# orm/attributes.py
# Copyright (C) 2005-2014 the SQLAlchemy authors and contributors <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""Defines instrumentation for class attributes and their interaction
with instances.

This module is usually not directly visible to user applications, but
defines a large part of the ORM's interactivity.


"""

import operator
from .. import util, event, inspection
from . import interfaces, collections, exc as orm_exc

from .base import instance_state, instance_dict, manager_of_class

from .base import PASSIVE_NO_RESULT, ATTR_WAS_SET, ATTR_EMPTY, NO_VALUE,\
            NEVER_SET, NO_CHANGE, CALLABLES_OK, SQL_OK, RELATED_OBJECT_OK,\
            INIT_OK, NON_PERSISTENT_OK, LOAD_AGAINST_COMMITTED, PASSIVE_OFF,\
            PASSIVE_RETURN_NEVER_SET, PASSIVE_NO_INITIALIZE, PASSIVE_NO_FETCH,\
            PASSIVE_NO_FETCH_RELATED, PASSIVE_ONLY_PERSISTENT, NO_AUTOFLUSH
from .base import state_str, instance_str

@inspection._self_inspects
class QueryableAttribute(interfaces._MappedAttribute,
                            interfaces._InspectionAttr,
                            interfaces.PropComparator):
    """Base class for :term:`descriptor` objects that intercept
    attribute events on behalf of a :class:`.MapperProperty`
    object.  The actual :class:`.MapperProperty` is accessible
    via the :attr:`.QueryableAttribute.property`
    attribute.


    .. seealso::

        :class:`.InstrumentedAttribute`

        :class:`.MapperProperty`

        :attr:`.Mapper.all_orm_descriptors`

        :attr:`.Mapper.attrs`
    """

    is_attribute = True

    def __init__(self, class_, key, impl=None,
                        comparator=None, parententity=None,
                        of_type=None):
        self.class_ = class_
        self.key = key
        self.impl = impl
        self.comparator = comparator
        self._parententity = parententity
        self._of_type = of_type

        manager = manager_of_class(class_)
        # manager is None in the case of AliasedClass
        if manager:
            # propagate existing event listeners from
            # immediate superclass
            for base in manager._bases:
                if key in base:
                    self.dispatch._update(base[key].dispatch)

    @util.memoized_property
    def _supports_population(self):
        return self.impl.supports_population

    def get_history(self, instance, passive=PASSIVE_OFF):
        return self.impl.get_history(instance_state(instance),
                                        instance_dict(instance), passive)

    def __selectable__(self):
        # TODO: conditionally attach this method based on clause_element ?
        return self


    @util.memoized_property
    def info(self):
        """Return the 'info' dictionary for the underlying SQL element.

        The behavior here is as follows:

        * If the attribute is a column-mapped property, i.e.
          :class:`.ColumnProperty`, which is mapped directly
          to a schema-level :class:`.Column` object, this attribute
          will return the :attr:`.SchemaItem.info` dictionary associated
          with the core-level :class:`.Column` object.

        * If the attribute is a :class:`.ColumnProperty` but is mapped to
          any other kind of SQL expression other than a :class:`.Column`,
          the attribute will refer to the :attr:`.MapperProperty.info` dictionary
          associated directly with the :class:`.ColumnProperty`, assuming the SQL
          expression itself does not have it's own ``.info`` attribute
          (which should be the case, unless a user-defined SQL construct
          has defined one).

        * If the attribute refers to any other kind of :class:`.MapperProperty`,
          including :class:`.RelationshipProperty`, the attribute will refer
          to the :attr:`.MapperProperty.info` dictionary associated with
          that :class:`.MapperProperty`.

        * To access the :attr:`.MapperProperty.info` dictionary of the :class:`.MapperProperty`
          unconditionally, including for a :class:`.ColumnProperty` that's
          associated directly with a :class:`.schema.Column`, the attribute
          can be referred to using :attr:`.QueryableAttribute.property`
          attribute, as ``MyClass.someattribute.property.info``.

        .. versionadded:: 0.8.0

        .. seealso::

            :attr:`.SchemaItem.info`

            :attr:`.MapperProperty.info`

        """
        return self.comparator.info

    @util.memoized_property
    def parent(self):
        """Return an inspection instance representing the parent.

