text: # orm/session.py
  # Copyright (C) 2005-2017 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
  """Provides the Session class and related utilities."""
  
  
  import weakref
  from .. import util, sql, engine, exc as sa_exc
  from ..sql import util as sql_util, expression
  from . import (
      SessionExtension, attributes, exc, query,
      loading, identity
  )
  from ..inspection import inspect
  from .base import (
      object_mapper, class_mapper,
      _class_to_mapper, _state_mapper, object_state,
      _none_set, state_str, instance_str
  )
  import itertools
  from . import persistence
  from .unitofwork import UOWTransaction
  from . import state as statelib
  import sys
  
  __all__ = ['Session', 'SessionTransaction',
             'SessionExtension', 'sessionmaker']
  
  _sessions = weakref.WeakValueDictionary()
  """Weak-referencing dictionary of :class:`.Session` objects.
  """
  
  
  def _state_session(state):
      """Given an :class:`.InstanceState`, return the :class:`.Session`
          associated, if any.
      """
      if state.session_id:
          try:
              return _sessions[state.session_id]
          except KeyError:
              pass
      return None
  
  
  class _SessionClassMethods(object):
      """Class-level methods for :class:`.Session`, :class:`.sessionmaker`."""
  
      @classmethod
      def close_all(cls):
          """Close *all* sessions in memory."""
  
          for sess in _sessions.values():
              sess.close()
  
      @classmethod
      @util.dependencies("sqlalchemy.orm.util")
      def identity_key(cls, orm_util, *args, **kwargs):
          """Return an identity key.
  
          This is an alias of :func:`.util.identity_key`.
  
          """
          return orm_util.identity_key(*args, **kwargs)
  
      @classmethod
      def object_session(cls, instance):
          """Return the :class:`.Session` to which an object belongs.
  
          This is an alias of :func:`.object_session`.
  
          """
  
          return object_session(instance)
  
  
  ACTIVE = util.symbol('ACTIVE')
  PREPARED = util.symbol('PREPARED')
  COMMITTED = util.symbol('COMMITTED')
  DEACTIVE = util.symbol('DEACTIVE')
  CLOSED = util.symbol('CLOSED')
  
  
  class SessionTransaction(object):
      """A :class:`.Session`-level transaction.
  
      :class:`.SessionTransaction` is a mostly behind-the-scenes object
      not normally referenced directly by application code.   It coordinates
      among multiple :class:`.Connection` objects, maintaining a database
      transaction for each one individually, committing or rolling them
      back all at once.   It also provides optional two-phase commit behavior
      which can augment this coordination operation.
  
      The :attr:`.Session.transaction` attribute of :class:`.Session`
      refers to the current :class:`.SessionTransaction` object in use, if any.
      The :attr:`.SessionTransaction.parent` attribute refers to the parent
      :class:`.SessionTransaction` in the stack of :class:`.SessionTransaction`
      objects.  If this attribute is ``None``, then this is the top of the stack.
      If non-``None``, then this :class:`.SessionTransaction` refers either
      to a so-called "subtransaction" or a "nested" transaction.  A
      "subtransaction" is a scoping concept that demarcates an inner portion
      of the outermost "real" transaction.  A nested transaction, which
      is indicated when the :attr:`.SessionTransaction.nested`
      attribute is also True, indicates that this :class:`.SessionTransaction`
      corresponds to a SAVEPOINT.
  
      **Life Cycle**
  
      A :class:`.SessionTransaction` is associated with a :class:`.Session`
      in its default mode of ``autocommit=False`` immediately, associated
      with no database connections.  As the :class:`.Session` is called upon
      to emit SQL on behalf of various :class:`.Engine` or :class:`.Connection`
      objects, a corresponding :class:`.Connection` and associated
      :class:`.Transaction` is added to a collection within the
      :class:`.SessionTransaction` object, becoming one of the
      connection/transaction pairs maintained by the
      :class:`.SessionTransaction`.  The start of a :class:`.SessionTransaction`
      can be tracked using the :meth:`.SessionEvents.after_transaction_create`
      event.
  
      The lifespan of the :class:`.SessionTransaction` ends when the
      :meth:`.Session.commit`, :meth:`.Session.rollback` or
      :meth:`.Session.close` methods are called.  At this point, the
      :class:`.SessionTransaction` removes its association with its parent
      :class:`.Session`.   A :class:`.Session` that is in ``autocommit=False``
      mode will create a new :class:`.SessionTransaction` to replace it
      immediately, whereas a :class:`.Session` that's in ``autocommit=True``
      mode will remain without a :class:`.SessionTransaction` until the
      :meth:`.Session.begin` method is called.  The end of a
      :class:`.SessionTransaction` can be tracked using the
      :meth:`.SessionEvents.after_transaction_end` event.
  
      **Nesting and Subtransactions**
  
      Another detail of :class:`.SessionTransaction` behavior is that it is
      capable of "nesting".  This means that the :meth:`.Session.begin` method
      can be called while an existing :class:`.SessionTransaction` is already
      present, producing a new :class:`.SessionTransaction` that temporarily
      replaces the parent :class:`.SessionTransaction`.   When a
      :class:`.SessionTransaction` is produced as nested, it assigns itself to
      the :attr:`.Session.transaction` attribute, and it additionally will assign
      the previous :class:`.SessionTransaction` to its :attr:`.Session.parent`
      attribute.  The behavior is effectively a
      stack, where :attr:`.Session.transaction` refers to the current head of
      the stack, and the :attr:`.SessionTransaction.parent` attribute allows
      traversal up the stack until :attr:`.SessionTransaction.parent` is
      ``None``, indicating the top of the stack.
  
      When the scope of :class:`.SessionTransaction` is ended via
      :meth:`.Session.commit` or :meth:`.Session.rollback`, it restores its
      parent :class:`.SessionTransaction` back onto the
      :attr:`.Session.transaction` attribute.
  
      The purpose of this stack is to allow nesting of
      :meth:`.Session.rollback` or :meth:`.Session.commit` calls in context
      with various flavors of :meth:`.Session.begin`. This nesting behavior
      applies to when :meth:`.Session.begin_nested` is used to emit a
      SAVEPOINT transaction, and is also used to produce a so-called
      "subtransaction" which allows a block of code to use a
      begin/rollback/commit sequence regardless of whether or not its enclosing
      code block has begun a transaction.  The :meth:`.flush` method, whether
      called explicitly or via autoflush, is the primary consumer of the
      "subtransaction" feature, in that it wishes to guarantee that it works
      within in a transaction block regardless of whether or not the
      :class:`.Session` is in transactional mode when the method is called.
  
      Note that the flush process that occurs within the "autoflush" feature
      as well as when the :meth:`.Session.flush` method is used **always**
      creates a :class:`.SessionTransaction` object.   This object is normally
      a subtransaction, unless the :class:`.Session` is in autocommit mode
      and no transaction exists at all, in which case it's the outermost
      transaction.   Any event-handling logic or other inspection logic
      needs to take into account whether a :class:`.SessionTransaction`
      is the outermost transaction, a subtransaction, or a "nested" / SAVEPOINT
      transaction.
  
      .. seealso::
  
      :meth:`.Session.rollback`
  
      :meth:`.Session.commit`
  
      :meth:`.Session.begin`
  
      :meth:`.Session.begin_nested`
  
      :attr:`.Session.is_active`
  
      :meth:`.SessionEvents.after_transaction_create`
  
      :meth:`.SessionEvents.after_transaction_end`
  
      :meth:`.SessionEvents.after_commit`
  
      :meth:`.SessionEvents.after_rollback`
  
      :meth:`.SessionEvents.after_soft_rollback`
  
      """
  
      _rollback_exception = None
  
      def __init__(self, session, parent=None, nested=False):
          self.session = session
          self._connections = {}
          self._parent = parent
          self.nested = nested
          self._state = ACTIVE
          if not parent and nested:
              raise sa_exc.InvalidRequestError(
                  "Can't start a SAVEPOINT transaction when no existing "
                  "transaction is in progress")
  
          if self.session._enable_transaction_accounting:
              self._take_snapshot()
  
          self.session.dispatch.after_transaction_create(self.session, self)
  
      @property
      def parent(self):
          """The parent :class:`.SessionTransaction` of this
          :class:`.SessionTransaction`.
  
          If this attribute is ``None``, indicates this
          :class:`.SessionTransaction` is at the top of the stack, and
          corresponds to a real "COMMIT"/"ROLLBACK"
          block.  If non-``None``, then this is either a "subtransaction"
          or a "nested" / SAVEPOINT transaction.  If the
          :attr:`.SessionTransaction.nested` attribute is ``True``, then
          this is a SAVEPOINT, and if ``False``, indicates this a subtransaction.
  
          .. versionadded:: 1.0.16 - use ._parent for previous versions
  
          """
          return self._parent
  
      nested = False
      """Indicates if this is a nested, or SAVEPOINT, transaction.
  
      When :attr:`.SessionTransaction.nested` is True, it is expected
      that :attr:`.SessionTransaction.parent` will be True as well.
  
