path: root/lib/sqlalchemy/orm/attributes.py

# attributes.py - manages object attributes
# Copyright (C) 2005, 2006, 2007, 2008 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.util import EMPTY_SET
from sqlalchemy.orm import interfaces, collections, exc
import sqlalchemy.exceptions as sa_exc

# lazy imports
_entity_info = None
identity_equal = 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
resultion 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, impl, comparator=None, parententity=None):
        """Construct an InstrumentedAttribute.

          comparator
            a sql.Comparator to which class-level compare/math events will be sent
        """

        self.impl = impl
        self.comparator = comparator
        self.parententity = parententity

        if parententity:
            mapper, selectable, is_aliased_class = _entity_info(parententity, compile=False)
            self.property = mapper._get_property(self.impl.key)
        else:
            self.property = None

    def get_history(self, instance, **kwargs):
        return self.impl.get_history(instance_state(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

class InstrumentedAttribute(QueryableAttribute):
    """Public-facing descriptor, placed in the mapped class dictionary."""

    def __set__(self, instance, value):
        self.impl.set(instance_state(instance), value, None)

    def __delete__(self, instance):
        self.impl.delete(instance_state(instance))

    def __get__(self, instance, owner):
        if instance is None:
            return self
        return self.impl.get(instance_state(instance))

class _ProxyImpl(object):
    accepts_scalar_loader = 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)

        @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)
                    )

        def _property(self):
            return self._parententity.get_property(self.key, resolve_synonyms=True)
        property = property(_property)

    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_, class_manager, trackparent=False, extension=None,
                    compare_function=None, active_history=False, **kwargs):
        """Construct an AttributeImpl.

        \class_
          the class to be instrumented.

        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.

        """

        self.class_ = class_
        self.key = key
        self.callable_ = callable_
        self.class_manager = class_manager
        self.trackparent = trackparent
        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

    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), 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)] = 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, 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):
        """Initialize this attribute on the given object instance with an empty value."""

        state.dict[self.key] = None
        return None

    def get(self, state, 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 state.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, value)
                    else:
                        if self.key not in state.dict:
                            return self.get(state, passive=passive)
                        return state.dict[self.key]

            # Return a new, empty value
            return self.initialize(state)

    def append(self, state, value, initiator, passive=PASSIVE_OFF):
        self.set(state, value, initiator)

    def remove(self, state, value, initiator, passive=PASSIVE_OFF):
        self.set(state, None, initiator)

    def set(self, state, value, initiator):
        raise NotImplementedError()

    def get_committed_value(self, state, 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, passive=passive)

    def set_committed_value(self, state, value):
        """set an attribute value on the given instance and 'commit' it."""

        state.commit([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):

        # TODO: catch key errors, convert to attributeerror?
        if self.active_history or self.extensions:
            old = self.get(state)
        else:
            old = state.dict.get(self.key, NO_VALUE)

        state.modified_event(self, False, old)

        if self.extensions:
            self.fire_remove_event(state, old, None)
            del state.dict[self.key]
        else:
            del state.dict[self.key]

    def get_history(self, state, passive=PASSIVE_OFF):
        return History.from_attribute(
            self, state, state.dict.get(self.key, NO_VALUE))

    def set(self, state, value, initiator):
        if initiator is self:
            return

        if self.active_history or self.extensions:
            old = self.get(state)
        else:
            old = state.dict.get(self.key, NO_VALUE)

        state.modified_event(self, False, old)

        if self.extensions:
            value = self.fire_replace_event(state, value, old, initiator)
            state.dict[self.key] = value
        else:
            state.dict[self.key] = value

    def fire_replace_event(self, state, 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, 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_,
                                class_manager, 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, passive=PASSIVE_OFF):
        return History.from_attribute(
            self, state, state.dict.get(self.key, NO_VALUE))

    def commit_to_state(self, state, dest):
        dest[self.key] = self.copy(state.dict[self.key])

    def check_mutable_modified(self, state):
        (added, unchanged, deleted) = self.get_history(state, passive=PASSIVE_NO_INITIALIZE)
        return bool(added or deleted)

    def set(self, state, value, initiator):
        if initiator is self:
            return

        state.modified_event(self, True, NEVER_SET)

