text: # attributes.py - manages object attributes
  # Copyright (C) 2005, 2006, 2007, 2008, 2009 Michael Bayer mike_mp@zzzcomputing.com
  #
  # This module is part of SQLAlchemy and is released under
  # the MIT License: http://www.opensource.org/licenses/mit-license.php
  """Defines SQLAlchemy's system of class instrumentation..
  
  This module is usually not directly visible to user applications, but
  defines a large part of the ORM's interactivity.
  
  SQLA's instrumentation system is completely customizable, in which
  case an understanding of the general mechanics of this module is helpful.
  An example of full customization is in /examples/custom_attributes.
  
  """
  
  import operator
  from operator import attrgetter, itemgetter
  import types
  import weakref
  
  from sqlalchemy import util
  from sqlalchemy.orm import interfaces, collections, exc
  import sqlalchemy.exceptions as sa_exc
  
  # lazy imports
  _entity_info = None
  identity_equal = None
  state = None
  
  PASSIVE_NORESULT = util.symbol('PASSIVE_NORESULT')
  ATTR_WAS_SET = util.symbol('ATTR_WAS_SET')
  NO_VALUE = util.symbol('NO_VALUE')
  NEVER_SET = util.symbol('NEVER_SET')
  
  # "passive" get settings
  # TODO: the True/False values need to be factored out
  # of the rest of ORM code
  # don't fire off any callables, and don't initialize the attribute to
  # an empty value
  PASSIVE_NO_INITIALIZE = True #util.symbol('PASSIVE_NO_INITIALIZE')
  
  # don't fire off any callables, but if no callables present
  # then initialize to an empty value/collection
  # this is used by backrefs.
  PASSIVE_NO_CALLABLES = util.symbol('PASSIVE_NO_CALLABLES')
  
  # fire callables/initialize as needed
  PASSIVE_OFF = False #util.symbol('PASSIVE_OFF')
  
  INSTRUMENTATION_MANAGER = '__sa_instrumentation_manager__'
  """Attribute, elects custom instrumentation when present on a mapped class.
  
  Allows a class to specify a slightly or wildly different technique for
  tracking changes made to mapped attributes and collections.
  
  Only one instrumentation implementation is allowed in a given object
  inheritance hierarchy.
  
  The value of this attribute must be a callable and will be passed a class
  object.  The callable must return one of:
  
    - An instance of an interfaces.InstrumentationManager or subclass
    - An object implementing all or some of InstrumentationManager (TODO)
    - A dictionary of callables, implementing all or some of the above (TODO)
    - An instance of a ClassManager or subclass
  
  interfaces.InstrumentationManager is public API and will remain stable
  between releases.  ClassManager is not public and no guarantees are made
  about stability.  Caveat emptor.
  
  This attribute is consulted by the default SQLAlchemy instrumentation
  resolution code.  If custom finders are installed in the global
  instrumentation_finders list, they may or may not choose to honor this
  attribute.
  
  """
  
  instrumentation_finders = []
  """An extensible sequence of instrumentation implementation finding callables.
  
  Finders callables will be passed a class object.  If None is returned, the
  next finder in the sequence is consulted.  Otherwise the return must be an
  instrumentation factory that follows the same guidelines as
  INSTRUMENTATION_MANAGER.
  
  By default, the only finder is find_native_user_instrumentation_hook, which
  searches for INSTRUMENTATION_MANAGER.  If all finders return None, standard
  ClassManager instrumentation is used.
  
  """
  
  class QueryableAttribute(interfaces.PropComparator):
  
      def __init__(self, key, impl=None, comparator=None, parententity=None):
          """Construct an InstrumentedAttribute.
  
            comparator
              a sql.Comparator to which class-level compare/math events will be sent
          """
          self.key = key
          self.impl = impl
          self.comparator = comparator
          self.parententity = parententity
  
      def get_history(self, instance, **kwargs):
          return self.impl.get_history(instance_state(instance), instance_dict(instance), **kwargs)
  
      def __selectable__(self):
          # TODO: conditionally attach this method based on clause_element ?
          return self
  
      def __clause_element__(self):
          return self.comparator.__clause_element__()
  
      def label(self, name):
          return self.__clause_element__().label(name)
  
      def operate(self, op, *other, **kwargs):
          return op(self.comparator, *other, **kwargs)
  
      def reverse_operate(self, op, other, **kwargs):
          return op(other, self.comparator, **kwargs)
  
      def hasparent(self, state, optimistic=False):
          return self.impl.hasparent(state, optimistic=optimistic)
      
      def __getattr__(self, key):
          try:
              return getattr(self.comparator, key)
          except AttributeError:
              raise AttributeError('Neither %r object nor %r object has an attribute %r' % (
                      type(self).__name__, 
                      type(self.comparator).__name__, 
                      key)
              )
          
      def __str__(self):
          return repr(self.parententity) + "." + self.property.key
  
      @property
      def property(self):
          return self.comparator.property
  
  
  class InstrumentedAttribute(QueryableAttribute):
      """Public-facing descriptor, placed in the mapped class dictionary."""
  
      def __set__(self, instance, value):
          self.impl.set(instance_state(instance), instance_dict(instance), value, None)
  
      def __delete__(self, instance):
          self.impl.delete(instance_state(instance), instance_dict(instance))
  
      def __get__(self, instance, owner):
          if instance is None:
              return self
          return self.impl.get(instance_state(instance), instance_dict(instance))
  
  class _ProxyImpl(object):
      accepts_scalar_loader = False
      dont_expire_missing = False
      
      def __init__(self, key):
          self.key = key
  
  def proxied_attribute_factory(descriptor):
      """Create an InstrumentedAttribute / user descriptor hybrid.
  