        This will be either an instance of :class:`.Mapper`
        or :class:`.AliasedInsp`, depending upon the nature
        of the parent entity which this attribute is associated
        with.

        """
        return inspection.inspect(self._parententity)

    @property
    def expression(self):
        return self.comparator.__clause_element__()

    def __clause_element__(self):
        return self.comparator.__clause_element__()

    def _query_clause_element(self):
        """like __clause_element__(), but called specifically
        by :class:`.Query` to allow special behavior."""

        return self.comparator._query_clause_element()

    def adapt_to_entity(self, adapt_to_entity):
        assert not self._of_type
        return self.__class__(adapt_to_entity.entity, self.key, impl=self.impl,
                           comparator=self.comparator.adapt_to_entity(adapt_to_entity),
                           parententity=adapt_to_entity)

    def of_type(self, cls):
        return QueryableAttribute(
                    self.class_,
                    self.key,
                    self.impl,
                    self.comparator.of_type(cls),
                    self._parententity,
                    of_type=cls)

    def label(self, name):
        return self._query_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) is not False

    def __getattr__(self, key):
        try:
            return getattr(self.comparator, key)
        except AttributeError:
            raise AttributeError(
                    'Neither %r object nor %r object associated with %s '
                    'has an attribute %r' % (
                    type(self).__name__,
                    type(self.comparator).__name__,
                    self,
                    key)
            )

    def __str__(self):
        return "%s.%s" % (self.class_.__name__, self.key)

    @util.memoized_property
    def property(self):
        """Return the :class:`.MapperProperty` associated with this
        :class:`.QueryableAttribute`.


        Return values here will commonly be instances of
        :class:`.ColumnProperty` or :class:`.RelationshipProperty`.


        """
        return self.comparator.property


class InstrumentedAttribute(QueryableAttribute):
    """Class bound instrumented attribute which adds basic
    :term:`descriptor` methods.

    See :class:`.QueryableAttribute` for a description of most features.


    """

    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

        dict_ = instance_dict(instance)
        if self._supports_population and self.key in dict_:
            return dict_[self.key]
        else:
            return self.impl.get(instance_state(instance), dict_)


def create_proxied_attribute(descriptor):
    """Create an QueryableAttribute / user descriptor hybrid.

    Returns a new QueryableAttribute type that delegates descriptor
    behavior and getattr() to the given descriptor.
    """

    # TODO: can move this to descriptor_props if the need for this
    # function is removed from ext/hybrid.py

    class Proxy(QueryableAttribute):
        """Presents the :class:`.QueryableAttribute` interface as a
        proxy on top of a Python descriptor / :class:`.PropComparator`
        combination.

        """

        def __init__(self, class_, key, descriptor,
                                comparator,
                                adapt_to_entity=None, doc=None,
                                original_property=None):
            self.class_ = class_
            self.key = key
            self.descriptor = descriptor
            self.original_property = original_property
            self._comparator = comparator
            self._adapt_to_entity = adapt_to_entity
            self.__doc__ = doc

        @property
        def property(self):
            return self.comparator.property

        @util.memoized_property
        def comparator(self):
            if util.callable(self._comparator):
                self._comparator = self._comparator()
            if self._adapt_to_entity:
                self._comparator = self._comparator.adapt_to_entity(
                                    self._adapt_to_entity)
            return self._comparator

        def adapt_to_entity(self, adapt_to_entity):
            return self.__class__(adapt_to_entity.entity, self.key, self.descriptor,
                                       self._comparator,
                                       adapt_to_entity)

        def __get__(self, instance, owner):
            if instance is None:
                return self
            else:
                return self.descriptor.__get__(instance, owner)

        def __str__(self):
            return "%s.%s" % (self.class_.__name__, self.key)

        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 associated with %s '
                        'has an attribute %r' % (
                    type(descriptor).__name__,
                    type(self.comparator).__name__,
                    self,
                    attribute)
                    )

    Proxy.__name__ = type(descriptor).__name__ + 'Proxy'

    util.monkeypatch_proxied_specials(Proxy, type(descriptor),
                                      name='descriptor',
                                      from_instance=descriptor)
    return Proxy

OP_REMOVE = util.symbol("REMOVE")
OP_APPEND = util.symbol("APPEND")
OP_REPLACE = util.symbol("REPLACE")

class Event(object):
    """A token propagated throughout the course of a chain of attribute
    events.