        if self.extensions:
            old = self.get(state)
            value = self.fire_replace_event(state, value, old, initiator)
            state.dict[self.key] = value
        else:
            state.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_, class_manager,
                    trackparent=False, extension=None, copy_function=None,
                    compare_function=None, **kwargs):
        super(ScalarObjectAttributeImpl, self).__init__(class_, key,
          callable_, class_manager, trackparent=trackparent, extension=extension,
          compare_function=compare_function, **kwargs)
        if compare_function is None:
            self.is_equal = identity_equal

    def delete(self, state):
        old = self.get(state)
        self.fire_remove_event(state, old, self)
        del state.dict[self.key]

    def get_history(self, state, passive=PASSIVE_OFF):
        if self.key in state.dict:
            return History.from_attribute(self, state, state.dict[self.key])
        else:
            current = self.get(state, passive=passive)
            if current is PASSIVE_NORESULT:
                return HISTORY_BLANK
            else:
                return History.from_attribute(self, state, current)

    def set(self, state, 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)
        value = self.fire_replace_event(state, value, old, initiator)
        state.dict[self.key] = value

    def fire_remove_event(self, state, value, initiator):
        state.modified_event(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, value, previous, initiator):
        state.modified_event(self, False, previous)

        if self.trackparent:
            if value is not None:
                self.sethasparent(instance_state(value), True)
            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)
        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_, class_manager,
                    typecallable=None, trackparent=False, extension=None,
                    copy_function=None, compare_function=None, **kwargs):
        super(CollectionAttributeImpl, self).__init__(class_,
          key, callable_, class_manager, 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, passive=PASSIVE_OFF):
        current = self.get(state, passive=passive)
        if current is PASSIVE_NORESULT:
            return HISTORY_BLANK
        else:
            return History.from_attribute(self, state, current)

    def fire_append_event(self, state, value, initiator):
        state.modified_event(self, True, NEVER_SET, passive=PASSIVE_NO_INITIALIZE)

        if self.trackparent and value is not None:
            self.sethasparent(instance_state(value), True)

        for ext in self.extensions:
            value = ext.append(state, value, initiator or self)
        return value

    def fire_pre_remove_event(self, state, initiator):
        state.modified_event(self, True, NEVER_SET, passive=PASSIVE_NO_INITIALIZE)

    def fire_remove_event(self, state, value, initiator):
        state.modified_event(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):
        if self.key not in state.dict:
            return

        state.modified_event(self, True, NEVER_SET)

        collection = self.get_collection(state)
        collection.clear_with_event()
        # TODO: catch key errors, convert to attributeerror?
        del state.dict[self.key]

    def initialize(self, state):
        """Initialize this attribute with an empty collection."""

        _, user_data = self._initialize_collection(state)
        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, value, initiator, passive=PASSIVE_OFF):
        if initiator is self:
            return

        collection = self.get_collection(state, passive=passive)
        if collection is PASSIVE_NORESULT:
            value = self.fire_append_event(state, value, initiator)
            state.get_pending(self.key).append(value)
        else:
            collection.append_with_event(value, initiator)

    def remove(self, state, value, initiator, passive=PASSIVE_OFF):
        if initiator is self:
            return

        collection = self.get_collection(state, passive=passive)
        if collection is PASSIVE_NORESULT:
            self.fire_remove_event(state, value, initiator)
            state.get_pending(self.key).remove(value)
        else:
            collection.remove_with_event(value, initiator)

    def set(self, state, 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, value,
            lambda adapter, i: adapter.adapt_like_to_iterable(i))

    def _set_iterable(self, state, 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)

        # 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(self, True, old)

        old_collection = self.get_collection(state, old)

        state.dict[self.key] = user_data

        collections.bulk_replace(new_values, old_collection, new_collection)
        old_collection.unlink(old)


    def set_committed_value(self, state, 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([self.key])

        if self.key in state.pending:
            # pending items exist.  issue a modified event,
            # add/remove new items.
            state.modified_event(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, 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, 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 = instance_state(oldchild)
            impl = old_state.get_impl(self.key)
            try:
                impl.remove(old_state, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
            except (ValueError, KeyError, IndexError):
                pass
        if child is not None:
            new_state = instance_state(child)
            new_state.get_impl(self.key).append(new_state, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
        return child

    def append(self, state, child, initiator):
        child_state = instance_state(child)
        child_state.get_impl(self.key).append(child_state, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)
        return child

    def remove(self, state, child, initiator):
        if child is not None:
            child_state = instance_state(child)
            child_state.get_impl(self.key).remove(child_state, state.obj(), initiator, passive=PASSIVE_NO_CALLABLES)


class InstanceState(object):
    """tracks state information at the instance level."""