      Returns a new InstrumentedAttribute type that delegates descriptor
      behavior and getattr() to the given descriptor.
      """
  
      class Proxy(InstrumentedAttribute):
          """A combination of InsturmentedAttribute and a regular descriptor."""
  
          def __init__(self, key, descriptor, comparator, parententity):
              self.key = key
              # maintain ProxiedAttribute.user_prop compatability.
              self.descriptor = self.user_prop = descriptor
              self._comparator = comparator
              self._parententity = parententity
              self.impl = _ProxyImpl(key)
  
          @util.memoized_property
          def comparator(self):
              if util.callable(self._comparator):
                  self._comparator = self._comparator()
              return self._comparator
  
          def __get__(self, instance, owner):
              """Delegate __get__ to the original descriptor."""
              if instance is None:
                  descriptor.__get__(instance, owner)
                  return self
              return descriptor.__get__(instance, owner)
  
          def __set__(self, instance, value):
              """Delegate __set__ to the original descriptor."""
              return descriptor.__set__(instance, value)
  
          def __delete__(self, instance):
              """Delegate __delete__ to the original descriptor."""
              return descriptor.__delete__(instance)
  
          def __getattr__(self, attribute):
              """Delegate __getattr__ to the original descriptor and/or comparator."""
              
              try:
                  return getattr(descriptor, attribute)
              except AttributeError:
                  try:
                      return getattr(self._comparator, attribute)
                  except AttributeError:
                      raise AttributeError('Neither %r object nor %r object has an attribute %r' % (
                              type(descriptor).__name__, 
                              type(self._comparator).__name__, 
                              attribute)
                      )
  
      Proxy.__name__ = type(descriptor).__name__ + 'Proxy'
  
      util.monkeypatch_proxied_specials(Proxy, type(descriptor),
                                        name='descriptor',
                                        from_instance=descriptor)
      return Proxy
  
  class AttributeImpl(object):
      """internal implementation for instrumented attributes."""
  
      def __init__(self, class_, key,
                      callable_, trackparent=False, extension=None,
                      compare_function=None, active_history=False, parent_token=None, 
                      dont_expire_missing=False,
                      **kwargs):
          """Construct an AttributeImpl.
  
          \class_
            associated class
            
          key
            string name of the attribute
  
          \callable_
            optional function which generates a callable based on a parent
            instance, which produces the "default" values for a scalar or
            collection attribute when it's first accessed, if not present
            already.
  
          trackparent
            if True, attempt to track if an instance has a parent attached
            to it via this attribute.
  
          extension
            a single or list of AttributeExtension object(s) which will
            receive set/delete/append/remove/etc. events.
  
          compare_function
            a function that compares two values which are normally
            assignable to this attribute.
  
          active_history
            indicates that get_history() should always return the "old" value,
            even if it means executing a lazy callable upon attribute change.
            This flag is set to True if any extensions are present.
  
          parent_token
            Usually references the MapperProperty, used as a key for
            the hasparent() function to identify an "owning" attribute.
            Allows multiple AttributeImpls to all match a single 
            owner attribute.
            
          dont_expire_missing
            if True, don't add an "expiry" callable to this attribute
            during state.expire_attributes(None), if no value is present 
            for this key.
            
          """
          self.class_ = class_
          self.key = key
          self.callable_ = callable_
          self.trackparent = trackparent
          self.parent_token = parent_token or self
          if compare_function is None:
              self.is_equal = operator.eq
          else:
              self.is_equal = compare_function
          self.extensions = util.to_list(extension or [])
          self.active_history = active_history
          self.dont_expire_missing = dont_expire_missing
          
      def hasparent(self, state, optimistic=False):
          """Return the boolean value of a `hasparent` flag attached to the given item.
  
          The `optimistic` flag determines what the default return value
          should be if no `hasparent` flag can be located.
  
          As this function is used to determine if an instance is an
          *orphan*, instances that were loaded from storage should be
          assumed to not be orphans, until a True/False value for this
          flag is set.
  
          An instance attribute that is loaded by a callable function
          will also not have a `hasparent` flag.
  
          """
          return state.parents.get(id(self.parent_token), optimistic)
  
      def sethasparent(self, state, value):
          """Set a boolean flag on the given item corresponding to
          whether or not it is attached to a parent object via the
          attribute represented by this ``InstrumentedAttribute``.
  
          """
          state.parents[id(self.parent_token)] = value
  
      def set_callable(self, state, callable_):
          """Set a callable function for this attribute on the given object.
  
          This callable will be executed when the attribute is next
          accessed, and is assumed to construct part of the instances
          previously stored state. When its value or values are loaded,
          they will be established as part of the instance's *committed
          state*.  While *trackparent* information will be assembled for
          these instances, attribute-level event handlers will not be
          fired.
  
          The callable overrides the class level callable set in the
          ``InstrumentedAttribute`` constructor.
  
          """
          if callable_ is None:
              self.initialize(state)
          else:
              state.callables[self.key] = callable_
  
      def get_history(self, state, dict_, passive=PASSIVE_OFF):
          raise NotImplementedError()
  
      def _get_callable(self, state):
          if self.key in state.callables:
              return state.callables[self.key]
          elif self.callable_ is not None:
              return self.callable_(state)
          else:
              return None
  
      def initialize(self, state, dict_):
          """Initialize this attribute on the given object instance with an empty value."""
  
          dict_[self.key] = None
          return None
  
      def get(self, state, dict_, passive=PASSIVE_OFF):
          """Retrieve a value from the given object.
  
          If a callable is assembled on this object's attribute, and
          passive is False, the callable will be executed and the
          resulting value will be set as the new value for this attribute.
          """
  
          try:
              return dict_[self.key]
          except KeyError:
              # if no history, check for lazy callables, etc.
              if state.committed_state.get(self.key, NEVER_SET) is NEVER_SET:
                  if passive is PASSIVE_NO_INITIALIZE:
                      return PASSIVE_NORESULT
                      
                  callable_ = self._get_callable(state)
                  if callable_ is not None:
                      if passive is not PASSIVE_OFF:
                          return PASSIVE_NORESULT
                      value = callable_()
                      if value is not ATTR_WAS_SET:
                          return self.set_committed_value(state, dict_, value)
                      else:
                          if self.key not in dict_:
                              return self.get(state, dict_, passive=passive)
                          return dict_[self.key]
  
              # Return a new, empty value
              return self.initialize(state, dict_)
  
      def append(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
          self.set(state, dict_, value, initiator)
  
      def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
          self.set(state, dict_, None, initiator)
  
      def set(self, state, dict_, value, initiator):
          raise NotImplementedError()
  
      def get_committed_value(self, state, dict_, passive=PASSIVE_OFF):
          """return the unchanged value of this attribute"""
  
          if self.key in state.committed_state:
              if state.committed_state[self.key] is NO_VALUE:
                  return None
              else:
                  return state.committed_state.get(self.key)
          else:
              return self.get(state, dict_, passive=passive)
  
      def set_committed_value(self, state, dict_, value):
          """set an attribute value on the given instance and 'commit' it."""
  