    Serves as an indicator of the source of the event and also provides
    a means of controlling propagation across a chain of attribute
    operations.

    The :class:`.Event` object is sent as the ``initiator`` argument
    when dealing with the :meth:`.AttributeEvents.append`,
    :meth:`.AttributeEvents.set`,
    and :meth:`.AttributeEvents.remove` events.

    The :class:`.Event` object is currently interpreted by the backref
    event handlers, and is used to control the propagation of operations
    across two mutually-dependent attributes.

    .. versionadded:: 0.9.0

    """

    impl = None
    """The :class:`.AttributeImpl` which is the current event initiator.
    """

    op = None
    """The symbol :attr:`.OP_APPEND`, :attr:`.OP_REMOVE` or :attr:`.OP_REPLACE`,
    indicating the source operation.

    """

    def __init__(self, attribute_impl, op):
        self.impl = attribute_impl
        self.op = op
        self.parent_token = self.impl.parent_token


    @property
    def key(self):
        return self.impl.key

    def hasparent(self, state):
        return self.impl.hasparent(state)

class AttributeImpl(object):
    """internal implementation for instrumented attributes."""

    def __init__(self, class_, key,
                    callable_, dispatch, trackparent=False, extension=None,
                    compare_function=None, active_history=False,
                    parent_token=None, expire_missing=True,
                    send_modified_events=True,
                    **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.  Deprecated.
          The event package is now used.

        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.

        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.

        expire_missing
          if False, don't add an "expiry" callable to this attribute
          during state.expire_attributes(None), if no value is present
          for this key.

        send_modified_events
          if False, the InstanceState._modified_event method will have no effect;
          this means the attribute will never show up as changed in a
          history entry.
        """
        self.class_ = class_
        self.key = key
        self.callable_ = callable_
        self.dispatch = dispatch
        self.trackparent = trackparent
        self.parent_token = parent_token or self
        self.send_modified_events = send_modified_events
        if compare_function is None:
            self.is_equal = operator.eq
        else:
            self.is_equal = compare_function

        # TODO: pass in the manager here
        # instead of doing a lookup
        attr = manager_of_class(class_)[key]

        for ext in util.to_list(extension or []):
            ext._adapt_listener(attr, ext)

        if active_history:
            self.dispatch._active_history = True

        self.expire_missing = expire_missing

    def __str__(self):
        return "%s.%s" % (self.class_.__name__, self.key)

    def _get_active_history(self):
        """Backwards compat for impl.active_history"""

        return self.dispatch._active_history

    def _set_active_history(self, value):
        self.dispatch._active_history = value

    active_history = property(_get_active_history, _set_active_history)

    def hasparent(self, state, optimistic=False):
        """Return the boolean value of a `hasparent` flag attached to
        the given state.

        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.

        """
        msg = "This AttributeImpl is not configured to track parents."
        assert self.trackparent, msg

        return state.parents.get(id(self.parent_token), optimistic) \
                is not False

    def sethasparent(self, state, parent_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``.

        """
        msg = "This AttributeImpl is not configured to track parents."
        assert self.trackparent, msg

        id_ = id(self.parent_token)
        if value:
            state.parents[id_] = parent_state
        else:
            if id_ in state.parents:
                last_parent = state.parents[id_]

                if last_parent is not False and \
                    last_parent.key != parent_state.key:

                    if last_parent.obj() is None:
                        raise orm_exc.StaleDataError(
                            "Removing state %s from parent "
                            "state %s along attribute '%s', "
                            "but the parent record "
                            "has gone stale, can't be sure this "
                            "is the most recent parent." %
                            (state_str(state),
                            state_str(parent_state),
                            self.key))

                    return

            state.parents[id_] = False

    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.

        """
        state.callables[self.key] = callable_

    def get_history(self, state, dict_, passive=PASSIVE_OFF):
        raise NotImplementedError()

    def get_all_pending(self, state, dict_):
        """Return a list of tuples of (state, obj)
        for all objects in this attribute's current state
        + history.

        Only applies to object-based attributes.