    session_id = None
    key = None
    runid = None
    expired_attributes = EMPTY_SET
    insert_order = None
    
    def __init__(self, obj, manager):
        self.class_ = obj.__class__
        self.manager = manager
        self.obj = weakref.ref(obj, self._cleanup)
        self.dict = obj.__dict__
        self.modified = False
        self.callables = {}
        self.expired = False
        self.committed_state = {}
        self.pending = {}
        self.parents = {}
        
    def detach(self):
        if self.session_id:
            del self.session_id

    def dispose(self):
        if self.session_id:
            del self.session_id
        del self.obj
        del self.dict
    
    def _cleanup(self, ref):
        self.dispose()
    
    def obj(self):
        return None
    
    @util.memoized_property
    def dict(self):
        # return a blank dict
        # if none is available, so that asynchronous gc
        # doesn't blow up expiration operations in progress
        # (usually expire_attributes)
        return {}
    
    @property
    def sort_key(self):
        return self.key and self.key[1] or (self.insert_order, )

    def check_modified(self):
        if self.modified:
            return True
        else:
            for key in self.manager.mutable_attributes:
                if self.manager[key].impl.check_mutable_modified(self):
                    return True
            else:
                return False

    def initialize_instance(*mixed, **kwargs):
        self, instance, args = mixed[0], mixed[1], mixed[2:]
        manager = self.manager

        for fn in manager.events.on_init:
            fn(self, instance, args, kwargs)
        try:
            return manager.events.original_init(*mixed[1:], **kwargs)
        except:
            for fn in manager.events.on_init_failure:
                fn(self, instance, args, kwargs)
            raise

    def get_history(self, key, **kwargs):
        return self.manager.get_impl(key).get_history(self, **kwargs)

    def get_impl(self, key):
        return self.manager.get_impl(key)

    def get_pending(self, key):
        if key not in self.pending:
            self.pending[key] = PendingCollection()
        return self.pending[key]

    def value_as_iterable(self, key, passive=PASSIVE_OFF):
        """return an InstanceState attribute as a list,
        regardless of it being a scalar or collection-based
        attribute.

        returns None if passive is not PASSIVE_OFF and the getter returns
        PASSIVE_NORESULT.
        """

        impl = self.get_impl(key)
        x = impl.get(self, passive=passive)
        if x is PASSIVE_NORESULT:

            return None
        elif hasattr(impl, 'get_collection'):
            return impl.get_collection(self, x, passive=passive)
        elif isinstance(x, list):
            return x
        else:
            return [x]

    def _run_on_load(self, instance=None):
        if instance is None:
            instance = self.obj()
        self.manager.events.run('on_load', instance)

    def __getstate__(self):
        return {'key': self.key,
                'committed_state': self.committed_state,
                'pending': self.pending,
                'parents': self.parents,
                'modified': self.modified,
                'expired':self.expired,
                'instance': self.obj(),
                'expired_attributes':self.expired_attributes,
                'callables': self.callables}

    def __setstate__(self, state):
        self.committed_state = state['committed_state']
        self.parents = state['parents']
        self.key = state['key']
        self.session_id = None
        self.pending = state['pending']
        self.modified = state['modified']
        self.obj = weakref.ref(state['instance'])
        self.class_ = self.obj().__class__
        self.manager = manager_of_class(self.class_)
        self.dict = self.obj().__dict__
        self.callables = state['callables']
        self.runid = None
        self.expired = state['expired']
        self.expired_attributes = state['expired_attributes']

    def initialize(self, key):
        self.manager.get_impl(key).initialize(self)

    def set_callable(self, key, callable_):
        self.dict.pop(key, None)
        self.callables[key] = callable_

    def __call__(self):
        """__call__ allows the InstanceState to act as a deferred
        callable for loading expired attributes, which is also
        serializable (picklable).