          state.commit(dict_, [self.key])
  
          state.callables.pop(self.key, None)
          state.dict[self.key] = value
  
          return value
  
  class ScalarAttributeImpl(AttributeImpl):
      """represents a scalar value-holding InstrumentedAttribute."""
  
      accepts_scalar_loader = True
      uses_objects = False
  
      def delete(self, state, dict_):
  
          # TODO: catch key errors, convert to attributeerror?
          if self.active_history or self.extensions:
              old = self.get(state, dict_)
          else:
              old = dict_.get(self.key, NO_VALUE)
  
          state.modified_event(dict_, self, False, old)
  
          if self.extensions:
              self.fire_remove_event(state, dict_, old, None)
          del dict_[self.key]
  
      def get_history(self, state, dict_, passive=PASSIVE_OFF):
          return History.from_attribute(
              self, state, dict_.get(self.key, NO_VALUE))
  
      def set(self, state, dict_, value, initiator):
          if initiator is self:
              return
  
          if self.active_history or self.extensions:
              old = self.get(state, dict_)
          else:
              old = dict_.get(self.key, NO_VALUE)
  
          state.modified_event(dict_, self, False, old)
  
          if self.extensions:
              value = self.fire_replace_event(state, dict_, value, old, initiator)
          dict_[self.key] = value
  
      def fire_replace_event(self, state, dict_, value, previous, initiator):
          for ext in self.extensions:
              value = ext.set(state, value, previous, initiator or self)
          return value
  
      def fire_remove_event(self, state, dict_, value, initiator):
          for ext in self.extensions:
              ext.remove(state, value, initiator or self)
  
      @property
      def type(self):
          self.property.columns[0].type
  
  
  class MutableScalarAttributeImpl(ScalarAttributeImpl):
      """represents a scalar value-holding InstrumentedAttribute, which can detect
      changes within the value itself.
      """
  
      uses_objects = False
  
      def __init__(self, class_, key, callable_,
                      class_manager, copy_function=None,
                      compare_function=None, **kwargs):
          super(ScalarAttributeImpl, self).__init__(class_, key, callable_,
                                  compare_function=compare_function, **kwargs)
          class_manager.mutable_attributes.add(key)
          if copy_function is None:
              raise sa_exc.ArgumentError("MutableScalarAttributeImpl requires a copy function")
          self.copy = copy_function
  
      def get_history(self, state, dict_, passive=PASSIVE_OFF):
          if not dict_:
              v = state.committed_state.get(self.key, NO_VALUE)
          else:
              v = dict_.get(self.key, NO_VALUE)
              
          return History.from_attribute(
              self, state, v)
  
      def commit_to_state(self, state, dict_, dest):
          dest[self.key] = self.copy(dict_[self.key])
  
      def check_mutable_modified(self, state, dict_):
          (added, unchanged, deleted) = self.get_history(state, dict_, passive=PASSIVE_NO_INITIALIZE)
          return bool(added or deleted)
  
      def get(self, state, dict_, passive=PASSIVE_OFF):
          if self.key not in state.mutable_dict:
              ret = ScalarAttributeImpl.get(self, state, dict_, passive=passive)
              if ret is not PASSIVE_NORESULT:
                  state.mutable_dict[self.key] = ret
              return ret
          else:
              return state.mutable_dict[self.key]
  
      def delete(self, state, dict_):
          ScalarAttributeImpl.delete(self, state, dict_)
          state.mutable_dict.pop(self.key)
  
      def set(self, state, dict_, value, initiator):
          if initiator is self:
              return
  
          state.modified_event(dict_, self, True, NEVER_SET)
          
          if self.extensions:
              old = self.get(state, dict_)
              value = self.fire_replace_event(state, dict_, value, old, initiator)
              dict_[self.key] = value
          else:
              dict_[self.key] = value
          state.mutable_dict[self.key] = value
  
  
  class ScalarObjectAttributeImpl(ScalarAttributeImpl):
      """represents a scalar-holding InstrumentedAttribute, where the target object is also instrumented.
  
      Adds events to delete/set operations.
      """
  
      accepts_scalar_loader = False
      uses_objects = True
  
      def __init__(self, class_, key, callable_, 
                      trackparent=False, extension=None, copy_function=None,
                      compare_function=None, **kwargs):
          super(ScalarObjectAttributeImpl, self).__init__(class_, key,
            callable_, trackparent=trackparent, extension=extension,
            compare_function=compare_function, **kwargs)
          if compare_function is None:
              self.is_equal = identity_equal
  
      def delete(self, state, dict_):
          old = self.get(state, dict_)
          self.fire_remove_event(state, dict_, old, self)
          del dict_[self.key]
  
      def get_history(self, state, dict_, passive=PASSIVE_OFF):
          if self.key in dict_:
              return History.from_attribute(self, state, dict_[self.key])
          else:
              current = self.get(state, dict_, passive=passive)
              if current is PASSIVE_NORESULT:
                  return HISTORY_BLANK
              else:
                  return History.from_attribute(self, state, current)
  
      def set(self, state, dict_, value, initiator):
          """Set a value on the given InstanceState.
  
          `initiator` is the ``InstrumentedAttribute`` that initiated the
          ``set()`` operation and is used to control the depth of a circular
          setter operation.
  
          """
          if initiator is self:
              return
  
          # may want to add options to allow the get() here to be passive
          old = self.get(state, dict_)
          value = self.fire_replace_event(state, dict_, value, old, initiator)
          dict_[self.key] = value
  
      def fire_remove_event(self, state, dict_, value, initiator):
          state.modified_event(dict_, self, False, value)
  
          if self.trackparent and value is not None:
              self.sethasparent(instance_state(value), False)
  
          for ext in self.extensions:
              ext.remove(state, value, initiator or self)
  
      def fire_replace_event(self, state, dict_, value, previous, initiator):
          state.modified_event(dict_, self, False, previous)
  
          if self.trackparent:
              if previous is not value and previous is not None:
                  self.sethasparent(instance_state(previous), False)
  
          for ext in self.extensions:
              value = ext.set(state, value, previous, initiator or self)
  
          if self.trackparent:
              if value is not None:
                  self.sethasparent(instance_state(value), True)
  
          return value
  
  
  class CollectionAttributeImpl(AttributeImpl):
      """A collection-holding attribute that instruments changes in membership.
  
      Only handles collections of instrumented objects.
  
      InstrumentedCollectionAttribute holds an arbitrary, user-specified
      container object (defaulting to a list) and brokers access to the
      CollectionAdapter, a "view" onto that object that presents consistent
      bag semantics to the orm layer independent of the user data implementation.
  