        This is an inlining of existing functionality
        which roughly corresponds to:

            get_state_history(
                        state,
                        key,
                        passive=PASSIVE_NO_INITIALIZE).sum()

        """
        raise NotImplementedError()

    def initialize(self, state, dict_):
        """Initialize the given state's attribute 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.
        """
        if self.key in dict_:
            return dict_[self.key]
        else:
            # if history present, don't load
            key = self.key
            if key not in state.committed_state or \
                state.committed_state[key] is NEVER_SET:
                if not passive & CALLABLES_OK:
                    return PASSIVE_NO_RESULT

                if key in state.callables:
                    callable_ = state.callables[key]
                    value = callable_(state, passive)
                elif self.callable_:
                    value = self.callable_(state, passive)
                else:
                    value = ATTR_EMPTY

                if value is PASSIVE_NO_RESULT or value is NEVER_SET:
                    return value
                elif value is ATTR_WAS_SET:
                    try:
                        return dict_[key]
                    except KeyError:
                        # TODO: no test coverage here.
                        raise KeyError(
                                "Deferred loader for attribute "
                                "%r failed to populate "
                                "correctly" % key)
                elif value is not ATTR_EMPTY:
                    return self.set_committed_value(state, dict_, value)

            if not passive & INIT_OK:
                return NEVER_SET
            else:
                # 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, passive=passive)

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

    def pop(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
        self.set(state, dict_, None, initiator,
                    passive=passive, check_old=value, pop=True)

    def set(self, state, dict_, value, initiator,
                passive=PASSIVE_OFF, check_old=None, pop=False):
        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:
            value = state.committed_state[self.key]
            if value is NO_VALUE:
                return None
            else:
                return value
        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."""

        dict_[self.key] = value
        state._commit(dict_, [self.key])
        return value


class ScalarAttributeImpl(AttributeImpl):
    """represents a scalar value-holding InstrumentedAttribute."""

    accepts_scalar_loader = True
    uses_objects = False
    supports_population = True
    collection = False

    def delete(self, state, dict_):

        # TODO: catch key errors, convert to attributeerror?
        if self.dispatch._active_history:
            old = self.get(state, dict_, PASSIVE_RETURN_NEVER_SET)
        else:
            old = dict_.get(self.key, NO_VALUE)

        if self.dispatch.remove:
            self.fire_remove_event(state, dict_, old, self._remove_token)
        state._modified_event(dict_, self, old)
        del dict_[self.key]

    def get_history(self, state, dict_, passive=PASSIVE_OFF):
        if self.key in dict_:
            return History.from_scalar_attribute(self, state, dict_[self.key])
        else:
            if passive & INIT_OK:
                passive ^= INIT_OK
            current = self.get(state, dict_, passive=passive)
            if current is PASSIVE_NO_RESULT:
                return HISTORY_BLANK
            else:
                return History.from_scalar_attribute(self, state, current)

    def set(self, state, dict_, value, initiator,
                passive=PASSIVE_OFF, check_old=None, pop=False):
        if self.dispatch._active_history:
            old = self.get(state, dict_, PASSIVE_RETURN_NEVER_SET)
        else:
            old = dict_.get(self.key, NO_VALUE)

        if self.dispatch.set:
            value = self.fire_replace_event(state, dict_,
                                                value, old, initiator)
        state._modified_event(dict_, self, old)
        dict_[self.key] = value

    @util.memoized_property
    def _replace_token(self):
        return Event(self, OP_REPLACE)

    @util.memoized_property
    def _append_token(self):
        return Event(self, OP_REPLACE)

    @util.memoized_property
    def _remove_token(self):
        return Event(self, OP_REMOVE)

    def fire_replace_event(self, state, dict_, value, previous, initiator):
        for fn in self.dispatch.set:
            value = fn(state, value, previous, initiator or self._replace_token)
        return value

    def fire_remove_event(self, state, dict_, value, initiator):
        for fn in self.dispatch.remove:
            fn(state, value, initiator or self._remove_token)

    @property
    def type(self):
        self.property.columns[0].type


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
    supports_population = True
    collection = False

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

    def get_history(self, state, dict_, passive=PASSIVE_OFF):
        if self.key in dict_:
            return History.from_object_attribute(self, state, dict_[self.key])
        else:
            if passive & INIT_OK:
                passive ^= INIT_OK
            current = self.get(state, dict_, passive=passive)
            if current is PASSIVE_NO_RESULT:
                return HISTORY_BLANK
            else:
                return History.from_object_attribute(self, state, current)

    def get_all_pending(self, state, dict_):
        if self.key in dict_:
            current = dict_[self.key]
            if current is not None:
                ret = [(instance_state(current), current)]
            else:
                ret = [(None, None)]

            if self.key in state.committed_state:
                original = state.committed_state[self.key]
                if original not in (NEVER_SET, PASSIVE_NO_RESULT, None) and \
                    original is not current:

                    ret.append((instance_state(original), original))
            return ret
        else:
            return []

    def set(self, state, dict_, value, initiator,
                passive=PASSIVE_OFF, check_old=None, pop=False):
        """Set a value on the given InstanceState.