        """
        unmodified = self.unmodified
        class_manager = self.manager
        class_manager.deferred_scalar_loader(self, [
            attr.impl.key for attr in class_manager.attributes if
                attr.impl.accepts_scalar_loader and
                attr.impl.key in self.expired_attributes and
                attr.impl.key in unmodified
            ])
        for k in self.expired_attributes:
            self.callables.pop(k, None)
        del self.expired_attributes
        return ATTR_WAS_SET

    @property
    def unmodified(self):
        """a set of keys which have no uncommitted changes"""

        return set(
            key for key in self.manager.iterkeys()
            if (key not in self.committed_state or
                (key in self.manager.mutable_attributes and
                 not self.manager[key].impl.check_mutable_modified(self))))

    @property
    def unloaded(self):
        """a set of keys which do not have a loaded value.

        This includes expired attributes and any other attribute that
        was never populated or modified.

        """
        return set(
            key for key in self.manager.iterkeys()
            if key not in self.committed_state and key not in self.dict)

    def expire_attributes(self, attribute_names):
        self.expired_attributes = set(self.expired_attributes)

        if attribute_names is None:
            attribute_names = self.manager.keys()
            self.expired = True
            self.modified = False
        for key in attribute_names:
            self.dict.pop(key, None)
            self.committed_state.pop(key, None)
            self.expired_attributes.add(key)
            if self.manager.get_impl(key).accepts_scalar_loader:
                self.callables[key] = self

    def reset(self, key):
        """remove the given attribute and any callables associated with it."""

        self.dict.pop(key, None)
        self.callables.pop(key, None)

    def modified_event(self, attr, should_copy, previous, passive=PASSIVE_OFF):
        needs_committed = attr.key not in self.committed_state

        if needs_committed:
            if previous is NEVER_SET:
                if passive:
                    if attr.key in self.dict:
                        previous = self.dict[attr.key]
                else:
                    previous = attr.get(self)

            if should_copy and previous not in (None, NO_VALUE, NEVER_SET):
                previous = attr.copy(previous)

            if needs_committed:
                self.committed_state[attr.key] = previous

        self.modified = True

    def commit(self, keys):
        """Commit attributes.

        This is used by a partial-attribute load operation to mark committed
        those attributes which were refreshed from the database.

        Attributes marked as "expired" can potentially remain "expired" after
        this step if a value was not populated in state.dict.

        """
        class_manager = self.manager
        for key in keys:
            if key in self.dict and key in class_manager.mutable_attributes:
                class_manager[key].impl.commit_to_state(self, self.committed_state)
            else:
                self.committed_state.pop(key, None)

        self.expired = False
        # unexpire attributes which have loaded
        for key in self.expired_attributes.intersection(keys):
            if key in self.dict:
                self.expired_attributes.remove(key)
                self.callables.pop(key, None)

    def commit_all(self):
        """commit all attributes unconditionally.

        This is used after a flush() or a full load/refresh
        to remove all pending state from the instance.

         - all attributes are marked as "committed"
         - the "strong dirty reference" is removed
         - the "modified" flag is set to False
         - any "expired" markers/callables are removed.

        Attributes marked as "expired" can potentially remain "expired" after this step
        if a value was not populated in state.dict.

        """
        
        self.committed_state = {}
        self.pending = {}
        
        # unexpire attributes which have loaded
        if self.expired_attributes:
            for key in self.expired_attributes.intersection(self.dict):
                self.callables.pop(key, None)
            self.expired_attributes.difference_update(self.dict)

        for key in self.manager.mutable_attributes:
            if key in self.dict:
                self.manager[key].impl.commit_to_state(self, self.committed_state)

        self.modified = self.expired = False
        self._strong_obj = None


class Events(object):
    def __init__(self):
        self.original_init = object.__init__
        self.on_init = ()
        self.on_init_failure = ()
        self.on_load = ()