      """
      accepts_scalar_loader = False
      uses_objects = True
  
      def __init__(self, class_, key, callable_, 
                      typecallable=None, trackparent=False, extension=None,
                      copy_function=None, compare_function=None, **kwargs):
          super(CollectionAttributeImpl, self).__init__(class_, key, callable_, trackparent=trackparent,
            extension=extension, compare_function=compare_function, **kwargs)
  
          if copy_function is None:
              copy_function = self.__copy
          self.copy = copy_function
  
          self.collection_factory = typecallable
          # may be removed in 0.5:
          self.collection_interface = \
            util.duck_type_collection(self.collection_factory())
  
      def __copy(self, item):
          return [y for y in list(collections.collection_adapter(item))]
  
      def get_history(self, state, dict_, passive=PASSIVE_OFF):
          current = self.get(state, dict_, passive=passive)
          if current is PASSIVE_NORESULT:
              return HISTORY_BLANK
          else:
              return History.from_attribute(self, state, current)
  
      def fire_append_event(self, state, dict_, value, initiator):
          state.modified_event(dict_, self, True, NEVER_SET, passive=PASSIVE_NO_INITIALIZE)
  
          for ext in self.extensions:
              value = ext.append(state, value, initiator or self)
  
          if self.trackparent and value is not None:
              self.sethasparent(instance_state(value), True)
  
          return value
  
      def fire_pre_remove_event(self, state, dict_, initiator):
          state.modified_event(dict_, self, True, NEVER_SET, passive=PASSIVE_NO_INITIALIZE)
  
      def fire_remove_event(self, state, dict_, value, initiator):
          state.modified_event(dict_, self, True, NEVER_SET, passive=PASSIVE_NO_INITIALIZE)
  
          if self.trackparent and value is not None:
              self.sethasparent(instance_state(value), False)
  
          for ext in self.extensions:
              ext.remove(state, value, initiator or self)
  
      def delete(self, state, dict_):
          if self.key not in dict_:
              return
  
          state.modified_event(dict_, self, True, NEVER_SET)
  
          collection = self.get_collection(state, state.dict)
          collection.clear_with_event()
          # TODO: catch key errors, convert to attributeerror?
          del dict_[self.key]
  
      def initialize(self, state, dict_):
          """Initialize this attribute with an empty collection."""
  
          _, user_data = self._initialize_collection(state)
          dict_[self.key] = user_data
          return user_data
  
      def _initialize_collection(self, state):
          return state.manager.initialize_collection(
              self.key, state, self.collection_factory)
  
      def append(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
          if initiator is self:
              return
  
          collection = self.get_collection(state, dict_, passive=passive)
          if collection is PASSIVE_NORESULT:
              value = self.fire_append_event(state, dict_, value, initiator)
              state.get_pending(self.key).append(value)
          else:
              collection.append_with_event(value, initiator)
  
      def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
          if initiator is self:
              return
  
          collection = self.get_collection(state, state.dict, passive=passive)
          if collection is PASSIVE_NORESULT:
              self.fire_remove_event(state, dict_, value, initiator)
              state.get_pending(self.key).remove(value)
          else:
              collection.remove_with_event(value, initiator)
  
      def set(self, state, dict_, value, initiator):
          """Set a value on the given object.
  
          `initiator` is the ``InstrumentedAttribute`` that initiated the
          ``set()`` operation and is used to control the depth of a circular
          setter operation.
          """
  
          if initiator is self:
              return
  
          self._set_iterable(
              state, dict_, value,
              lambda adapter, i: adapter.adapt_like_to_iterable(i))
  
      def _set_iterable(self, state, dict_, iterable, adapter=None):
          """Set a collection value from an iterable of state-bearers.
  
          ``adapter`` is an optional callable invoked with a CollectionAdapter
          and the iterable.  Should return an iterable of state-bearing
          instances suitable for appending via a CollectionAdapter.  Can be used
          for, e.g., adapting an incoming dictionary into an iterator of values
          rather than keys.
  
          """
          # pulling a new collection first so that an adaptation exception does
          # not trigger a lazy load of the old collection.
          new_collection, user_data = self._initialize_collection(state)
          if adapter:
              new_values = list(adapter(new_collection, iterable))
          else:
              new_values = list(iterable)
  
          old = self.get(state, dict_)
  
          # ignore re-assignment of the current collection, as happens
          # implicitly with in-place operators (foo.collection |= other)
          if old is iterable:
              return
  
          state.modified_event(dict_, self, True, old)
  
          old_collection = self.get_collection(state, dict_, old)
  
          dict_[self.key] = user_data
  
          collections.bulk_replace(new_values, old_collection, new_collection)
          old_collection.unlink(old)
  
  
      def set_committed_value(self, state, dict_, value):
          """Set an attribute value on the given instance and 'commit' it."""
  
          collection, user_data = self._initialize_collection(state)
  
          if value:
              for item in value:
                  collection.append_without_event(item)
  
          state.callables.pop(self.key, None)
          state.dict[self.key] = user_data
  
          state.commit(dict_, [self.key])
  
          if self.key in state.pending:
              
              # pending items exist.  issue a modified event,
              # add/remove new items.
              state.modified_event(dict_, self, True, user_data)
  
              pending = state.pending.pop(self.key)
              added = pending.added_items
              removed = pending.deleted_items
              for item in added:
                  collection.append_without_event(item)
              for item in removed:
                  collection.remove_without_event(item)
  
          return user_data
  
      def get_collection(self, state, dict_, user_data=None, passive=PASSIVE_OFF):
          """Retrieve the CollectionAdapter associated with the given state.
  
          Creates a new CollectionAdapter if one does not exist.
  
          """
          if user_data is None:
              user_data = self.get(state, dict_, passive=passive)
              if user_data is PASSIVE_NORESULT:
                  return user_data
  
          return getattr(user_data, '_sa_adapter')
  
  class GenericBackrefExtension(interfaces.AttributeExtension):
      """An extension which synchronizes a two-way relationship.
  
      A typical two-way relationship is a parent object containing a list of
      child objects, where each child object references the parent.  The other
      are two objects which contain scalar references to each other.
  