      """
  
      @property
      def is_active(self):
          return self.session is not None and self._state is ACTIVE
  
      def _assert_active(self, prepared_ok=False,
                         rollback_ok=False,
                         deactive_ok=False,
                         closed_msg="This transaction is closed"):
          if self._state is COMMITTED:
              raise sa_exc.InvalidRequestError(
                  "This session is in 'committed' state; no further "
                  "SQL can be emitted within this transaction."
              )
          elif self._state is PREPARED:
              if not prepared_ok:
                  raise sa_exc.InvalidRequestError(
                      "This session is in 'prepared' state; no further "
                      "SQL can be emitted within this transaction."
                  )
          elif self._state is DEACTIVE:
              if not deactive_ok and not rollback_ok:
                  if self._rollback_exception:
                      raise sa_exc.InvalidRequestError(
                          "This Session's transaction has been rolled back "
                          "due to a previous exception during flush."
                          " To begin a new transaction with this Session, "
                          "first issue Session.rollback()."
                          " Original exception was: %s"
                          % self._rollback_exception
                      )
                  elif not deactive_ok:
                      raise sa_exc.InvalidRequestError(
                          "This Session's transaction has been rolled back "
                          "by a nested rollback() call.  To begin a new "
                          "transaction, issue Session.rollback() first."
                      )
          elif self._state is CLOSED:
              raise sa_exc.ResourceClosedError(closed_msg)
  
      @property
      def _is_transaction_boundary(self):
          return self.nested or not self._parent
  
      def connection(self, bindkey, execution_options=None, **kwargs):
          self._assert_active()
          bind = self.session.get_bind(bindkey, **kwargs)
          return self._connection_for_bind(bind, execution_options)
  
      def _begin(self, nested=False):
          self._assert_active()
          return SessionTransaction(
              self.session, self, nested=nested)
  
      def _iterate_self_and_parents(self, upto=None):
  
          current = self
          result = ()
          while current:
              result += (current, )
              if current._parent is upto:
                  break
              elif current._parent is None:
                  raise sa_exc.InvalidRequestError(
                      "Transaction %s is not on the active transaction list" % (
                          upto))
              else:
                  current = current._parent
  
          return result
  
      def _take_snapshot(self):
          if not self._is_transaction_boundary:
              self._new = self._parent._new
              self._deleted = self._parent._deleted
              self._dirty = self._parent._dirty
              self._key_switches = self._parent._key_switches
              return
  
          if not self.session._flushing:
              self.session.flush()
  
          self._new = weakref.WeakKeyDictionary()
          self._deleted = weakref.WeakKeyDictionary()
          self._dirty = weakref.WeakKeyDictionary()
          self._key_switches = weakref.WeakKeyDictionary()
  
      def _restore_snapshot(self, dirty_only=False):
          """Restore the restoration state taken before a transaction began.
  
          Corresponds to a rollback.
  
          """
          assert self._is_transaction_boundary
  
          self.session._expunge_states(
              set(self._new).union(self.session._new),
              to_transient=True)
  
          for s, (oldkey, newkey) in self._key_switches.items():
              self.session.identity_map.safe_discard(s)
              s.key = oldkey
              self.session.identity_map.replace(s)
  
          for s in set(self._deleted).union(self.session._deleted):
              self.session._update_impl(s, revert_deletion=True)
  
          assert not self.session._deleted
  
          for s in self.session.identity_map.all_states():
              if not dirty_only or s.modified or s in self._dirty:
                  s._expire(s.dict, self.session.identity_map._modified)
  
      def _remove_snapshot(self):
          """Remove the restoration state taken before a transaction began.
  
          Corresponds to a commit.
  
          """
          assert self._is_transaction_boundary
  
          if not self.nested and self.session.expire_on_commit:
              for s in self.session.identity_map.all_states():
                  s._expire(s.dict, self.session.identity_map._modified)
  
              statelib.InstanceState._detach_states(
                  list(self._deleted), self.session)
              self._deleted.clear()
          elif self.nested:
              self._parent._new.update(self._new)
              self._parent._dirty.update(self._dirty)
              self._parent._deleted.update(self._deleted)
              self._parent._key_switches.update(self._key_switches)
  
      def _connection_for_bind(self, bind, execution_options):
          self._assert_active()
  
          if bind in self._connections:
              if execution_options:
                  util.warn(
                      "Connection is already established for the "
                      "given bind; execution_options ignored")
              return self._connections[bind][0]
  
          if self._parent:
              conn = self._parent._connection_for_bind(bind, execution_options)
              if not self.nested:
                  return conn
          else:
              if isinstance(bind, engine.Connection):
                  conn = bind
                  if conn.engine in self._connections:
                      raise sa_exc.InvalidRequestError(
                          "Session already has a Connection associated for the "
                          "given Connection's Engine")
              else:
                  conn = bind.contextual_connect()
  
          if execution_options:
              conn = conn.execution_options(**execution_options)
  
          if self.session.twophase and self._parent is None:
              transaction = conn.begin_twophase()
          elif self.nested:
              transaction = conn.begin_nested()
          else:
              transaction = conn.begin()
  
          self._connections[conn] = self._connections[conn.engine] = \
              (conn, transaction, conn is not bind)
          self.session.dispatch.after_begin(self.session, self, conn)
          return conn
  
      def prepare(self):
          if self._parent is not None or not self.session.twophase:
              raise sa_exc.InvalidRequestError(
                  "'twophase' mode not enabled, or not root transaction; "
                  "can't prepare.")
          self._prepare_impl()
  
      def _prepare_impl(self):
          self._assert_active()
          if self._parent is None or self.nested:
              self.session.dispatch.before_commit(self.session)
  
          stx = self.session.transaction
          if stx is not self:
              for subtransaction in stx._iterate_self_and_parents(upto=self):
                  subtransaction.commit()
  
          if not self.session._flushing:
              for _flush_guard in range(100):
                  if self.session._is_clean():
                      break
                  self.session.flush()
              else:
                  raise exc.FlushError(
                      "Over 100 subsequent flushes have occurred within "
                      "session.commit() - is an after_flush() hook "
                      "creating new objects?")
  
          if self._parent is None and self.session.twophase:
              try:
                  for t in set(self._connections.values()):
                      t[1].prepare()
              except:
                  with util.safe_reraise():
                      self.rollback()
  
          self._state = PREPARED
  
      def commit(self):
          self._assert_active(prepared_ok=True)
          if self._state is not PREPARED:
              self._prepare_impl()
  
          if self._parent is None or self.nested:
              for t in set(self._connections.values()):
                  t[1].commit()
  
              self._state = COMMITTED
              self.session.dispatch.after_commit(self.session)
  
              if self.session._enable_transaction_accounting:
                  self._remove_snapshot()
  
          self.close()
          return self._parent
  
      def rollback(self, _capture_exception=False):
          self._assert_active(prepared_ok=True, rollback_ok=True)
  
          stx = self.session.transaction
          if stx is not self:
              for subtransaction in stx._iterate_self_and_parents(upto=self):
                  subtransaction.close()
  
          boundary = self
          rollback_err = None
          if self._state in (ACTIVE, PREPARED):
              for transaction in self._iterate_self_and_parents():
                  if transaction._parent is None or transaction.nested:
                      try:
                          transaction._rollback_impl()
                      except:
                          rollback_err = sys.exc_info()
                      transaction._state = DEACTIVE
                      boundary = transaction
                      break
                  else:
                      transaction._state = DEACTIVE
  
          sess = self.session
  
          if not rollback_err and sess._enable_transaction_accounting and \
                  not sess._is_clean():
  
              # if items were added, deleted, or mutated
              # here, we need to re-restore the snapshot
              util.warn(
                  "Session's state has been changed on "
                  "a non-active transaction - this state "
                  "will be discarded.")
              boundary._restore_snapshot(dirty_only=boundary.nested)
  
          self.close()
  
          if self._parent and _capture_exception:
              self._parent._rollback_exception = sys.exc_info()[1]
  
          if rollback_err:
              util.reraise(*rollback_err)
  
          sess.dispatch.after_soft_rollback(sess, self)
  
          return self._parent
  
      def _rollback_impl(self):
          try:
              for t in set(self._connections.values()):
                  t[1].rollback()
          finally:
              if self.session._enable_transaction_accounting:
                  self._restore_snapshot(dirty_only=self.nested)
  
          self.session.dispatch.after_rollback(self.session)
  
      def close(self, invalidate=False):
          self.session.transaction = self._parent
          if self._parent is None:
              for connection, transaction, autoclose in \
                      set(self._connections.values()):
                  if invalidate:
                      connection.invalidate()
                  if autoclose:
                      connection.close()
                  else:
                      transaction.close()
  
          self._state = CLOSED
          self.session.dispatch.after_transaction_end(self.session, self)
  
          if self._parent is None:
              if not self.session.autocommit:
                  self.session.begin()
          self.session = None
          self._connections = None
  
      def __enter__(self):
          return self
  
      def __exit__(self, type, value, traceback):
          self._assert_active(deactive_ok=True, prepared_ok=True)
          if self.session.transaction is None:
              return
          if type is None:
              try:
                  self.commit()
              except:
                  with util.safe_reraise():
                      self.rollback()
          else:
              self.rollback()
  
  
  class Session(_SessionClassMethods):
      """Manages persistence operations for ORM-mapped objects.
  
      The Session's usage paradigm is described at :doc:`/orm/session`.
  
  
      """
  
      public_methods = (
          '__contains__', '__iter__', 'add', 'add_all', 'begin', 'begin_nested',
          'close', 'commit', 'connection', 'delete', 'execute', 'expire',
          'expire_all', 'expunge', 'expunge_all', 'flush', 'get_bind',
          'is_modified', 'bulk_save_objects', 'bulk_insert_mappings',
          'bulk_update_mappings',
          'merge', 'query', 'refresh', 'rollback',
          'scalar')
  
      def __init__(self, bind=None, autoflush=True, expire_on_commit=True,
                   _enable_transaction_accounting=True,
                   autocommit=False, twophase=False,
                   weak_identity_map=True, binds=None, extension=None,
                   info=None,
                   query_cls=query.Query):
          r"""Construct a new Session.
  
          See also the :class:`.sessionmaker` function which is used to
          generate a :class:`.Session`-producing callable with a given
          set of arguments.
  
          :param autocommit:
  
            .. warning::
  
               The autocommit flag is **not for general use**, and if it is
               used, queries should only be invoked within the span of a
               :meth:`.Session.begin` / :meth:`.Session.commit` pair.  Executing
               queries outside of a demarcated transaction is a legacy mode
               of usage, and can in some cases lead to concurrent connection
               checkouts.
  
            Defaults to ``False``. When ``True``, the
            :class:`.Session` does not keep a persistent transaction running,
            and will acquire connections from the engine on an as-needed basis,
            returning them immediately after their use. Flushes will begin and
            commit (or possibly rollback) their own transaction if no
            transaction is present. When using this mode, the
            :meth:`.Session.begin` method is used to explicitly start
            transactions.
  
            .. seealso::
  
              :ref:`session_autocommit`
  
          :param autoflush: When ``True``, all query operations will issue a
             :meth:`~.Session.flush` call to this ``Session`` before proceeding.
             This is a convenience feature so that :meth:`~.Session.flush` need
             not be called repeatedly in order for database queries to retrieve
             results. It's typical that ``autoflush`` is used in conjunction
             with ``autocommit=False``. In this scenario, explicit calls to
             :meth:`~.Session.flush` are rarely needed; you usually only need to
             call :meth:`~.Session.commit` (which flushes) to finalize changes.
  