    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
    instance_state_factory = InstanceState

    def __init__(self, class_):
        self.class_ = class_
        self.factory = None  # where we came from, for inheritance bookkeeping
        self.info = {}
        self.mapper = 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.registered = False
        self._instantiable = False
        self.events = self.event_registry_factory()

    def instantiable(self, boolean):
        # experiment, probably won't stay in this form
        assert boolean ^ self._instantiable, (boolean, self._instantiable)
        if boolean:
            self.events.original_init = self.class_.__init__
            new_init = _generate_init(self.class_, self)
            self.install_member('__init__', new_init)
        else:
            self.uninstall_member('__init__')
        self._instantiable = bool(boolean)
    instantiable = property(lambda s: s._instantiable, instantiable)

    def manage(self):
        """Mark this instance as the manager for its class."""
        setattr(self.class_, self.MANAGER_ATTR, self)

    def dispose(self):
        """Dissasociate this instance 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 = manager_of_class(cls)
            if manager is None:
                manager = create_manager_for_cls(cls)
            manager.instrument_attribute(key, inst, True)

    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 = manager_of_class(cls)
            if manager is None:
                manager = create_manager_for_cls(cls)
            manager.uninstrument_attribute(key, True)

    def unregister(self):
        for key in list(self):
            if key in self.local_attrs:
                self.uninstrument_attribute(key)
        self.registered = False

    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()

    @classmethod
    def deferred_scalar_loader(cls, state, keys):
        """Apply a scalar loader to the given state.
        
        Unimplemented by default, is patched
        by the mapper.
        
        """

    ## InstanceState management

    def new_instance(self, state=None):
        instance = self.class_.__new__(self.class_)
        setattr(instance, self.STATE_ATTR, state or self.instance_state_factory(instance, self))
        return instance

    def setup_instance(self, instance, state=None):
        setattr(instance, self.STATE_ATTR, state or self.instance_state_factory(instance, self))
        
    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.instance_state_factory(instance, self)
            setattr(instance, self.STATE_ATTR, state)
            return state
    
    def state_of(self, instance):
        return getattr(instance, self.STATE_ATTR)
        
    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 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):
        ClassManager.__init__(self, class_)
        self._adapted = override

    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 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.instance_state_factory(instance, self)
            
        # the given instance is assumed to have no state
        self._adapted.install_state(self.class_, instance, state)
        state.dict = self._adapted.get_instance_dict(self.class_, instance)
        return state

    def state_of(self, instance):
        if hasattr(self._adapted, 'state_of'):
            return self._adapted.state_of(self.class_, instance)
        else:
            getter = self._adapted.state_getter(self.class_)
            return getter(instance)

    def has_state(self, instance):
        if hasattr(self._adapted, 'has_state'):
            return self._adapted.has_state(self.class_, instance)
        else:
            try:
                state = self.state_of(instance)
                return True
            except exc.NO_STATE:
                return False

    def state_getter(self):
        return self._adapted.state_getter(self.class_)


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, 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)

class PendingCollection(object):
    """A writable placeholder for an unloaded collection.

    Stores items appended to and removed from a collection that has not yet
    been loaded. When the collection is loaded, the changes stored in
    PendingCollection are applied to it to produce the final result.

    """
    def __init__(self):
        self.deleted_items = util.IdentitySet()
        self.added_items = util.OrderedIdentitySet()

    def append(self, value):
        if value in self.deleted_items:
            self.deleted_items.remove(value)
        self.added_items.add(value)

    def remove(self, value):
        if value in self.added_items:
            self.added_items.remove(value)
        self.deleted_items.add(value)


def get_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_):
    """TODO"""

    # TODO: what's this function for ?  why would I call this and not
    # create_manager_for_cls ?

    manager = manager_of_class(class_)
    if manager is None:
        manager = create_manager_for_cls(class_)
    if not manager.instantiable:
        manager.instantiable = True

def unregister_class(class_):
    """TODO"""
    manager = manager_of_class(class_)
    assert manager
    assert manager.instantiable
    manager.instantiable = False
    manager.unregister()

def register_attribute(class_, key, uselist, useobject,
                        callable_=None, proxy_property=None,
                        mutable_scalars=False, impl_class=None, **kwargs):
    manager = manager_of_class(class_)
    if manager.is_instrumented(key):
        return

    if uselist:
        factory = kwargs.pop('typecallable', None)
        typecallable = manager.instrument_collection_class(
            key, factory or list)
    else:
        typecallable = kwargs.pop('typecallable', None)

    comparator = kwargs.pop('comparator', None)
    parententity = kwargs.pop('parententity', None)