        """
        if self.dispatch._active_history:
            old = self.get(state, dict_, passive=PASSIVE_ONLY_PERSISTENT | NO_AUTOFLUSH)
        else:
            old = self.get(state, dict_, passive=PASSIVE_NO_FETCH)

        if check_old is not None and \
             old is not PASSIVE_NO_RESULT and \
             check_old is not old:
            if pop:
                return
            else:
                raise ValueError(
                    "Object %s not associated with %s on attribute '%s'" % (
                    instance_str(check_old),
                   state_str(state),
                   self.key
                ))
        value = self.fire_replace_event(state, dict_, value, old, initiator)
        dict_[self.key] = value


    def fire_remove_event(self, state, dict_, value, initiator):
        if self.trackparent and value is not None:
            self.sethasparent(instance_state(value), state, False)

        for fn in self.dispatch.remove:
            fn(state, value, initiator or self._remove_token)

        state._modified_event(dict_, self, value)

    def fire_replace_event(self, state, dict_, value, previous, initiator):
        if self.trackparent:
            if (previous is not value and
                previous is not None and
                previous is not PASSIVE_NO_RESULT):
                self.sethasparent(instance_state(previous), state, False)

        for fn in self.dispatch.set:
            value = fn(state, value, previous, initiator or self._replace_token)

        state._modified_event(dict_, self, previous)

        if self.trackparent:
            if value is not None:
                self.sethasparent(instance_state(value), state, 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
    supports_population = True
    collection = True

    def __init__(self, class_, key, callable_, dispatch,
                    typecallable=None, trackparent=False, extension=None,
                    copy_function=None, compare_function=None, **kwargs):
        super(CollectionAttributeImpl, self).__init__(
                                            class_,
                                            key,
                                            callable_, dispatch,
                                            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

    def __copy(self, item):
        return [y for y in collections.collection_adapter(item)]

    def get_history(self, state, dict_, passive=PASSIVE_OFF):
        current = self.get(state, dict_, passive=passive)
        if current is PASSIVE_NO_RESULT:
            return HISTORY_BLANK
        else:
            return History.from_collection(self, state, current)

    def get_all_pending(self, state, dict_):
        if self.key not in dict_:
            return []

        current = dict_[self.key]
        current = getattr(current, '_sa_adapter')

        if self.key in state.committed_state:
            original = state.committed_state[self.key]
            if original not in (NO_VALUE, NEVER_SET):
                current_states = [((c is not None) and
                                    instance_state(c) or None, c)
                                    for c in current]
                original_states = [((c is not None) and
                                    instance_state(c) or None, c)
                                    for c in original]

                current_set = dict(current_states)
                original_set = dict(original_states)

                return \
                    [(s, o) for s, o in current_states
                        if s not in original_set] + \
                    [(s, o) for s, o in current_states
                        if s in original_set] + \
                    [(s, o) for s, o in original_states
                        if s not in current_set]

        return [(instance_state(o), o) for o in current]

    @util.memoized_property
    def _append_token(self):
        return Event(self, OP_APPEND)

    @util.memoized_property
    def _remove_token(self):
        return Event(self, OP_REMOVE)

    def fire_append_event(self, state, dict_, value, initiator):
        for fn in self.dispatch.append:
            value = fn(state, value, initiator or self._append_token)

        state._modified_event(dict_, self, NEVER_SET, True)

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

        return value

    def fire_pre_remove_event(self, state, dict_, initiator):
        state._modified_event(dict_, self, NEVER_SET, True)

    def fire_remove_event(self, state, dict_, value, initiator):
        if self.trackparent and value is not None:
            self.sethasparent(instance_state(value), state, False)

        for fn in self.dispatch.remove:
            fn(state, value, initiator or self._remove_token)

        state._modified_event(dict_, self, NEVER_SET, True)

    def delete(self, state, dict_):
        if self.key not in dict_:
            return

        state._modified_event(dict_, self, NEVER_SET, True)