      """
      def __init__(self, key):
          self.key = key
  
      def set(self, state, child, oldchild, initiator):
          if oldchild is child:
              return child
          if oldchild is not None:
              # With lazy=None, there's no guarantee that the full collection is
              # present when updating via a backref.
              old_state, old_dict = instance_state(oldchild), instance_dict(oldchild)
              impl = old_state.get_impl(self.key)
              try:
                  impl.remove(old_state, old_dict, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
              except (ValueError, KeyError, IndexError):
                  pass
          if child is not None:
              new_state,  new_dict = instance_state(child), instance_dict(child)
              new_state.get_impl(self.key).append(new_state, new_dict, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
          return child
  
      def append(self, state, child, initiator):
          child_state, child_dict = instance_state(child), instance_dict(child)
          child_state.get_impl(self.key).append(child_state, child_dict, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
          return child
  
      def remove(self, state, child, initiator):
          if child is not None:
              child_state, child_dict = instance_state(child), instance_dict(child)
              child_state.get_impl(self.key).remove(child_state, child_dict, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
  
  
  class Events(object):
      def __init__(self):
          self.original_init = object.__init__
          self.on_init = ()
          self.on_init_failure = ()
          self.on_load = ()
          self.on_resurrect = ()
  
      def run(self, event, *args, **kwargs):
          for fn in getattr(self, event):
              fn(*args, **kwargs)
  
      def add_listener(self, event, listener):
          # not thread safe... problem?  mb: nope
          bucket = getattr(self, event)
          if bucket == ():
              setattr(self, event, [listener])
          else:
              bucket.append(listener)
  
      def remove_listener(self, event, listener):
          bucket = getattr(self, event)
          bucket.remove(listener)
  
  
  class ClassManager(dict):
      """tracks state information at the class level."""
  
      MANAGER_ATTR = '_sa_class_manager'
      STATE_ATTR = '_sa_instance_state'
  
      event_registry_factory = Events
      deferred_scalar_loader = None
      
      def __init__(self, class_):
          self.class_ = class_
          self.factory = None  # where we came from, for inheritance bookkeeping
          self.info = {}
          self.mapper = None
          self.new_init = None
          self.mutable_attributes = set()
          self.local_attrs = {}
          self.originals = {}
          for base in class_.__mro__[-2:0:-1]:   # reverse, skipping 1st and last
              if not isinstance(base, type):
                  continue
              cls_state = manager_of_class(base)
              if cls_state:
                  self.update(cls_state)
          self.events = self.event_registry_factory()
          self.manage()
          self._instrument_init()
      
      def _configure_create_arguments(self, 
                              _source=None, 
                              deferred_scalar_loader=None):
          """Accept extra **kw arguments passed to create_manager_for_cls.
          
          The current contract of ClassManager and other managers is that they
          take a single "cls" argument in their constructor (as per 
          test/orm/instrumentation.py InstrumentationCollisionTest).  This
          is to provide consistency with the current API of "class manager"
          callables and such which may return various ClassManager and 
          ClassManager-like instances.   So create_manager_for_cls sends
          in ClassManager-specific arguments via this method once the 
          non-proxied ClassManager is available.
          
          """
          if _source:
              deferred_scalar_loader = _source.deferred_scalar_loader
  
          if deferred_scalar_loader:
              self.deferred_scalar_loader = deferred_scalar_loader
      
      def _subclass_manager(self, cls):
          """Create a new ClassManager for a subclass of this ClassManager's class.
          
          This is called automatically when attributes are instrumented so that
          the attributes can be propagated to subclasses against their own
          class-local manager, without the need for mappers etc. to have already
          pre-configured managers for the full class hierarchy.   Mappers
          can post-configure the auto-generated ClassManager when needed.
          
          """
          manager = manager_of_class(cls)
          if manager is None:
              manager = _create_manager_for_cls(cls, _source=self)
          return manager
          
      def _instrument_init(self):
          # TODO: self.class_.__init__ is often the already-instrumented
          # __init__ from an instrumented superclass.  We still need to make 
          # our own wrapper, but it would
          # be nice to wrap the original __init__ and not our existing wrapper
          # of such, since this adds method overhead.
          self.events.original_init = self.class_.__init__
          self.new_init = _generate_init(self.class_, self)
          self.install_member('__init__', self.new_init)
          
      def _uninstrument_init(self):
          if self.new_init:
              self.uninstall_member('__init__')
              self.new_init = None
      
      def _create_instance_state(self, instance):
          global state
          if state is None:
              from sqlalchemy.orm import state
          if self.mutable_attributes:
              return state.MutableAttrInstanceState(instance, self)
          else:
              return state.InstanceState(instance, self)
          
      def manage(self):
          """Mark this instance as the manager for its class."""
          
          setattr(self.class_, self.MANAGER_ATTR, self)
  
      def dispose(self):
          """Dissasociate this manager from its class."""
          
          delattr(self.class_, self.MANAGER_ATTR)
  
      def manager_getter(self):
          return attrgetter(self.MANAGER_ATTR)
  
      def instrument_attribute(self, key, inst, propagated=False):
          if propagated:
              if key in self.local_attrs:
                  return  # don't override local attr with inherited attr
          else:
              self.local_attrs[key] = inst
              self.install_descriptor(key, inst)
          self[key] = inst
          for cls in self.class_.__subclasses__():
              if isinstance(cls, types.ClassType):
                  continue
              manager = self._subclass_manager(cls)
              manager.instrument_attribute(key, inst, True)
  
      def post_configure_attribute(self, key):
          pass
          
      def uninstrument_attribute(self, key, propagated=False):
          if key not in self:
              return
          if propagated:
              if key in self.local_attrs:
                  return  # don't get rid of local attr
          else:
              del self.local_attrs[key]
              self.uninstall_descriptor(key)
          del self[key]
          if key in self.mutable_attributes:
              self.mutable_attributes.remove(key)
          for cls in self.class_.__subclasses__():
              if isinstance(cls, types.ClassType):
                  continue
              manager = self._subclass_manager(cls)
              manager.uninstrument_attribute(key, True)
  
      def unregister(self):
          """remove all instrumentation established by this ClassManager."""
          