          :param bind: An optional :class:`.Engine` or :class:`.Connection` to
             which this ``Session`` should be bound. When specified, all SQL
             operations performed by this session will execute via this
             connectable.
  
          :param binds: An optional dictionary which contains more granular
             "bind" information than the ``bind`` parameter provides. This
             dictionary can map individual :class`.Table`
             instances as well as :class:`~.Mapper` instances to individual
             :class:`.Engine` or :class:`.Connection` objects. Operations which
             proceed relative to a particular :class:`.Mapper` will consult this
             dictionary for the direct :class:`.Mapper` instance as
             well as the mapper's ``mapped_table`` attribute in order to locate
             a connectable to use. The full resolution is described in the
             :meth:`.Session.get_bind`.
             Usage looks like::
  
              Session = sessionmaker(binds={
                  SomeMappedClass: create_engine('postgresql://engine1'),
                  somemapper: create_engine('postgresql://engine2'),
                  some_table: create_engine('postgresql://engine3'),
                  })
  
            Also see the :meth:`.Session.bind_mapper`
            and :meth:`.Session.bind_table` methods.
  
          :param \class_: Specify an alternate class other than
             ``sqlalchemy.orm.session.Session`` which should be used by the
             returned class. This is the only argument that is local to the
             :class:`.sessionmaker` function, and is not sent directly to the
             constructor for ``Session``.
  
          :param _enable_transaction_accounting:  Defaults to ``True``.  A
             legacy-only flag which when ``False`` disables *all* 0.5-style
             object accounting on transaction boundaries, including auto-expiry
             of instances on rollback and commit, maintenance of the "new" and
             "deleted" lists upon rollback, and autoflush of pending changes
             upon :meth:`~.Session.begin`, all of which are interdependent.
  
          :param expire_on_commit:  Defaults to ``True``. When ``True``, all
             instances will be fully expired after each :meth:`~.commit`,
             so that all attribute/object access subsequent to a completed
             transaction will load from the most recent database state.
  
          :param extension: An optional
             :class:`~.SessionExtension` instance, or a list
             of such instances, which will receive pre- and post- commit and
             flush events, as well as a post-rollback event. **Deprecated.**
             Please see :class:`.SessionEvents`.
  
          :param info: optional dictionary of arbitrary data to be associated
             with this :class:`.Session`.  Is available via the
             :attr:`.Session.info` attribute.  Note the dictionary is copied at
             construction time so that modifications to the per-
             :class:`.Session` dictionary will be local to that
             :class:`.Session`.
  
             .. versionadded:: 0.9.0
  
          :param query_cls:  Class which should be used to create new Query
            objects, as returned by the :meth:`~.Session.query` method.
            Defaults to :class:`.Query`.
  
          :param twophase:  When ``True``, all transactions will be started as
              a "two phase" transaction, i.e. using the "two phase" semantics
              of the database in use along with an XID.  During a
              :meth:`~.commit`, after :meth:`~.flush` has been issued for all
              attached databases, the :meth:`~.TwoPhaseTransaction.prepare`
              method on each database's :class:`.TwoPhaseTransaction` will be
              called. This allows each database to roll back the entire
              transaction, before each transaction is committed.
  
          :param weak_identity_map:  Defaults to ``True`` - when set to
             ``False``, objects placed in the :class:`.Session` will be
             strongly referenced until explicitly removed or the
             :class:`.Session` is closed.  **Deprecated** - The strong
             reference identity map is legacy.  See the
             recipe at :ref:`session_referencing_behavior` for
             an event-based approach to maintaining strong identity
             references.
  
          """
  
          if weak_identity_map:
              self._identity_cls = identity.WeakInstanceDict
          else:
              util.warn_deprecated(
                  "weak_identity_map=False is deprecated.  "
                  "See the documentation on 'Session Referencing Behavior' "
                  "for an event-based approach to maintaining strong identity "
                  "references.")
  
              self._identity_cls = identity.StrongInstanceDict
          self.identity_map = self._identity_cls()
  
          self._new = {}   # InstanceState->object, strong refs object
          self._deleted = {}  # same
          self.bind = bind
          self.__binds = {}
          self._flushing = False
          self._warn_on_events = False
          self.transaction = None
          self.hash_key = _new_sessionid()
          self.autoflush = autoflush
          self.autocommit = autocommit
          self.expire_on_commit = expire_on_commit
          self._enable_transaction_accounting = _enable_transaction_accounting
          self.twophase = twophase
          self._query_cls = query_cls
          if info:
              self.info.update(info)
  
          if extension:
              for ext in util.to_list(extension):
                  SessionExtension._adapt_listener(self, ext)
  
          if binds is not None:
              for key, bind in binds.items():
                  self._add_bind(key, bind)
  
          if not self.autocommit:
              self.begin()
          _sessions[self.hash_key] = self
  
      connection_callable = None
  
      transaction = None
      """The current active or inactive :class:`.SessionTransaction`."""
  
      @util.memoized_property
      def info(self):
          """A user-modifiable dictionary.
  
          The initial value of this dictionary can be populated using the
          ``info`` argument to the :class:`.Session` constructor or
          :class:`.sessionmaker` constructor or factory methods.  The dictionary
          here is always local to this :class:`.Session` and can be modified
          independently of all other :class:`.Session` objects.
  
          .. versionadded:: 0.9.0
  
          """
          return {}
  
      def begin(self, subtransactions=False, nested=False):
          """Begin a transaction on this :class:`.Session`.
  
          The :meth:`.Session.begin` method is only
          meaningful if this session is in **autocommit mode** prior to
          it being called; see :ref:`session_autocommit` for background
          on this setting.
  
          The method will raise an error if this :class:`.Session`
          is already inside of a transaction, unless
          :paramref:`.Session.begin.subtransactions` or
          :paramref:`.Session.begin.nested` are specified.
  
          :param subtransactions: if True, indicates that this
           :meth:`~.Session.begin` can create a subtransaction if a transaction
           is already in progress. For documentation on subtransactions, please
           see :ref:`session_subtransactions`.
  
          :param nested: if True, begins a SAVEPOINT transaction and is equivalent
           to calling :meth:`~.Session.begin_nested`. For documentation on
           SAVEPOINT transactions, please see :ref:`session_begin_nested`.
  
          :return: the :class:`.SessionTransaction` object.  Note that
           :class:`.SessionTransaction`
           acts as a Python context manager, allowing :meth:`.Session.begin`
           to be used in a "with" block.  See :ref:`session_autocommit` for
           an example.
  
          .. seealso::
  
              :ref:`session_autocommit`
  
              :meth:`.Session.begin_nested`
  
  
          """
          if self.transaction is not None:
              if subtransactions or nested:
                  self.transaction = self.transaction._begin(
                      nested=nested)
              else:
                  raise sa_exc.InvalidRequestError(
                      "A transaction is already begun.  Use "
                      "subtransactions=True to allow subtransactions.")
          else:
              self.transaction = SessionTransaction(
                  self, nested=nested)
          return self.transaction  # needed for __enter__/__exit__ hook
  
      def begin_nested(self):
          """Begin a "nested" transaction on this Session, e.g. SAVEPOINT.
  
          The target database(s) and associated drivers must support SQL
          SAVEPOINT for this method to function correctly.
  
          For documentation on SAVEPOINT
          transactions, please see :ref:`session_begin_nested`.
  
          :return: the :class:`.SessionTransaction` object.  Note that
           :class:`.SessionTransaction` acts as a context manager, allowing
           :meth:`.Session.begin_nested` to be used in a "with" block.
           See :ref:`session_begin_nested` for a usage example.
  
          .. seealso::
  
              :ref:`session_begin_nested`
  
              :ref:`pysqlite_serializable` - special workarounds required
              with the SQLite driver in order for SAVEPOINT to work
              correctly.
  
          """
          return self.begin(nested=True)
  
      def rollback(self):
          """Rollback the current transaction in progress.
  
          If no transaction is in progress, this method is a pass-through.
  
          This method rolls back the current transaction or nested transaction
          regardless of subtransactions being in effect.  All subtransactions up
          to the first real transaction are closed.  Subtransactions occur when
          :meth:`.begin` is called multiple times.
  
          .. seealso::
  
              :ref:`session_rollback`
  
          """
          if self.transaction is None:
              pass
          else:
              self.transaction.rollback()
  
      def commit(self):
          """Flush pending changes and commit the current transaction.
  
          If no transaction is in progress, this method raises an
          :exc:`~sqlalchemy.exc.InvalidRequestError`.
  
          By default, the :class:`.Session` also expires all database
          loaded state on all ORM-managed attributes after transaction commit.
          This so that subsequent operations load the most recent
          data from the database.   This behavior can be disabled using
          the ``expire_on_commit=False`` option to :class:`.sessionmaker` or
          the :class:`.Session` constructor.
  
          If a subtransaction is in effect (which occurs when begin() is called
          multiple times), the subtransaction will be closed, and the next call
          to ``commit()`` will operate on the enclosing transaction.
  
          When using the :class:`.Session` in its default mode of
          ``autocommit=False``, a new transaction will
          be begun immediately after the commit, but note that the newly begun
          transaction does *not* use any connection resources until the first
          SQL is actually emitted.
  
          .. seealso::
  
              :ref:`session_committing`
  
          """
          if self.transaction is None:
              if not self.autocommit:
                  self.begin()
              else:
                  raise sa_exc.InvalidRequestError("No transaction is begun.")
  
          self.transaction.commit()
  
      def prepare(self):
          """Prepare the current transaction in progress for two phase commit.
  
          If no transaction is in progress, this method raises an
          :exc:`~sqlalchemy.exc.InvalidRequestError`.
  
          Only root transactions of two phase sessions can be prepared. If the
          current transaction is not such, an
          :exc:`~sqlalchemy.exc.InvalidRequestError` is raised.
  
          """
          if self.transaction is None:
              if not self.autocommit:
                  self.begin()
              else:
                  raise sa_exc.InvalidRequestError("No transaction is begun.")
  
          self.transaction.prepare()
  
      def connection(self, mapper=None, clause=None,
                     bind=None,
                     close_with_result=False,
                     execution_options=None,
                     **kw):
          r"""Return a :class:`.Connection` object corresponding to this
          :class:`.Session` object's transactional state.
  