        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):
        collection = self.get_collection(state, dict_, passive=passive)
        if collection is PASSIVE_NO_RESULT:
            value = self.fire_append_event(state, dict_, value, initiator)
            assert self.key not in dict_, \
                    "Collection was loaded during event handling."
            state._get_pending_mutation(self.key).append(value)
        else:
            collection.append_with_event(value, initiator)

    def remove(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
        collection = self.get_collection(state, state.dict, passive=passive)
        if collection is PASSIVE_NO_RESULT:
            self.fire_remove_event(state, dict_, value, initiator)
            assert self.key not in dict_, \
                    "Collection was loaded during event handling."
            state._get_pending_mutation(self.key).remove(value)
        else:
            collection.remove_with_event(value, initiator)

    def pop(self, state, dict_, value, initiator, passive=PASSIVE_OFF):
        try:
            # TODO: better solution here would be to add
            # a "popper" role to collections.py to complement
            # "remover".
            self.remove(state, dict_, value, initiator, passive=passive)
        except (ValueError, KeyError, IndexError):
            pass

    def set(self, state, dict_, value, initiator,
                    passive=PASSIVE_OFF, pop=False):
        """Set a value on the given object.

        """

        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_, passive=PASSIVE_ONLY_PERSISTENT)
        if old is PASSIVE_NO_RESULT:
            old = self.initialize(state, dict_)
        elif old is iterable:
            # ignore re-assignment of the current collection, as happens
            # implicitly with in-place operators (foo.collection |= other)
            return

        # place a copy of "old" in state.committed_state
        state._modified_event(dict_, self, old, True)

        old_collection = getattr(old, '_sa_adapter')

        dict_[self.key] = user_data

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

    def _invalidate_collection(self, collection):
        adapter = getattr(collection, '_sa_adapter')
        adapter.invalidated = True

    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:
            collection.append_multiple_without_event(value)

        state.dict[self.key] = user_data

        state._commit(dict_, [self.key])

        if self.key in state._pending_mutations:
            # pending items exist.  issue a modified event,
            # add/remove new items.
            state._modified_event(dict_, self, user_data, True)

            pending = state._pending_mutations.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_NO_RESULT:
                return user_data

        return getattr(user_data, '_sa_adapter')


def backref_listeners(attribute, key, uselist):
    """Apply listeners to synchronize a two-way relationship."""

    # use easily recognizable names for stack traces

    parent_token = attribute.impl.parent_token
    parent_impl = attribute.impl

    def _acceptable_key_err(child_state, initiator, child_impl):
        raise ValueError(
            "Bidirectional attribute conflict detected: "
            'Passing object %s to attribute "%s" '
            'triggers a modify event on attribute "%s" '
            'via the backref "%s".' % (
                state_str(child_state),
                initiator.parent_token,
                child_impl.parent_token,
                attribute.impl.parent_token
            )
        )

    def emit_backref_from_scalar_set_event(state, child, oldchild, initiator):
        if oldchild is child:
            return child
        if oldchild is not None and oldchild is not PASSIVE_NO_RESULT:
            # 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.manager[key].impl

            if initiator.impl is not impl or \
                    initiator.op not in (OP_REPLACE, OP_REMOVE):
                impl.pop(old_state,
                            old_dict,
                            state.obj(),
                            parent_impl._append_token,
                            passive=PASSIVE_NO_FETCH)

        if child is not None:
            child_state, child_dict = instance_state(child),\
                                        instance_dict(child)
            child_impl = child_state.manager[key].impl
            if initiator.parent_token is not parent_token and \
                    initiator.parent_token is not child_impl.parent_token:
                _acceptable_key_err(state, initiator, child_impl)
            elif initiator.impl is not child_impl or \
                    initiator.op not in (OP_APPEND, OP_REPLACE):
                child_impl.append(
                                    child_state,
                                    child_dict,
                                    state.obj(),
                                    initiator,
                                    passive=PASSIVE_NO_FETCH)
        return child

    def emit_backref_from_collection_append_event(state, child, initiator):
        if child is None:
            return

        child_state, child_dict = instance_state(child), \
                                    instance_dict(child)
        child_impl = child_state.manager[key].impl