          self._uninstrument_init()
  
          self.mapper = self.events = None
          self.info.clear()
          
          for key in list(self):
              if key in self.local_attrs:
                  self.uninstrument_attribute(key)
  
      def install_descriptor(self, key, inst):
          if key in (self.STATE_ATTR, self.MANAGER_ATTR):
              raise KeyError("%r: requested attribute name conflicts with "
                             "instrumentation attribute of the same name." % key)
          setattr(self.class_, key, inst)
  
      def uninstall_descriptor(self, key):
          delattr(self.class_, key)
  
      def install_member(self, key, implementation):
          if key in (self.STATE_ATTR, self.MANAGER_ATTR):
              raise KeyError("%r: requested attribute name conflicts with "
                             "instrumentation attribute of the same name." % key)
          self.originals.setdefault(key, getattr(self.class_, key, None))
          setattr(self.class_, key, implementation)
  
      def uninstall_member(self, key):
          original = self.originals.pop(key, None)
          if original is not None:
              setattr(self.class_, key, original)
  
      def instrument_collection_class(self, key, collection_class):
          return collections.prepare_instrumentation(collection_class)
  
      def initialize_collection(self, key, state, factory):
          user_data = factory()
          adapter = collections.CollectionAdapter(
              self.get_impl(key), state, user_data)
          return adapter, user_data
  
      def is_instrumented(self, key, search=False):
          if search:
              return key in self
          else:
              return key in self.local_attrs
  
      def get_impl(self, key):
          return self[key].impl
  
      @property
      def attributes(self):
          return self.itervalues()
  
      ## InstanceState management
  
      def new_instance(self, state=None):
          instance = self.class_.__new__(self.class_)
          setattr(instance, self.STATE_ATTR, state or self._create_instance_state(instance))
          return instance
  
      def setup_instance(self, instance, state=None):
          setattr(instance, self.STATE_ATTR, state or self._create_instance_state(instance))
      
      def teardown_instance(self, instance):
          delattr(instance, self.STATE_ATTR)
          
      def _new_state_if_none(self, instance):
          """Install a default InstanceState if none is present.
  
          A private convenience method used by the __init__ decorator.
          
          """
          if hasattr(instance, self.STATE_ATTR):
              return False
          else:
              state = self._create_instance_state(instance)
              setattr(instance, self.STATE_ATTR, state)
              return state
      
      def state_getter(self):
          """Return a (instance) -> InstanceState callable.
  
          "state getter" callables should raise either KeyError or
          AttributeError if no InstanceState could be found for the
          instance.
          """
  
          return attrgetter(self.STATE_ATTR)
      
      def dict_getter(self):
          return attrgetter('__dict__')
          
      def has_state(self, instance):
          return hasattr(instance, self.STATE_ATTR)
          
      def has_parent(self, state, key, optimistic=False):
          """TODO"""
          return self.get_impl(key).hasparent(state, optimistic=optimistic)
  
      def __nonzero__(self):
          """All ClassManagers are non-zero regardless of attribute state."""
          return True
  
      def __repr__(self):
          return '<%s of %r at %x>' % (
              self.__class__.__name__, self.class_, id(self))
  
  class _ClassInstrumentationAdapter(ClassManager):
      """Adapts a user-defined InstrumentationManager to a ClassManager."""
  
      def __init__(self, class_, override, **kw):
          self._adapted = override
          self._get_state = self._adapted.state_getter(class_)
          self._get_dict = self._adapted.dict_getter(class_)
          
          ClassManager.__init__(self, class_, **kw)
  
      def manage(self):
          self._adapted.manage(self.class_, self)
  
      def dispose(self):
          self._adapted.dispose(self.class_)
  
      def manager_getter(self):
          return self._adapted.manager_getter(self.class_)
  
      def instrument_attribute(self, key, inst, propagated=False):
          ClassManager.instrument_attribute(self, key, inst, propagated)
          if not propagated:
              self._adapted.instrument_attribute(self.class_, key, inst)
  
      def post_configure_attribute(self, key):
          self._adapted.post_configure_attribute(self.class_, key, self[key])
  
      def install_descriptor(self, key, inst):
          self._adapted.install_descriptor(self.class_, key, inst)
  
      def uninstall_descriptor(self, key):
          self._adapted.uninstall_descriptor(self.class_, key)
  
      def install_member(self, key, implementation):
          self._adapted.install_member(self.class_, key, implementation)
  
      def uninstall_member(self, key):
          self._adapted.uninstall_member(self.class_, key)
  
      def instrument_collection_class(self, key, collection_class):
          return self._adapted.instrument_collection_class(
              self.class_, key, collection_class)
  
      def initialize_collection(self, key, state, factory):
          delegate = getattr(self._adapted, 'initialize_collection', None)
          if delegate:
              return delegate(key, state, factory)
          else:
              return ClassManager.initialize_collection(self, key, state, factory)
  
      def new_instance(self, state=None):
          instance = self.class_.__new__(self.class_)
          self.setup_instance(instance, state)
          return instance
  
      def _new_state_if_none(self, instance):
          """Install a default InstanceState if none is present.
  
          A private convenience method used by the __init__ decorator.
          """
          if self.has_state(instance):
              return False
          else:
              return self.setup_instance(instance)
  
      def setup_instance(self, instance, state=None):
          self._adapted.initialize_instance_dict(self.class_, instance)
          
          if state is None:
              state = self._create_instance_state(instance)
              
          # the given instance is assumed to have no state
          self._adapted.install_state(self.class_, instance, state)
          return state
  
      def teardown_instance(self, instance):
          self._adapted.remove_state(self.class_, instance)
  
      def has_state(self, instance):
          try:
              state = self._get_state(instance)
              return True
          except exc.NO_STATE:
              return False
  
      def state_getter(self):
          return self._get_state
  
      def dict_getter(self):
          return self._get_dict
  
  class History(tuple):
      """A 3-tuple of added, unchanged and deleted values.
  
      Each tuple member is an iterable sequence.
  