          If this :class:`.Session` is configured with ``autocommit=False``,
          either the :class:`.Connection` corresponding to the current
          transaction is returned, or if no transaction is in progress, a new
          one is begun and the :class:`.Connection` returned (note that no
          transactional state is established with the DBAPI until the first
          SQL statement is emitted).
  
          Alternatively, if this :class:`.Session` is configured with
          ``autocommit=True``, an ad-hoc :class:`.Connection` is returned
          using :meth:`.Engine.contextual_connect` on the underlying
          :class:`.Engine`.
  
          Ambiguity in multi-bind or unbound :class:`.Session` objects can be
          resolved through any of the optional keyword arguments.   This
          ultimately makes usage of the :meth:`.get_bind` method for resolution.
  
          :param bind:
            Optional :class:`.Engine` to be used as the bind.  If
            this engine is already involved in an ongoing transaction,
            that connection will be used.  This argument takes precedence
            over ``mapper``, ``clause``.
  
          :param mapper:
            Optional :func:`.mapper` mapped class, used to identify
            the appropriate bind.  This argument takes precedence over
            ``clause``.
  
          :param clause:
              A :class:`.ClauseElement` (i.e. :func:`~.sql.expression.select`,
              :func:`~.sql.expression.text`,
              etc.) which will be used to locate a bind, if a bind
              cannot otherwise be identified.
  
          :param close_with_result: Passed to :meth:`.Engine.connect`,
            indicating the :class:`.Connection` should be considered
            "single use", automatically closing when the first result set is
            closed.  This flag only has an effect if this :class:`.Session` is
            configured with ``autocommit=True`` and does not already have a
            transaction in progress.
  
          :param execution_options: a dictionary of execution options that will
           be passed to :meth:`.Connection.execution_options`, **when the
           connection is first procured only**.   If the connection is already
           present within the :class:`.Session`, a warning is emitted and
           the arguments are ignored.
  
           .. versionadded:: 0.9.9
  
           .. seealso::
  
              :ref:`session_transaction_isolation`
  
          :param \**kw:
            Additional keyword arguments are sent to :meth:`get_bind()`,
            allowing additional arguments to be passed to custom
            implementations of :meth:`get_bind`.
  
          """
          if bind is None:
              bind = self.get_bind(mapper, clause=clause, **kw)
  
          return self._connection_for_bind(bind,
                                           close_with_result=close_with_result,
                                           execution_options=execution_options)
  
      def _connection_for_bind(self, engine, execution_options=None, **kw):
          if self.transaction is not None:
              return self.transaction._connection_for_bind(
                  engine, execution_options)
          else:
              conn = engine.contextual_connect(**kw)
              if execution_options:
                  conn = conn.execution_options(**execution_options)
              return conn
  
      def execute(self, clause, params=None, mapper=None, bind=None, **kw):
          r"""Execute a SQL expression construct or string statement within
          the current transaction.
  
          Returns a :class:`.ResultProxy` representing
          results of the statement execution, in the same manner as that of an
          :class:`.Engine` or
          :class:`.Connection`.
  
          E.g.::
  
              result = session.execute(
                          user_table.select().where(user_table.c.id == 5)
                      )
  
          :meth:`~.Session.execute` accepts any executable clause construct,
          such as :func:`~.sql.expression.select`,
          :func:`~.sql.expression.insert`,
          :func:`~.sql.expression.update`,
          :func:`~.sql.expression.delete`, and
          :func:`~.sql.expression.text`.  Plain SQL strings can be passed
          as well, which in the case of :meth:`.Session.execute` only
          will be interpreted the same as if it were passed via a
          :func:`~.expression.text` construct.  That is, the following usage::
  
              result = session.execute(
                          "SELECT * FROM user WHERE id=:param",
                          {"param":5}
                      )
  
          is equivalent to::
  
              from sqlalchemy import text
              result = session.execute(
                          text("SELECT * FROM user WHERE id=:param"),
                          {"param":5}
                      )
  
          The second positional argument to :meth:`.Session.execute` is an
          optional parameter set.  Similar to that of
          :meth:`.Connection.execute`, whether this is passed as a single
          dictionary, or a list of dictionaries, determines whether the DBAPI
          cursor's ``execute()`` or ``executemany()`` is used to execute the
          statement.   An INSERT construct may be invoked for a single row::
  
              result = session.execute(
                  users.insert(), {"id": 7, "name": "somename"})
  
          or for multiple rows::
  
              result = session.execute(users.insert(), [
                                      {"id": 7, "name": "somename7"},
                                      {"id": 8, "name": "somename8"},
                                      {"id": 9, "name": "somename9"}
                                  ])
  
          The statement is executed within the current transactional context of
          this :class:`.Session`.   The :class:`.Connection` which is used
          to execute the statement can also be acquired directly by
          calling the :meth:`.Session.connection` method.  Both methods use
          a rule-based resolution scheme in order to determine the
          :class:`.Connection`, which in the average case is derived directly
          from the "bind" of the :class:`.Session` itself, and in other cases
          can be based on the :func:`.mapper`
          and :class:`.Table` objects passed to the method; see the
          documentation for :meth:`.Session.get_bind` for a full description of
          this scheme.
  
          The :meth:`.Session.execute` method does *not* invoke autoflush.
  
          The :class:`.ResultProxy` returned by the :meth:`.Session.execute`
          method is returned with the "close_with_result" flag set to true;
          the significance of this flag is that if this :class:`.Session` is
          autocommitting and does not have a transaction-dedicated
          :class:`.Connection` available, a temporary :class:`.Connection` is
          established for the statement execution, which is closed (meaning,
          returned to the connection pool) when the :class:`.ResultProxy` has
          consumed all available data. This applies *only* when the
          :class:`.Session` is configured with autocommit=True and no
          transaction has been started.
  
          :param clause:
              An executable statement (i.e. an :class:`.Executable` expression
              such as :func:`.expression.select`) or string SQL statement
              to be executed.
  
          :param params:
              Optional dictionary, or list of dictionaries, containing
              bound parameter values.   If a single dictionary, single-row
              execution occurs; if a list of dictionaries, an
              "executemany" will be invoked.  The keys in each dictionary
              must correspond to parameter names present in the statement.
  
          :param mapper:
            Optional :func:`.mapper` or mapped class, used to identify
            the appropriate bind.  This argument takes precedence over
            ``clause`` when locating a bind.   See :meth:`.Session.get_bind`
            for more details.
  
          :param bind:
            Optional :class:`.Engine` to be used as the bind.  If
            this engine is already involved in an ongoing transaction,
            that connection will be used.  This argument takes
            precedence over ``mapper`` and ``clause`` when locating
            a bind.
  
          :param \**kw:
            Additional keyword arguments are sent to :meth:`.Session.get_bind()`
            to allow extensibility of "bind" schemes.
  
          .. seealso::
  
              :ref:`sqlexpression_toplevel` - Tutorial on using Core SQL
              constructs.
  
              :ref:`connections_toplevel` - Further information on direct
              statement execution.
  
              :meth:`.Connection.execute` - core level statement execution
              method, which is :meth:`.Session.execute` ultimately uses
              in order to execute the statement.
  
          """
          clause = expression._literal_as_text(clause)
  
          if bind is None:
              bind = self.get_bind(mapper, clause=clause, **kw)
  
          return self._connection_for_bind(
              bind, close_with_result=True).execute(clause, params or {})
  
      def scalar(self, clause, params=None, mapper=None, bind=None, **kw):
          """Like :meth:`~.Session.execute` but return a scalar result."""
  
          return self.execute(
              clause, params=params, mapper=mapper, bind=bind, **kw).scalar()
  
      def close(self):
          """Close this Session.
  
          This clears all items and ends any transaction in progress.
  
          If this session were created with ``autocommit=False``, a new
          transaction is immediately begun.  Note that this new transaction does
          not use any connection resources until they are first needed.
  
          """
          self._close_impl(invalidate=False)
  
      def invalidate(self):
          """Close this Session, using connection invalidation.
  
          This is a variant of :meth:`.Session.close` that will additionally
          ensure that the :meth:`.Connection.invalidate` method will be called
          on all :class:`.Connection` objects.  This can be called when
          the database is known to be in a state where the connections are
          no longer safe to be used.
  
          E.g.::
  
              try:
                  sess = Session()
                  sess.add(User())
                  sess.commit()
              except gevent.Timeout:
                  sess.invalidate()
                  raise
              except:
                  sess.rollback()
                  raise
  
          This clears all items and ends any transaction in progress.
  
          If this session were created with ``autocommit=False``, a new
          transaction is immediately begun.  Note that this new transaction does
          not use any connection resources until they are first needed.
  
          .. versionadded:: 0.9.9
  
          """
          self._close_impl(invalidate=True)
  
      def _close_impl(self, invalidate):
          self.expunge_all()
          if self.transaction is not None:
              for transaction in self.transaction._iterate_self_and_parents():
                  transaction.close(invalidate)
  
      def expunge_all(self):
          """Remove all object instances from this ``Session``.
  
          This is equivalent to calling ``expunge(obj)`` on all objects in this
          ``Session``.
  
          """
  
          all_states = self.identity_map.all_states() + list(self._new)
          self.identity_map = self._identity_cls()
          self._new = {}
          self._deleted = {}
  
          statelib.InstanceState._detach_states(
              all_states, self
          )
  
      def _add_bind(self, key, bind):
          try:
              insp = inspect(key)
          except sa_exc.NoInspectionAvailable:
              if not isinstance(key, type):
                  raise sa_exc.ArgumentError(
                      "Not an acceptable bind target: %s" % key)
              else:
                  self.__binds[key] = bind
          else:
              if insp.is_selectable:
                  self.__binds[insp] = bind
              elif insp.is_mapper:
                  self.__binds[insp.class_] = bind
                  for selectable in insp._all_tables:
                      self.__binds[selectable] = bind
              else:
                  raise sa_exc.ArgumentError(
                      "Not an acceptable bind target: %s" % key)
  
      def bind_mapper(self, mapper, bind):
          """Associate a :class:`.Mapper` with a "bind", e.g. a :class:`.Engine`
          or :class:`.Connection`.
  
          The given mapper is added to a lookup used by the
          :meth:`.Session.get_bind` method.
  