    if proxy_property:
        proxy_type = proxied_attribute_factory(proxy_property)
        descriptor = proxy_type(key, proxy_property, comparator, parententity)
    else:
        descriptor = InstrumentedAttribute(
            _create_prop(class_, key, uselist, callable_,
                    class_manager=manager,
                    useobject=useobject,
                    typecallable=typecallable,
                    mutable_scalars=mutable_scalars,
                    impl_class=impl_class,
                    **kwargs),
                comparator=comparator, parententity=parententity)

    manager.instrument_attribute(key, descriptor)

def unregister_attribute(class_, key):
    manager_of_class(class_).uninstrument_attribute(key)

def init_collection(state, key):
    """Initialize a collection attribute and return the collection adapter."""
    attr = state.get_impl(key)
    user_data = attr.initialize(state)
    return attr.get_collection(state, user_data)

def set_attribute(instance, key, value):
    state = instance_state(instance)
    state.get_impl(key).set(state, value, None)

def get_attribute(instance, key):
    state = instance_state(instance)
    return state.get_impl(key).get(state)

def del_attribute(instance, key):
    state = instance_state(instance)
    state.get_impl(key).delete(state)

def is_instrumented(instance, key):
    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()
    extended = False

    def create_manager_for_cls(self, class_):
        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 = collect_management_factories_for(class_)
        existing_factories.add(factory)
        if len(existing_factories) > 1:
            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.factory = factory
        manager.manage()
        self.manager_finders[class_] = manager.manager_getter()
        self.state_finders[class_] = manager.state_getter()
        return manager

    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):
        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 state_or_default(self, instance, default=None):
        if instance is None:
            return default
        try:
            finder = self.state_finders[instance.__class__]
        except KeyError:
            return default
        else:
            try:
                return finder(instance)
            except exc.NO_STATE:
                return default
            except:
                raise

    def unregister(self, class_):
        if class_ in self.manager_finders:
            manager = self.manager_of_class(class_)
            manager.dispose()
            del self.manager_finders[class_]
            del self.state_finders[class_]

# Create a registry singleton and prepare placeholders for lookup functions.

instrumentation_registry = InstrumentationRegistry()
create_manager_for_cls = None
manager_of_class = None
instance_state = None
_lookup_strategy = None

def _install_lookup_strategy(implementation):
    """Switch between native and extended instrumentation modes.

    Completely private.  Use the instrumentation_finders interface to
    inject global instrumentation behavior.

    """
    global manager_of_class, instance_state, create_manager_for_cls
    global _lookup_strategy

    # Using a symbol here to make debugging a little friendlier.
    if implementation is not util.symbol('native'):
        manager_of_class = implementation.manager_of_class
        instance_state = implementation.state_of
        create_manager_for_cls = implementation.create_manager_for_cls
    else:
        def manager_of_class(class_):
            return getattr(class_, ClassManager.MANAGER_ATTR, None)
        manager_of_class = instrumentation_registry.manager_of_class
        instance_state = attrgetter(ClassManager.STATE_ATTR)
        create_manager_for_cls = instrumentation_registry.create_manager_for_cls
    # TODO: maybe log an event when setting a strategy.
    _lookup_strategy = implementation

_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 collect_management_factories_for(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 _create_prop(class_, key, uselist, callable_, class_manager, typecallable, useobject, mutable_scalars, impl_class, **kwargs):
    if impl_class:
        return impl_class(class_, key, typecallable, class_manager=class_manager, **kwargs)
    elif uselist:
        return CollectionAttributeImpl(class_, key, callable_,
                                       typecallable=typecallable,
                                       class_manager=class_manager, **kwargs)
    elif useobject:
        return ScalarObjectAttributeImpl(class_, key, callable_,
                                         class_manager=class_manager, **kwargs)
    elif mutable_scalars:
        return MutableScalarAttributeImpl(class_, key, callable_,
                                          class_manager=class_manager, **kwargs)
    else:
        return ScalarAttributeImpl(class_, key, callable_,
                                   class_manager=class_manager, **kwargs)

def _generate_init(class_, class_manager):
    """Build an __init__ decorator that triggers ClassManager events."""

    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
    #TODO: log debug #print func_text

    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__