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Here's an example of a little typed environment. It's not
the most robust, but it gets you thinking.

import code
class TypedDictionary(dict):
 def __setitem__(self, key, value):
   if self.has_key(key):
     t = type(self[key])
     if t != type(value):
       try:
         value = t(value)
       except Exception:
         raise TypeError, \
             "illegal assignment to '%s':" \
             " %s cannot be coerced to %s" \
               % (key, type(value), t)
   dict.__setitem__(self, key, value)
code.interact(local=TypedDictionary())