          """
          self._add_bind(mapper, bind)
  
      def bind_table(self, table, bind):
          """Associate a :class:`.Table` with a "bind", e.g. a :class:`.Engine`
          or :class:`.Connection`.
  
          The given mapper is added to a lookup used by the
          :meth:`.Session.get_bind` method.
  
          """
          self._add_bind(table, bind)
  
      def get_bind(self, mapper=None, clause=None):
          """Return a "bind" to which this :class:`.Session` is bound.
  
          The "bind" is usually an instance of :class:`.Engine`,
          except in the case where the :class:`.Session` has been
          explicitly bound directly to a :class:`.Connection`.
  
          For a multiply-bound or unbound :class:`.Session`, the
          ``mapper`` or ``clause`` arguments are used to determine the
          appropriate bind to return.
  
          Note that the "mapper" argument is usually present
          when :meth:`.Session.get_bind` is called via an ORM
          operation such as a :meth:`.Session.query`, each
          individual INSERT/UPDATE/DELETE operation within a
          :meth:`.Session.flush`, call, etc.
  
          The order of resolution is:
  
          1. if mapper given and session.binds is present,
             locate a bind based on mapper.
          2. if clause given and session.binds is present,
             locate a bind based on :class:`.Table` objects
             found in the given clause present in session.binds.
          3. if session.bind is present, return that.
          4. if clause given, attempt to return a bind
             linked to the :class:`.MetaData` ultimately
             associated with the clause.
          5. if mapper given, attempt to return a bind
             linked to the :class:`.MetaData` ultimately
             associated with the :class:`.Table` or other
             selectable to which the mapper is mapped.
          6. No bind can be found, :exc:`~sqlalchemy.exc.UnboundExecutionError`
             is raised.
  
          :param mapper:
            Optional :func:`.mapper` mapped class or instance of
            :class:`.Mapper`.   The bind can be derived from a :class:`.Mapper`
            first by consulting the "binds" map associated with this
            :class:`.Session`, and secondly by consulting the :class:`.MetaData`
            associated with the :class:`.Table` to which the :class:`.Mapper`
            is mapped for a bind.
  
          :param clause:
              A :class:`.ClauseElement` (i.e. :func:`~.sql.expression.select`,
              :func:`~.sql.expression.text`,
              etc.).  If the ``mapper`` argument is not present or could not
              produce a bind, the given expression construct will be searched
              for a bound element, typically a :class:`.Table` associated with
              bound :class:`.MetaData`.
  
          """
  
          if mapper is clause is None:
              if self.bind:
                  return self.bind
              else:
                  raise sa_exc.UnboundExecutionError(
                      "This session is not bound to a single Engine or "
                      "Connection, and no context was provided to locate "
                      "a binding.")
  
          if mapper is not None:
              try:
                  mapper = inspect(mapper)
              except sa_exc.NoInspectionAvailable:
                  if isinstance(mapper, type):
                      raise exc.UnmappedClassError(mapper)
                  else:
                      raise
  
          if self.__binds:
              if mapper:
                  for cls in mapper.class_.__mro__:
                      if cls in self.__binds:
                          return self.__binds[cls]
                  if clause is None:
                      clause = mapper.mapped_table
  
              if clause is not None:
                  for t in sql_util.find_tables(clause, include_crud=True):
                      if t in self.__binds:
                          return self.__binds[t]
  
          if self.bind:
              return self.bind
  
          if isinstance(clause, sql.expression.ClauseElement) and clause.bind:
              return clause.bind
  
          if mapper and mapper.mapped_table.bind:
              return mapper.mapped_table.bind
  
          context = []
          if mapper is not None:
              context.append('mapper %s' % mapper)
          if clause is not None:
              context.append('SQL expression')
  
          raise sa_exc.UnboundExecutionError(
              "Could not locate a bind configured on %s or this Session" % (
                  ', '.join(context)))
  
      def query(self, *entities, **kwargs):
          """Return a new :class:`.Query` object corresponding to this
          :class:`.Session`."""
  
          return self._query_cls(entities, self, **kwargs)
  
      @property
      @util.contextmanager
      def no_autoflush(self):
          """Return a context manager that disables autoflush.
  
          e.g.::
  
              with session.no_autoflush:
  
                  some_object = SomeClass()
                  session.add(some_object)
                  # won't autoflush
                  some_object.related_thing = session.query(SomeRelated).first()
  
          Operations that proceed within the ``with:`` block
          will not be subject to flushes occurring upon query
          access.  This is useful when initializing a series
          of objects which involve existing database queries,
          where the uncompleted object should not yet be flushed.
  
          .. versionadded:: 0.7.6
  
          """
          autoflush = self.autoflush
          self.autoflush = False
          try:
              yield self
          finally:
              self.autoflush = autoflush
  
      def _autoflush(self):
          if self.autoflush and not self._flushing:
              try:
                  self.flush()
              except sa_exc.StatementError as e:
                  # note we are reraising StatementError as opposed to
                  # raising FlushError with "chaining" to remain compatible
                  # with code that catches StatementError, IntegrityError,
                  # etc.
                  e.add_detail(
                      "raised as a result of Query-invoked autoflush; "
                      "consider using a session.no_autoflush block if this "
                      "flush is occurring prematurely")
                  util.raise_from_cause(e)
  
      def refresh(self, instance, attribute_names=None, lockmode=None):
          """Expire and refresh the attributes on the given instance.
  
          A query will be issued to the database and all attributes will be
          refreshed with their current database value.
  
          Lazy-loaded relational attributes will remain lazily loaded, so that
          the instance-wide refresh operation will be followed immediately by
          the lazy load of that attribute.
  
          Eagerly-loaded relational attributes will eagerly load within the
          single refresh operation.
  
          Note that a highly isolated transaction will return the same values as
          were previously read in that same transaction, regardless of changes
          in database state outside of that transaction - usage of
          :meth:`~Session.refresh` usually only makes sense if non-ORM SQL
          statement were emitted in the ongoing transaction, or if autocommit
          mode is turned on.
  
          :param attribute_names: optional.  An iterable collection of
            string attribute names indicating a subset of attributes to
            be refreshed.
  
          :param lockmode: Passed to the :class:`~sqlalchemy.orm.query.Query`
            as used by :meth:`~sqlalchemy.orm.query.Query.with_lockmode`.
  
          .. seealso::
  
              :ref:`session_expire` - introductory material
  
              :meth:`.Session.expire`
  
              :meth:`.Session.expire_all`
  
          """
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
  
          self._expire_state(state, attribute_names)
  
          if loading.load_on_ident(
                  self.query(object_mapper(instance)),
                  state.key, refresh_state=state,
                  lockmode=lockmode,
                  only_load_props=attribute_names) is None:
              raise sa_exc.InvalidRequestError(
                  "Could not refresh instance '%s'" %
                  instance_str(instance))
  
      def expire_all(self):
          """Expires all persistent instances within this Session.
  
          When any attributes on a persistent instance is next accessed,
          a query will be issued using the
          :class:`.Session` object's current transactional context in order to
          load all expired attributes for the given instance.   Note that
          a highly isolated transaction will return the same values as were
          previously read in that same transaction, regardless of changes
          in database state outside of that transaction.
  
          To expire individual objects and individual attributes
          on those objects, use :meth:`Session.expire`.
  
          The :class:`.Session` object's default behavior is to
          expire all state whenever the :meth:`Session.rollback`
          or :meth:`Session.commit` methods are called, so that new
          state can be loaded for the new transaction.   For this reason,
          calling :meth:`Session.expire_all` should not be needed when
          autocommit is ``False``, assuming the transaction is isolated.
  
          .. seealso::
  
              :ref:`session_expire` - introductory material
  
              :meth:`.Session.expire`
  
              :meth:`.Session.refresh`
  
          """
          for state in self.identity_map.all_states():
              state._expire(state.dict, self.identity_map._modified)
  
      def expire(self, instance, attribute_names=None):
          """Expire the attributes on an instance.
  
          Marks the attributes of an instance as out of date. When an expired
          attribute is next accessed, a query will be issued to the
          :class:`.Session` object's current transactional context in order to
          load all expired attributes for the given instance.   Note that
          a highly isolated transaction will return the same values as were
          previously read in that same transaction, regardless of changes
          in database state outside of that transaction.
  
          To expire all objects in the :class:`.Session` simultaneously,
          use :meth:`Session.expire_all`.
  
          The :class:`.Session` object's default behavior is to
          expire all state whenever the :meth:`Session.rollback`
          or :meth:`Session.commit` methods are called, so that new
          state can be loaded for the new transaction.   For this reason,
          calling :meth:`Session.expire` only makes sense for the specific
          case that a non-ORM SQL statement was emitted in the current
          transaction.
  
          :param instance: The instance to be refreshed.
          :param attribute_names: optional list of string attribute names
            indicating a subset of attributes to be expired.
  
          .. seealso::
  
              :ref:`session_expire` - introductory material
  
              :meth:`.Session.expire`
  
              :meth:`.Session.refresh`
  
          """
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
          self._expire_state(state, attribute_names)
  
      def _expire_state(self, state, attribute_names):
          self._validate_persistent(state)
          if attribute_names:
              state._expire_attributes(state.dict, attribute_names)
          else:
              # pre-fetch the full cascade since the expire is going to
              # remove associations
              cascaded = list(state.manager.mapper.cascade_iterator(
                  'refresh-expire', state))
              self._conditional_expire(state)
              for o, m, st_, dct_ in cascaded:
                  self._conditional_expire(st_)
  
      def _conditional_expire(self, state):
          """Expire a state if persistent, else expunge if pending"""
  
          if state.key:
              state._expire(state.dict, self.identity_map._modified)
          elif state in self._new:
              self._new.pop(state)
              state._detach(self)
  
      @util.deprecated("0.7", "The non-weak-referencing identity map "
                       "feature is no longer needed.")
      def prune(self):
          """Remove unreferenced instances cached in the identity map.
  
          Note that this method is only meaningful if "weak_identity_map" is set
          to False.  The default weak identity map is self-pruning.
  
          Removes any object in this Session's identity map that is not
          referenced in user code, modified, new or scheduled for deletion.
          Returns the number of objects pruned.
  