        if initiator.parent_token is not parent_token and \
                initiator.parent_token is not child_impl.parent_token:
            _acceptable_key_err(state, initiator, child_impl)
        elif initiator.impl is not child_impl or \
                initiator.op not in (OP_APPEND, OP_REPLACE):
            child_impl.append(
                                child_state,
                                child_dict,
                                state.obj(),
                                initiator,
                                passive=PASSIVE_NO_FETCH)
        return child

    def emit_backref_from_collection_remove_event(state, child, initiator):
        if child is not None:
            child_state, child_dict = instance_state(child),\
                                        instance_dict(child)
            child_impl = child_state.manager[key].impl
            if initiator.impl is not child_impl or \
                    initiator.op not in (OP_REMOVE, OP_REPLACE):
                child_impl.pop(
                                child_state,
                                child_dict,
                                state.obj(),
                                initiator,
                                passive=PASSIVE_NO_FETCH)

    if uselist:
        event.listen(attribute, "append",
                    emit_backref_from_collection_append_event,
                    retval=True, raw=True)
    else:
        event.listen(attribute, "set",
                    emit_backref_from_scalar_set_event,
                    retval=True, raw=True)
    # TODO: need coverage in test/orm/ of remove event
    event.listen(attribute, "remove",
                    emit_backref_from_collection_remove_event,
                    retval=True, raw=True)

_NO_HISTORY = util.symbol('NO_HISTORY')
_NO_STATE_SYMBOLS = frozenset([
                        id(PASSIVE_NO_RESULT),
                        id(NO_VALUE),
                        id(NEVER_SET)])

History = util.namedtuple("History", [
        "added", "unchanged", "deleted"
    ])


class History(History):
    """A 3-tuple of added, unchanged and deleted values,
    representing the changes which have occurred on an instrumented
    attribute.

    The easiest way to get a :class:`.History` object for a particular
    attribute on an object is to use the :func:`.inspect` function::

        from sqlalchemy import inspect

        hist = inspect(myobject).attrs.myattribute.history

    Each tuple member is an iterable sequence:

    * ``added`` - the collection of items added to the attribute (the first
      tuple element).

    * ``unchanged`` - the collection of items that have not changed on the
      attribute (the second tuple element).

    * ``deleted`` - the collection of items that have been removed from the
      attribute (the third tuple element).

    """

    def __bool__(self):
        return self != HISTORY_BLANK
    __nonzero__ = __bool__

    def empty(self):
        """Return True if this :class:`.History` has no changes
        and no existing, unchanged state.

        """

        return not bool(
                        (self.added or self.deleted)
                        or self.unchanged and self.unchanged != [None]
                    )

    def sum(self):
        """Return a collection of added + unchanged + deleted."""

        return (self.added or []) +\
                (self.unchanged or []) +\
                (self.deleted or [])

    def non_deleted(self):
        """Return a collection of added + unchanged."""

        return (self.added or []) +\
                (self.unchanged or [])

    def non_added(self):
        """Return a collection of unchanged + deleted."""

        return (self.unchanged or []) +\
                (self.deleted or [])

    def has_changes(self):
        """Return True if this :class:`.History` has changes."""

        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_scalar_attribute(cls, attribute, state, current):
        original = state.committed_state.get(attribute.key, _NO_HISTORY)

        if original is _NO_HISTORY:
            if current is NEVER_SET:
                return cls((), (), ())
            else:
                return cls((), [current], ())
        # don't let ClauseElement expressions here trip things up
        elif attribute.is_equal(current, original) is True:
            return cls((), [current], ())
        else:
            # current convention on native scalars is to not
            # include information
            # about missing previous value in "deleted", but
            # we do include None, which helps in some primary
            # key situations
            if id(original) in _NO_STATE_SYMBOLS:
                deleted = ()
            else:
                deleted = [original]
            if current is NEVER_SET:
                return cls((), (), deleted)
            else:
                return cls([current], (), deleted)

