1,51 b''
+"""
+Shim to maintain backwards compatibility with old frontend imports.
+
+We have moved all contents of the old `frontend` subpackage into top-level
+subpackages (`html`, `qt` and `terminal`).  This will let code that was making
+`from IPython.frontend...` calls continue working, though a warning will be
+printed.
+"""
+
+#-----------------------------------------------------------------------------
+#  Copyright (c) 2013, IPython Development Team.
+#
+#  Distributed under the terms of the Modified BSD License.
+#
+#  The full license is in the file COPYING.txt, distributed with this software.
+#-----------------------------------------------------------------------------
+
+#-----------------------------------------------------------------------------
+# Imports
+#-----------------------------------------------------------------------------
+from __future__ import print_function
 import sys
 import types
 
+#-----------------------------------------------------------------------------
+# Class declarations
+#-----------------------------------------------------------------------------
 
 class ShimModule(types.ModuleType):
 
     def __getattribute__(self, key):
+        m = ("*** WARNING*** : The top-level `frontend` module has been deprecated.\n"
+        "Please import %s directly from the `IPython` level." % key)
+
+        # FIXME: I don't understand why, but if the print statement below is
+        # redirected to stderr, this shim module stops working.  It seems the
+        # Python import machinery has problem with redirected prints happening
+        # during the import process.  If we can't figure out a solution, we may
+        # need to leave it to print to default stdout.
+        print(m)
+        
+        # FIXME: this seems to work fine, but we should replace it with an
+        # __import__ call instead of using exec/eval.
         exec 'from IPython import %s' % key
         return eval(key)
 
+
+# Unconditionally insert the shim into sys.modules so that further import calls
+# trigger the custom attribute access above
+
 sys.modules['IPython.frontend'] = ShimModule('frontend')