          """
          return self.identity_map.prune()
  
      def expunge(self, instance):
          """Remove the `instance` from this ``Session``.
  
          This will free all internal references to the instance.  Cascading
          will be applied according to the *expunge* cascade rule.
  
          """
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
          if state.session_id is not self.hash_key:
              raise sa_exc.InvalidRequestError(
                  "Instance %s is not present in this Session" %
                  state_str(state))
  
          cascaded = list(state.manager.mapper.cascade_iterator(
              'expunge', state))
          self._expunge_states(
              [state] + [st_ for o, m, st_, dct_ in cascaded]
          )
  
      def _expunge_states(self, states, to_transient=False):
          for state in states:
              if state in self._new:
                  self._new.pop(state)
              elif self.identity_map.contains_state(state):
                  self.identity_map.safe_discard(state)
                  self._deleted.pop(state, None)
              elif self.transaction:
                  # state is "detached" from being deleted, but still present
                  # in the transaction snapshot
                  self.transaction._deleted.pop(state, None)
          statelib.InstanceState._detach_states(
              states, self, to_transient=to_transient)
  
      def _register_newly_persistent(self, states):
          pending_to_persistent = self.dispatch.pending_to_persistent or None
          for state in states:
              mapper = _state_mapper(state)
  
              # prevent against last minute dereferences of the object
              obj = state.obj()
              if obj is not None:
  
                  instance_key = mapper._identity_key_from_state(state)
  
                  if _none_set.intersection(instance_key[1]) and \
                          not mapper.allow_partial_pks or \
                          _none_set.issuperset(instance_key[1]):
                      raise exc.FlushError(
                          "Instance %s has a NULL identity key.  If this is an "
                          "auto-generated value, check that the database table "
                          "allows generation of new primary key values, and "
                          "that the mapped Column object is configured to "
                          "expect these generated values.  Ensure also that "
                          "this flush() is not occurring at an inappropriate "
                          "time, such as within a load() event."
                          % state_str(state)
                      )
  
                  if state.key is None:
                      state.key = instance_key
                  elif state.key != instance_key:
                      # primary key switch. use safe_discard() in case another
                      # state has already replaced this one in the identity
                      # map (see test/orm/test_naturalpks.py ReversePKsTest)
                      self.identity_map.safe_discard(state)
                      if state in self.transaction._key_switches:
                          orig_key = self.transaction._key_switches[state][0]
                      else:
                          orig_key = state.key
                      self.transaction._key_switches[state] = (
                          orig_key, instance_key)
                      state.key = instance_key
  
                  self.identity_map.replace(state)
  
          statelib.InstanceState._commit_all_states(
              ((state, state.dict) for state in states),
              self.identity_map
          )
  
          self._register_altered(states)
  
          if pending_to_persistent is not None:
              for state in states:
                  pending_to_persistent(self, state.obj())
  
          # remove from new last, might be the last strong ref
          for state in set(states).intersection(self._new):
              self._new.pop(state)
  
      def _register_altered(self, states):
          if self._enable_transaction_accounting and self.transaction:
              for state in states:
                  if state in self._new:
                      self.transaction._new[state] = True
                  else:
                      self.transaction._dirty[state] = True
  
      def _remove_newly_deleted(self, states):
          persistent_to_deleted = self.dispatch.persistent_to_deleted or None
          for state in states:
              if self._enable_transaction_accounting and self.transaction:
                  self.transaction._deleted[state] = True
  
              if persistent_to_deleted is not None:
                  # get a strong reference before we pop out of
                  # self._deleted
                  obj = state.obj()
  
              self.identity_map.safe_discard(state)
              self._deleted.pop(state, None)
              state._deleted = True
              # can't call state._detach() here, because this state
              # is still in the transaction snapshot and needs to be
              # tracked as part of that
              if persistent_to_deleted is not None:
                  persistent_to_deleted(self, obj)
  
      def add(self, instance, _warn=True):
          """Place an object in the ``Session``.
  
          Its state will be persisted to the database on the next flush
          operation.
  
          Repeated calls to ``add()`` will be ignored. The opposite of ``add()``
          is ``expunge()``.
  
          """
          if _warn and self._warn_on_events:
              self._flush_warning("Session.add()")
  
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
  
          self._save_or_update_state(state)
  
      def add_all(self, instances):
          """Add the given collection of instances to this ``Session``."""
  
          if self._warn_on_events:
              self._flush_warning("Session.add_all()")
  
          for instance in instances:
              self.add(instance, _warn=False)
  
      def _save_or_update_state(self, state):
          self._save_or_update_impl(state)
  
          mapper = _state_mapper(state)
          for o, m, st_, dct_ in mapper.cascade_iterator(
                  'save-update',
                  state,
                  halt_on=self._contains_state):
              self._save_or_update_impl(st_)
  
      def delete(self, instance):
          """Mark an instance as deleted.
  
          The database delete operation occurs upon ``flush()``.
  
          """
          if self._warn_on_events:
              self._flush_warning("Session.delete()")
  
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
  
          self._delete_impl(state, instance, head=True)
  
      def _delete_impl(self, state, obj, head):
  
          if state.key is None:
              if head:
                  raise sa_exc.InvalidRequestError(
                      "Instance '%s' is not persisted" %
                      state_str(state))
              else:
                  return
  
          to_attach = self._before_attach(state, obj)
  
          if state in self._deleted:
              return
  
          self.identity_map.add(state)
  
          if to_attach:
              self._after_attach(state, obj)
  
          if head:
              # grab the cascades before adding the item to the deleted list
              # so that autoflush does not delete the item
              # the strong reference to the instance itself is significant here
              cascade_states = list(state.manager.mapper.cascade_iterator(
                  'delete', state))
  
          self._deleted[state] = obj
  
          if head:
              for o, m, st_, dct_ in cascade_states:
                  self._delete_impl(st_, o, False)
  
      def merge(self, instance, load=True):
          """Copy the state of a given instance into a corresponding instance
          within this :class:`.Session`.
  
          :meth:`.Session.merge` examines the primary key attributes of the
          source instance, and attempts to reconcile it with an instance of the
          same primary key in the session.   If not found locally, it attempts
          to load the object from the database based on primary key, and if
          none can be located, creates a new instance.  The state of each
          attribute on the source instance is then copied to the target
          instance.  The resulting target instance is then returned by the
          method; the original source instance is left unmodified, and
          un-associated with the :class:`.Session` if not already.
  
          This operation cascades to associated instances if the association is
          mapped with ``cascade="merge"``.
  
          See :ref:`unitofwork_merging` for a detailed discussion of merging.
  
          .. versionchanged:: 1.1 - :meth:`.Session.merge` will now reconcile
             pending objects with overlapping primary keys in the same way
             as persistent.  See :ref:`change_3601` for discussion.
  
          :param instance: Instance to be merged.
          :param load: Boolean, when False, :meth:`.merge` switches into
           a "high performance" mode which causes it to forego emitting history
           events as well as all database access.  This flag is used for
           cases such as transferring graphs of objects into a :class:`.Session`
           from a second level cache, or to transfer just-loaded objects
           into the :class:`.Session` owned by a worker thread or process
           without re-querying the database.
  
           The ``load=False`` use case adds the caveat that the given
           object has to be in a "clean" state, that is, has no pending changes
           to be flushed - even if the incoming object is detached from any
           :class:`.Session`.   This is so that when
           the merge operation populates local attributes and
           cascades to related objects and
           collections, the values can be "stamped" onto the
           target object as is, without generating any history or attribute
           events, and without the need to reconcile the incoming data with
           any existing related objects or collections that might not
           be loaded.  The resulting objects from ``load=False`` are always
           produced as "clean", so it is only appropriate that the given objects
           should be "clean" as well, else this suggests a mis-use of the
           method.
  
  
          """
  
          if self._warn_on_events:
              self._flush_warning("Session.merge()")
  
          _recursive = {}
          _resolve_conflict_map = {}
  
          if load:
              # flush current contents if we expect to load data
              self._autoflush()
  
          object_mapper(instance)  # verify mapped
          autoflush = self.autoflush
          try:
              self.autoflush = False
              return self._merge(
                  attributes.instance_state(instance),
                  attributes.instance_dict(instance),
                  load=load, _recursive=_recursive,
                  _resolve_conflict_map=_resolve_conflict_map)
          finally:
              self.autoflush = autoflush
  
      def _merge(self, state, state_dict, load=True, _recursive=None,
                 _resolve_conflict_map=None):
          mapper = _state_mapper(state)
          if state in _recursive:
              return _recursive[state]
  
          new_instance = False
          key = state.key
  
          if key is None:
              if not load:
                  raise sa_exc.InvalidRequestError(
                      "merge() with load=False option does not support "
                      "objects transient (i.e. unpersisted) objects.  flush() "
                      "all changes on mapped instances before merging with "
                      "load=False.")
              key = mapper._identity_key_from_state(state)
              key_is_persistent = attributes.NEVER_SET not in key[1] and (
                  not _none_set.intersection(key[1]) or
                  (mapper.allow_partial_pks and not _none_set.issuperset(key[1]))
              )
          else:
              key_is_persistent = True
  
          if key in self.identity_map:
              try:
                  merged = self.identity_map[key]
              except KeyError:
                  # object was GC'ed right as we checked for it
                  merged = None
          else:
              merged = None
  
          if merged is None:
              if key_is_persistent and key in _resolve_conflict_map:
                  merged = _resolve_conflict_map[key]
  
              elif not load:
                  if state.modified:
                      raise sa_exc.InvalidRequestError(
                          "merge() with load=False option does not support "
                          "objects marked as 'dirty'.  flush() all changes on "
                          "mapped instances before merging with load=False.")
                  merged = mapper.class_manager.new_instance()
                  merged_state = attributes.instance_state(merged)
                  merged_state.key = key
                  self._update_impl(merged_state)
                  new_instance = True
  
              elif key_is_persistent:
                  merged = self.query(mapper.class_).get(key[1])
  
          if merged is None:
              merged = mapper.class_manager.new_instance()
              merged_state = attributes.instance_state(merged)
              merged_dict = attributes.instance_dict(merged)
              new_instance = True
              self._save_or_update_state(merged_state)
          else:
              merged_state = attributes.instance_state(merged)
              merged_dict = attributes.instance_dict(merged)
  