    @classmethod
    def from_object_attribute(cls, attribute, state, current):
        original = state.committed_state.get(attribute.key, _NO_HISTORY)

        if original is _NO_HISTORY:
            if current is NO_VALUE or current is NEVER_SET:
                return cls((), (), ())
            else:
                return cls((), [current], ())
        elif current is original:
            return cls((), [current], ())
        else:
            # current convention on related objects is to not
            # include information
            # about missing previous value in "deleted", and
            # to also not include None - the dependency.py rules
            # ignore the None in any case.
            if id(original) in _NO_STATE_SYMBOLS or original is None:
                deleted = ()
            else:
                deleted = [original]
            if current is NO_VALUE or current is NEVER_SET:
                return cls((), (), deleted)
            else:
                return cls([current], (), deleted)

    @classmethod
    def from_collection(cls, attribute, state, current):
        original = state.committed_state.get(attribute.key, _NO_HISTORY)

        if current is NO_VALUE or current is NEVER_SET:
            return cls((), (), ())

        current = getattr(current, '_sa_adapter')
        if original in (NO_VALUE, NEVER_SET):
            return cls(list(current), (), ())
        elif original is _NO_HISTORY:
            return cls((), list(current), ())
        else:

            current_states = [((c is not None) and instance_state(c)
                                or None, c)
                                for c in current
                                ]
            original_states = [((c is not None) and instance_state(c)
                                or None, c)
                                for c in original
                                ]

            current_set = dict(current_states)
            original_set = dict(original_states)

            return cls(
                [o for s, o in current_states if s not in original_set],
                [o for s, o in current_states if s in original_set],
                [o for s, o in original_states if s not in current_set]
            )

HISTORY_BLANK = History(None, None, None)


def get_history(obj, key, passive=PASSIVE_OFF):
    """Return a :class:`.History` record for the given object
    and attribute key.

    :param obj: an object whose class is instrumented by the
      attributes package.

    :param key: string attribute name.

    :param passive: indicates loading behavior for the attribute
       if the value is not already present.   This is a
       bitflag attribute, which defaults to the symbol
       :attr:`.PASSIVE_OFF` indicating all necessary SQL
       should be emitted.

    """
    if passive is True:
        util.warn_deprecated("Passing True for 'passive' is deprecated. "
                                "Use attributes.PASSIVE_NO_INITIALIZE")
        passive = PASSIVE_NO_INITIALIZE
    elif passive is False:
        util.warn_deprecated("Passing False for 'passive' is "
                                "deprecated.  Use attributes.PASSIVE_OFF")
        passive = PASSIVE_OFF

    return get_state_history(instance_state(obj), key, passive)


def get_state_history(state, key, passive=PASSIVE_OFF):
    return state.get_history(key, passive)


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_attribute(class_, key, **kw):
    comparator = kw.pop('comparator', None)
    parententity = kw.pop('parententity', None)
    doc = kw.pop('doc', None)
    desc = register_descriptor(class_, key,
                            comparator, parententity, doc=doc)
    register_attribute_impl(class_, key, **kw)
    return desc


def register_attribute_impl(class_, key,
        uselist=False, callable_=None,
        useobject=False,
        impl_class=None, backref=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)

    dispatch = manager[key].dispatch

    if impl_class:
        impl = impl_class(class_, key, typecallable, dispatch, **kw)
    elif uselist:
        impl = CollectionAttributeImpl(class_, key, callable_, dispatch,
                                       typecallable=typecallable, **kw)
    elif useobject:
        impl = ScalarObjectAttributeImpl(class_, key, callable_,
                                        dispatch, **kw)
    else:
        impl = ScalarAttributeImpl(class_, key, callable_, dispatch, **kw)

    manager[key].impl = impl

    if backref:
        backref_listeners(manager[key], backref, uselist)

    manager.post_configure_attribute(key)
    return manager[key]


def register_descriptor(class_, key, comparator=None,
                                parententity=None, doc=None):
    manager = manager_of_class(class_)

    descriptor = InstrumentedAttribute(class_, key, comparator=comparator,
                                            parententity=parententity)

    descriptor.__doc__ = doc

    manager.instrument_attribute(key, descriptor)
    return 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 = 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.manager[key].impl
    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.manager[key].impl.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.manager[key].impl.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.manager[key].impl.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.manager[key].impl.delete(state, dict_)


def flag_modified(instance, key):
    """Mark an attribute on an instance as 'modified'.

    This sets the 'modified' flag on the instance and
    establishes an unconditional change event for the given attribute.

    """
    state, dict_ = instance_state(instance), instance_dict(instance)
    impl = state.manager[key].impl
    state._modified_event(dict_, impl, NO_VALUE, force=True)