      """
  
      __slots__ = ()
  
      added = property(itemgetter(0))
      unchanged = property(itemgetter(1))
      deleted = property(itemgetter(2))
  
      def __new__(cls, added, unchanged, deleted):
          return tuple.__new__(cls, (added, unchanged, deleted))
      
      def __nonzero__(self):
          return self != HISTORY_BLANK
      
      def sum(self):
          return self.added + self.unchanged + self.deleted
      
      def non_deleted(self):
          return self.added + self.unchanged
      
      def non_added(self):
          return self.unchanged + self.deleted
      
      def has_changes(self):
          return bool(self.added or self.deleted)
          
      def as_state(self):
          return History(
              [c is not None and instance_state(c) or None for c in self.added],
              [c is not None and instance_state(c) or None for c in self.unchanged],
              [c is not None and instance_state(c) or None for c in self.deleted],
          )
      
      @classmethod
      def from_attribute(cls, attribute, state, current):
          original = state.committed_state.get(attribute.key, NEVER_SET)
  
          if hasattr(attribute, 'get_collection'):
              current = attribute.get_collection(state, state.dict, current)
              if original is NO_VALUE:
                  return cls(list(current), (), ())
              elif original is NEVER_SET:
                  return cls((), list(current), ())
              else:
                  current_set = util.IdentitySet(current)
                  original_set = util.IdentitySet(original)
  
                  # ensure duplicates are maintained
                  return cls(
                      [x for x in current if x not in original_set],
                      [x for x in current if x in original_set],
                      [x for x in original if x not in current_set]
                  )
          else:
              if current is NO_VALUE:
                  if original not in [None, NEVER_SET, NO_VALUE]:
                      deleted = [original]
                  else:
                      deleted = ()
                  return cls((), (), deleted)
              elif original is NO_VALUE:
                  return cls([current], (), ())
              elif (original is NEVER_SET or
                    attribute.is_equal(current, original) is True):
                  # dont let ClauseElement expressions here trip things up
                  return cls((), [current], ())
              else:
                  if original is not None:
                      deleted = [original]
                  else:
                      deleted = ()
                  return cls([current], (), deleted)
  
  HISTORY_BLANK = History(None, None, None)
  
  def _conditional_instance_state(obj):
      if not isinstance(obj, state.InstanceState):
          obj = instance_state(obj)
      return obj
          
  def get_history(obj, key, **kwargs):
      """Return a History record for the given object and attribute key.
      
      obj is an instrumented object instance.  An InstanceState
      is accepted directly for backwards compatibility but 
      this usage is deprecated.
      
      """
      return get_state_history(_conditional_instance_state(obj), key, **kwargs)
  
  def get_state_history(state, key, **kwargs):
      return state.get_history(key, **kwargs)
  
  def has_parent(cls, obj, key, optimistic=False):
      """TODO"""
      manager = manager_of_class(cls)
      state = instance_state(obj)
      return manager.has_parent(state, key, optimistic)
  
  def register_class(class_, **kw):
      """Register class instrumentation.
      
      Returns the existing or newly created class manager.
      """
  
      manager = manager_of_class(class_)
      if manager is None:
          manager = _create_manager_for_cls(class_, **kw)
      return manager
      
  def unregister_class(class_):
      """Unregister class instrumentation."""
      
      instrumentation_registry.unregister(class_)
  
  def register_attribute(class_, key, **kw):
  
      proxy_property = kw.pop('proxy_property', None)
      
      comparator = kw.pop('comparator', None)
      parententity = kw.pop('parententity', None)
      register_descriptor(class_, key, proxy_property, comparator, parententity)
      if not proxy_property:
          register_attribute_impl(class_, key, **kw)
      
  def register_attribute_impl(class_, key,         
          uselist=False, callable_=None, 
          useobject=False, mutable_scalars=False, 
          impl_class=None, **kw):
      
      manager = manager_of_class(class_)
      if uselist:
          factory = kw.pop('typecallable', None)
          typecallable = manager.instrument_collection_class(
              key, factory or list)
      else:
          typecallable = kw.pop('typecallable', None)
  
      if impl_class:
          impl = impl_class(class_, key, typecallable, **kw)
      elif uselist:
          impl = CollectionAttributeImpl(class_, key, callable_,
                                         typecallable=typecallable, **kw)
      elif useobject:
          impl = ScalarObjectAttributeImpl(class_, key, callable_, **kw)
      elif mutable_scalars:
          impl = MutableScalarAttributeImpl(class_, key, callable_,
                                            class_manager=manager, **kw)
      else:
          impl = ScalarAttributeImpl(class_, key, callable_, **kw)
  
      manager[key].impl = impl
      
      manager.post_configure_attribute(key)
      
  def register_descriptor(class_, key, proxy_property=None, comparator=None, parententity=None, property_=None):
      manager = manager_of_class(class_)
  
      if proxy_property:
          proxy_type = proxied_attribute_factory(proxy_property)
          descriptor = proxy_type(key, proxy_property, comparator, parententity)
      else:
          descriptor = InstrumentedAttribute(key, comparator=comparator, parententity=parententity)
  
      manager.instrument_attribute(key, descriptor)
  
  def unregister_attribute(class_, key):
      manager_of_class(class_).uninstrument_attribute(key)
  
  def init_collection(obj, key):
      """Initialize a collection attribute and return the collection adapter.
      
      This function is used to provide direct access to collection internals
      for a previously unloaded attribute.  e.g.::
          
          collection_adapter = init_collection(someobject, 'elements')
          for elem in values:
              collection_adapter.append_without_event(elem)
      
      For an easier way to do the above, see :func:`~sqlalchemy.orm.attributes.set_committed_value`.
      
      obj is an instrumented object instance.  An InstanceState
      is accepted directly for backwards compatibility but 
      this usage is deprecated.
      
      """
      state = _conditional_instance_state(obj)
      dict_ = state.dict
      return init_state_collection(state, dict_, key)
      
  def init_state_collection(state, dict_, key):
      """Initialize a collection attribute and return the collection adapter."""
      
      attr = state.get_impl(key)
      user_data = attr.initialize(state, dict_)
      return attr.get_collection(state, dict_, user_data)
  
  def set_committed_value(instance, key, value):
      """Set the value of an attribute with no history events.
      
      Cancels any previous history present.  The value should be 
      a scalar value for scalar-holding attributes, or
      an iterable for any collection-holding attribute.
  
      This is the same underlying method used when a lazy loader
      fires off and loads additional data from the database.
      In particular, this method can be used by application code
      which has loaded additional attributes or collections through
      separate queries, which can then be attached to an instance
      as though it were part of its original loaded state.
      
      """
      state, dict_ = instance_state(instance), instance_dict(instance)
      state.get_impl(key).set_committed_value(state, dict_, value)
      
  def set_attribute(instance, key, value):
      """Set the value of an attribute, firing history events.
      
      This function may be used regardless of instrumentation
      applied directly to the class, i.e. no descriptors are required.
      Custom attribute management schemes will need to make usage
      of this method to establish attribute state as understood
      by SQLAlchemy.
      
      """
      state, dict_ = instance_state(instance), instance_dict(instance)
      state.get_impl(key).set(state, dict_, value, None)
  
  def get_attribute(instance, key):
      """Get the value of an attribute, firing any callables required.
  