          _recursive[state] = merged
          _resolve_conflict_map[key] = merged
  
          # check that we didn't just pull the exact same
          # state out.
          if state is not merged_state:
              # version check if applicable
              if mapper.version_id_col is not None:
                  existing_version = mapper._get_state_attr_by_column(
                      state,
                      state_dict,
                      mapper.version_id_col,
                      passive=attributes.PASSIVE_NO_INITIALIZE)
  
                  merged_version = mapper._get_state_attr_by_column(
                      merged_state,
                      merged_dict,
                      mapper.version_id_col,
                      passive=attributes.PASSIVE_NO_INITIALIZE)
  
                  if existing_version is not attributes.PASSIVE_NO_RESULT and \
                          merged_version is not attributes.PASSIVE_NO_RESULT and \
                          existing_version != merged_version:
                      raise exc.StaleDataError(
                          "Version id '%s' on merged state %s "
                          "does not match existing version '%s'. "
                          "Leave the version attribute unset when "
                          "merging to update the most recent version."
                          % (
                              existing_version,
                              state_str(merged_state),
                              merged_version
                          ))
  
              merged_state.load_path = state.load_path
              merged_state.load_options = state.load_options
  
              # since we are copying load_options, we need to copy
              # the callables_ that would have been generated by those
              # load_options.
              # assumes that the callables we put in state.callables_
              # are not instance-specific (which they should not be)
              merged_state._copy_callables(state)
  
              for prop in mapper.iterate_properties:
                  prop.merge(self, state, state_dict,
                             merged_state, merged_dict,
                             load, _recursive, _resolve_conflict_map)
  
          if not load:
              # remove any history
              merged_state._commit_all(merged_dict, self.identity_map)
  
          if new_instance:
              merged_state.manager.dispatch.load(merged_state, None)
          return merged
  
      def _validate_persistent(self, state):
          if not self.identity_map.contains_state(state):
              raise sa_exc.InvalidRequestError(
                  "Instance '%s' is not persistent within this Session" %
                  state_str(state))
  
      def _save_impl(self, state):
          if state.key is not None:
              raise sa_exc.InvalidRequestError(
                  "Object '%s' already has an identity - "
                  "it can't be registered as pending" % state_str(state))
  
          obj = state.obj()
          to_attach = self._before_attach(state, obj)
          if state not in self._new:
              self._new[state] = obj
              state.insert_order = len(self._new)
          if to_attach:
              self._after_attach(state, obj)
  
      def _update_impl(self, state, revert_deletion=False):
          if state.key is None:
              raise sa_exc.InvalidRequestError(
                  "Instance '%s' is not persisted" %
                  state_str(state))
  
          if state._deleted:
              if revert_deletion:
                  if not state._attached:
                      return
                  del state._deleted
              else:
                  raise sa_exc.InvalidRequestError(
                      "Instance '%s' has been deleted.  "
                      "Use the make_transient() "
                      "function to send this object back "
                      "to the transient state." %
                      state_str(state)
                  )
  
          obj = state.obj()
  
          # check for late gc
          if obj is None:
              return
  
          to_attach = self._before_attach(state, obj)
  
          self._deleted.pop(state, None)
          if revert_deletion:
              self.identity_map.replace(state)
          else:
              self.identity_map.add(state)
  
          if to_attach:
              self._after_attach(state, obj)
          elif revert_deletion:
              self.dispatch.deleted_to_persistent(self, obj)
  
      def _save_or_update_impl(self, state):
          if state.key is None:
              self._save_impl(state)
          else:
              self._update_impl(state)
  
      def enable_relationship_loading(self, obj):
          """Associate an object with this :class:`.Session` for related
          object loading.
  
          .. warning::
  
              :meth:`.enable_relationship_loading` exists to serve special
              use cases and is not recommended for general use.
  
          Accesses of attributes mapped with :func:`.relationship`
          will attempt to load a value from the database using this
          :class:`.Session` as the source of connectivity.  The values
          will be loaded based on foreign key values present on this
          object - it follows that this functionality
          generally only works for many-to-one-relationships.
  
          The object will be attached to this session, but will
          **not** participate in any persistence operations; its state
          for almost all purposes will remain either "transient" or
          "detached", except for the case of relationship loading.
  
          Also note that backrefs will often not work as expected.
          Altering a relationship-bound attribute on the target object
          may not fire off a backref event, if the effective value
          is what was already loaded from a foreign-key-holding value.
  
          The :meth:`.Session.enable_relationship_loading` method is
          similar to the ``load_on_pending`` flag on :func:`.relationship`.
          Unlike that flag, :meth:`.Session.enable_relationship_loading` allows
          an object to remain transient while still being able to load
          related items.
  
          To make a transient object associated with a :class:`.Session`
          via :meth:`.Session.enable_relationship_loading` pending, add
          it to the :class:`.Session` using :meth:`.Session.add` normally.
  
          :meth:`.Session.enable_relationship_loading` does not improve
          behavior when the ORM is used normally - object references should be
          constructed at the object level, not at the foreign key level, so
          that they are present in an ordinary way before flush()
          proceeds.  This method is not intended for general use.
  
          .. versionadded:: 0.8
  
          .. seealso::
  
              ``load_on_pending`` at :func:`.relationship` - this flag
              allows per-relationship loading of many-to-ones on items that
              are pending.
  
          """
          state = attributes.instance_state(obj)
          to_attach = self._before_attach(state, obj)
          state._load_pending = True
          if to_attach:
              self._after_attach(state, obj)
  
      def _before_attach(self, state, obj):
          if state.session_id == self.hash_key:
              return False
  
          if state.session_id and state.session_id in _sessions:
              raise sa_exc.InvalidRequestError(
                  "Object '%s' is already attached to session '%s' "
                  "(this is '%s')" % (state_str(state),
                                      state.session_id, self.hash_key))
  
          self.dispatch.before_attach(self, obj)
  
          return True
  
      def _after_attach(self, state, obj):
          state.session_id = self.hash_key
          if state.modified and state._strong_obj is None:
              state._strong_obj = obj
          self.dispatch.after_attach(self, obj)
  
          if state.key:
              self.dispatch.detached_to_persistent(self, obj)
          else:
              self.dispatch.transient_to_pending(self, obj)
  
      def __contains__(self, instance):
          """Return True if the instance is associated with this session.
  
          The instance may be pending or persistent within the Session for a
          result of True.
  
          """
          try:
              state = attributes.instance_state(instance)
          except exc.NO_STATE:
              raise exc.UnmappedInstanceError(instance)
          return self._contains_state(state)
  
      def __iter__(self):
          """Iterate over all pending or persistent instances within this
          Session.
  
          """
          return iter(
              list(self._new.values()) + list(self.identity_map.values()))
  
      def _contains_state(self, state):
          return state in self._new or self.identity_map.contains_state(state)
  
      def flush(self, objects=None):
          """Flush all the object changes to the database.
  
          Writes out all pending object creations, deletions and modifications
          to the database as INSERTs, DELETEs, UPDATEs, etc.  Operations are
          automatically ordered by the Session's unit of work dependency
          solver.
  
          Database operations will be issued in the current transactional
          context and do not affect the state of the transaction, unless an
          error occurs, in which case the entire transaction is rolled back.
          You may flush() as often as you like within a transaction to move
          changes from Python to the database's transaction buffer.
  
          For ``autocommit`` Sessions with no active manual transaction, flush()
          will create a transaction on the fly that surrounds the entire set of
          operations into the flush.
  
          :param objects: Optional; restricts the flush operation to operate
            only on elements that are in the given collection.
  
            This feature is for an extremely narrow set of use cases where
            particular objects may need to be operated upon before the
            full flush() occurs.  It is not intended for general use.
  
          """
  
          if self._flushing:
              raise sa_exc.InvalidRequestError("Session is already flushing")
  
          if self._is_clean():
              return
          try:
              self._flushing = True
              self._flush(objects)
          finally:
              self._flushing = False
  
      def _flush_warning(self, method):
          util.warn(
              "Usage of the '%s' operation is not currently supported "
              "within the execution stage of the flush process. "
              "Results may not be consistent.  Consider using alternative "
              "event listeners or connection-level operations instead."
              % method)
  
      def _is_clean(self):
          return not self.identity_map.check_modified() and \
              not self._deleted and \
              not self._new
  
      def _flush(self, objects=None):
  
          dirty = self._dirty_states
          if not dirty and not self._deleted and not self._new:
              self.identity_map._modified.clear()
              return
  
          flush_context = UOWTransaction(self)
  
          if self.dispatch.before_flush:
              self.dispatch.before_flush(self, flush_context, objects)
              # re-establish "dirty states" in case the listeners
              # added
              dirty = self._dirty_states
  
          deleted = set(self._deleted)
          new = set(self._new)
  
          dirty = set(dirty).difference(deleted)
  
          # create the set of all objects we want to operate upon
          if objects:
              # specific list passed in
              objset = set()
              for o in objects:
                  try:
                      state = attributes.instance_state(o)
                  except exc.NO_STATE:
                      raise exc.UnmappedInstanceError(o)
                  objset.add(state)
          else:
              objset = None
  
          # store objects whose fate has been decided
          processed = set()
  
          # put all saves/updates into the flush context.  detect top-level
          # orphans and throw them into deleted.
          if objset:
              proc = new.union(dirty).intersection(objset).difference(deleted)
          else:
              proc = new.union(dirty).difference(deleted)
  
          for state in proc:
              is_orphan = (
                  _state_mapper(state)._is_orphan(state) and state.has_identity)
              _reg = flush_context.register_object(state, isdelete=is_orphan)
              assert _reg, "Failed to add object to the flush context!"
              processed.add(state)
  
          # put all remaining deletes into the flush context.
          if objset:
              proc = deleted.intersection(objset).difference(processed)
          else:
              proc = deleted.difference(processed)
          for state in proc:
              _reg = flush_context.register_object(state, isdelete=True)
              assert _reg, "Failed to add object to the flush context!"
  
          if not flush_context.has_work:
              return
  
          flush_context.transaction = transaction = self.begin(
              subtransactions=True)
          try:
              self._warn_on_events = True
              try:
                  flush_context.execute()
              finally:
                  self._warn_on_events = False
  
              self.dispatch.after_flush(self, flush_context)
  
              flush_context.finalize_flush_changes()
  
              if not objects and self.identity_map._modified:
                  len_ = len(self.identity_map._modified)
  
                  statelib.InstanceState._commit_all_states(
                      [(state, state.dict) for state in
                       self.identity_map._modified],
                      instance_dict=self.identity_map)
                  util.warn("Attribute history events accumulated on %d "
                            "previously clean instances "
                            "within inner-flush event handlers have been "
                            "reset, and will not result in database updates. "
                            "Consider using set_committed_value() within "
                            "inner-flush event handlers to avoid this warning."
                            % len_)
  
              # useful assertions:
              # if not objects:
              #    assert not self.identity_map._modified
              # else:
              #    assert self.identity_map._modified == \
              #            self.identity_map._modified.difference(objects)
  
              self.dispatch.after_flush_postexec(self, flush_context)
  
              transaction.commit()
  
          except:
              with util.safe_reraise():
                  transaction.rollback(_capture_exception=True)
  
      def bulk_save_objects(
              self, objects, return_defaults=False, update_changed_only=True):
          """Perform a bulk save of the given list of objects.
  
