I've got a class:

class odict(dict):

   def __init__(self, d={}):
      self._keys = d.keys()
      dict.__init__(self, d)

   def __setitem__(self, key, item):
        dict.__setitem__(self, key, item)
        if key not in self._keys:
            self._keys.append(key)

   ...

When I copy.deepcopy one of these it blows up in
__setitem__ with an AttributeError on _keys, because
__setitem__ is called without __init__ ever having been
called.  Presumably this thing looks and smells like a
dictionary, so deepcopy thinks it is one.