      This function may be used regardless of instrumentation
      applied directly to the class, i.e. no descriptors are required.
      Custom attribute management schemes will need to make usage
      of this method to make usage of attribute state as understood
      by SQLAlchemy.
      
      """
      state, dict_ = instance_state(instance), instance_dict(instance)
      return state.get_impl(key).get(state, dict_)
  
  def del_attribute(instance, key):
      """Delete the value of an attribute, firing history events.
  
      This function may be used regardless of instrumentation
      applied directly to the class, i.e. no descriptors are required.
      Custom attribute management schemes will need to make usage
      of this method to establish attribute state as understood
      by SQLAlchemy.
      
      """
      state, dict_ = instance_state(instance), instance_dict(instance)
      state.get_impl(key).delete(state, dict_)
  
  def is_instrumented(instance, key):
      """Return True if the given attribute on the given instance is instrumented
      by the attributes package.
      
      This function may be used regardless of instrumentation
      applied directly to the class, i.e. no descriptors are required.
      
      """
      return manager_of_class(instance.__class__).is_instrumented(key, search=True)
  
  class InstrumentationRegistry(object):
      """Private instrumentation registration singleton."""
  
      _manager_finders = weakref.WeakKeyDictionary()
      _state_finders = util.WeakIdentityMapping()
      _dict_finders = util.WeakIdentityMapping()
      _extended = False
  
      def create_manager_for_cls(self, class_, **kw):
          assert class_ is not None
          assert manager_of_class(class_) is None
  
          for finder in instrumentation_finders:
              factory = finder(class_)
              if factory is not None:
                  break
          else:
              factory = ClassManager
  
          existing_factories = self._collect_management_factories_for(class_).\
                                  difference([factory])
          if existing_factories:
              raise TypeError(
                  "multiple instrumentation implementations specified "
                  "in %s inheritance hierarchy: %r" % (
                      class_.__name__, list(existing_factories)))
  
          manager = factory(class_)
          if not isinstance(manager, ClassManager):
              manager = _ClassInstrumentationAdapter(class_, manager)
              
          if factory != ClassManager and not self._extended:
              self._extended = True
              _install_lookup_strategy(self)
          
          manager._configure_create_arguments(**kw)
  
          manager.factory = factory
          self._manager_finders[class_] = manager.manager_getter()
          self._state_finders[class_] = manager.state_getter()
          self._dict_finders[class_] = manager.dict_getter()
          return manager
  
      def _collect_management_factories_for(self, cls):
          """Return a collection of factories in play or specified for a hierarchy.
  
          Traverses the entire inheritance graph of a cls and returns a collection
          of instrumentation factories for those classes.  Factories are extracted
          from active ClassManagers, if available, otherwise
          instrumentation_finders is consulted.
  
          """
          hierarchy = util.class_hierarchy(cls)
          factories = set()
          for member in hierarchy:
              manager = manager_of_class(member)
              if manager is not None:
                  factories.add(manager.factory)
              else:
                  for finder in instrumentation_finders:
                      factory = finder(member)
                      if factory is not None:
                          break
                  else:
                      factory = None
                  factories.add(factory)
          factories.discard(None)
          return factories
  
      def manager_of_class(self, cls):
          if cls is None:
              return None
          try:
              finder = self._manager_finders[cls]
          except KeyError:
              return None
          else:
              return finder(cls)
  
      def state_of(self, instance):
          # this is only called when alternate instrumentation has been established
          if instance is None:
              raise AttributeError("None has no persistent state.")
          try:
              return self._state_finders[instance.__class__](instance)
          except KeyError:
              raise AttributeError("%r is not instrumented" % instance.__class__)
  
      def dict_of(self, instance):
          # this is only called when alternate instrumentation has been established
          if instance is None:
              raise AttributeError("None has no persistent state.")
          try:
              return self._dict_finders[instance.__class__](instance)
          except KeyError:
              raise AttributeError("%r is not instrumented" % instance.__class__)
          
      def unregister(self, class_):
          if class_ in self._manager_finders:
              manager = self.manager_of_class(class_)
              manager.unregister()
              manager.dispose()
              del self._manager_finders[class_]
              del self._state_finders[class_]
              del self._dict_finders[class_]
  
  instrumentation_registry = InstrumentationRegistry()
  
  def _install_lookup_strategy(implementation):
      """Replace global class/object management functions
      with either faster or more comprehensive implementations,
      based on whether or not extended class instrumentation
      has been detected.
      
      This function is called only by InstrumentationRegistry()
      and unit tests specific to this behavior.
      
      """
      global instance_state, instance_dict
      if implementation is util.symbol('native'):
          instance_state = attrgetter(ClassManager.STATE_ATTR)
          instance_dict = attrgetter("__dict__")
      else:
          instance_state = instrumentation_registry.state_of
          instance_dict = instrumentation_registry.dict_of
          
  manager_of_class = instrumentation_registry.manager_of_class
  _create_manager_for_cls = instrumentation_registry.create_manager_for_cls
  _install_lookup_strategy(util.symbol('native'))
  
  def find_native_user_instrumentation_hook(cls):
      """Find user-specified instrumentation management for a class."""
      return getattr(cls, INSTRUMENTATION_MANAGER, None)
  instrumentation_finders.append(find_native_user_instrumentation_hook)
  
  def _generate_init(class_, class_manager):
      """Build an __init__ decorator that triggers ClassManager events."""
  
      # TODO: we should use the ClassManager's notion of the 
      # original '__init__' method, once ClassManager is fixed
      # to always reference that.
      original__init__ = class_.__init__
      assert original__init__
  
      # Go through some effort here and don't change the user's __init__
      # calling signature.
      # FIXME: need to juggle local names to avoid constructor argument
      # clashes.
      func_body = """\
  def __init__(%(apply_pos)s):
      new_state = class_manager._new_state_if_none(%(self_arg)s)
      if new_state:
          return new_state.initialize_instance(%(apply_kw)s)
      else:
          return original__init__(%(apply_kw)s)
  """
      func_vars = util.format_argspec_init(original__init__, grouped=False)
      func_text = func_body % func_vars
  
      func = getattr(original__init__, 'im_func', original__init__)
      func_defaults = getattr(func, 'func_defaults', None)
  
      env = locals().copy()
      exec func_text in env
      __init__ = env['__init__']
      __init__.__doc__ = original__init__.__doc__
      if func_defaults:
          __init__.func_defaults = func_defaults
      return __init__
  