          The bulk save feature allows mapped objects to be used as the
          source of simple INSERT and UPDATE operations which can be more easily
          grouped together into higher performing "executemany"
          operations; the extraction of data from the objects is also performed
          using a lower-latency process that ignores whether or not attributes
          have actually been modified in the case of UPDATEs, and also ignores
          SQL expressions.
  
          The objects as given are not added to the session and no additional
          state is established on them, unless the ``return_defaults`` flag
          is also set, in which case primary key attributes and server-side
          default values will be populated.
  
          .. versionadded:: 1.0.0
  
          .. warning::
  
              The bulk save feature allows for a lower-latency INSERT/UPDATE
              of rows at the expense of most other unit-of-work features.
              Features such as object management, relationship handling,
              and SQL clause support are **silently omitted** in favor of raw
              INSERT/UPDATES of records.
  
              **Please read the list of caveats at** :ref:`bulk_operations`
              **before using this method, and fully test and confirm the
              functionality of all code developed using these systems.**
  
          :param objects: a list of mapped object instances.  The mapped
           objects are persisted as is, and are **not** associated with the
           :class:`.Session` afterwards.
  
           For each object, whether the object is sent as an INSERT or an
           UPDATE is dependent on the same rules used by the :class:`.Session`
           in traditional operation; if the object has the
           :attr:`.InstanceState.key`
           attribute set, then the object is assumed to be "detached" and
           will result in an UPDATE.  Otherwise, an INSERT is used.
  
           In the case of an UPDATE, statements are grouped based on which
           attributes have changed, and are thus to be the subject of each
           SET clause.  If ``update_changed_only`` is False, then all
           attributes present within each object are applied to the UPDATE
           statement, which may help in allowing the statements to be grouped
           together into a larger executemany(), and will also reduce the
           overhead of checking history on attributes.
  
          :param return_defaults: when True, rows that are missing values which
           generate defaults, namely integer primary key defaults and sequences,
           will be inserted **one at a time**, so that the primary key value
           is available.  In particular this will allow joined-inheritance
           and other multi-table mappings to insert correctly without the need
           to provide primary key values ahead of time; however,
           :paramref:`.Session.bulk_save_objects.return_defaults` **greatly
           reduces the performance gains** of the method overall.
  
          :param update_changed_only: when True, UPDATE statements are rendered
           based on those attributes in each state that have logged changes.
           When False, all attributes present are rendered into the SET clause
           with the exception of primary key attributes.
  
          .. seealso::
  
              :ref:`bulk_operations`
  
              :meth:`.Session.bulk_insert_mappings`
  
              :meth:`.Session.bulk_update_mappings`
  
          """
          for (mapper, isupdate), states in itertools.groupby(
              (attributes.instance_state(obj) for obj in objects),
              lambda state: (state.mapper, state.key is not None)
          ):
              self._bulk_save_mappings(
                  mapper, states, isupdate, True,
                  return_defaults, update_changed_only, False)
  
      def bulk_insert_mappings(
              self, mapper, mappings, return_defaults=False, render_nulls=False):
          """Perform a bulk insert of the given list of mapping dictionaries.
  
          The bulk insert feature allows plain Python dictionaries to be used as
          the source of simple INSERT operations which can be more easily
          grouped together into higher performing "executemany"
          operations.  Using dictionaries, there is no "history" or session
          state management features in use, reducing latency when inserting
          large numbers of simple rows.
  
          The values within the dictionaries as given are typically passed
          without modification into Core :meth:`.Insert` constructs, after
          organizing the values within them across the tables to which
          the given mapper is mapped.
  
          .. versionadded:: 1.0.0
  
          .. warning::
  
              The bulk insert feature allows for a lower-latency INSERT
              of rows at the expense of most other unit-of-work features.
              Features such as object management, relationship handling,
              and SQL clause support are **silently omitted** in favor of raw
              INSERT of records.
  
              **Please read the list of caveats at** :ref:`bulk_operations`
              **before using this method, and fully test and confirm the
              functionality of all code developed using these systems.**
  
          :param mapper: a mapped class, or the actual :class:`.Mapper` object,
           representing the single kind of object represented within the mapping
           list.
  
          :param mappings: a list of dictionaries, each one containing the state
           of the mapped row to be inserted, in terms of the attribute names
           on the mapped class.   If the mapping refers to multiple tables,
           such as a joined-inheritance mapping, each dictionary must contain
           all keys to be populated into all tables.
  
          :param return_defaults: when True, rows that are missing values which
           generate defaults, namely integer primary key defaults and sequences,
           will be inserted **one at a time**, so that the primary key value
           is available.  In particular this will allow joined-inheritance
           and other multi-table mappings to insert correctly without the need
           to provide primary
           key values ahead of time; however,
           :paramref:`.Session.bulk_insert_mappings.return_defaults`
           **greatly reduces the performance gains** of the method overall.
           If the rows
           to be inserted only refer to a single table, then there is no
           reason this flag should be set as the returned default information
           is not used.
  
          :param render_nulls: When True, a value of ``None`` will result
           in a NULL value being included in the INSERT statement, rather
           than the column being omitted from the INSERT.   This allows all
           the rows being INSERTed to have the identical set of columns which
           allows the full set of rows to be batched to the DBAPI.  Normally,
           each column-set that contains a different combination of NULL values
           than the previous row must omit a different series of columns from
           the rendered INSERT statement, which means it must be emitted as a
           separate statement.   By passing this flag, the full set of rows
           are guaranteed to be batchable into one batch; the cost however is
           that server-side defaults which are invoked by an omitted column will
           be skipped, so care must be taken to ensure that these are not
           necessary.
  
           .. warning::
  
              When this flag is set, **server side default SQL values will
              not be invoked** for those columns that are inserted as NULL;
              the NULL value will be sent explicitly.   Care must be taken
              to ensure that no server-side default functions need to be
              invoked for the operation as a whole.
  
           .. versionadded:: 1.1
  
          .. seealso::
  
              :ref:`bulk_operations`
  
              :meth:`.Session.bulk_save_objects`
  
              :meth:`.Session.bulk_update_mappings`
  
          """
          self._bulk_save_mappings(
              mapper, mappings, False, False,
              return_defaults, False, render_nulls)
  
      def bulk_update_mappings(self, mapper, mappings):
          """Perform a bulk update of the given list of mapping dictionaries.
  
          The bulk update feature allows plain Python dictionaries to be used as
          the source of simple UPDATE operations which can be more easily
          grouped together into higher performing "executemany"
          operations.  Using dictionaries, there is no "history" or session
          state management features in use, reducing latency when updating
          large numbers of simple rows.
  
          .. versionadded:: 1.0.0
  
          .. warning::
  
              The bulk update feature allows for a lower-latency UPDATE
              of rows at the expense of most other unit-of-work features.
              Features such as object management, relationship handling,
              and SQL clause support are **silently omitted** in favor of raw
              UPDATES of records.
  
              **Please read the list of caveats at** :ref:`bulk_operations`
              **before using this method, and fully test and confirm the
              functionality of all code developed using these systems.**
  
          :param mapper: a mapped class, or the actual :class:`.Mapper` object,
           representing the single kind of object represented within the mapping
           list.
  
          :param mappings: a list of dictionaries, each one containing the state
           of the mapped row to be updated, in terms of the attribute names
           on the mapped class.   If the mapping refers to multiple tables,
           such as a joined-inheritance mapping, each dictionary may contain
           keys corresponding to all tables.   All those keys which are present
           and are not part of the primary key are applied to the SET clause
           of the UPDATE statement; the primary key values, which are required,
           are applied to the WHERE clause.
  
  
          .. seealso::
  
              :ref:`bulk_operations`
  
              :meth:`.Session.bulk_insert_mappings`
  
              :meth:`.Session.bulk_save_objects`
  
          """
          self._bulk_save_mappings(
              mapper, mappings, True, False, False, False, False)
  
      def _bulk_save_mappings(
              self, mapper, mappings, isupdate, isstates,
              return_defaults, update_changed_only, render_nulls):
          mapper = _class_to_mapper(mapper)
          self._flushing = True
  
          transaction = self.begin(
              subtransactions=True)
          try:
              if isupdate:
                  persistence._bulk_update(
                      mapper, mappings, transaction,
                      isstates, update_changed_only)
              else:
                  persistence._bulk_insert(
                      mapper, mappings, transaction,
                      isstates, return_defaults, render_nulls)
              transaction.commit()
  
          except:
              with util.safe_reraise():
                  transaction.rollback(_capture_exception=True)
          finally:
              self._flushing = False
  
      def is_modified(self, instance, include_collections=True,
                      passive=True):
          r"""Return ``True`` if the given instance has locally
          modified attributes.
  
          This method retrieves the history for each instrumented
          attribute on the instance and performs a comparison of the current
          value to its previously committed value, if any.
  
          It is in effect a more expensive and accurate
          version of checking for the given instance in the
          :attr:`.Session.dirty` collection; a full test for
          each attribute's net "dirty" status is performed.
  
          E.g.::
  
              return session.is_modified(someobject)
  
          .. versionchanged:: 0.8
              When using SQLAlchemy 0.7 and earlier, the ``passive``
              flag should **always** be explicitly set to ``True``,
              else SQL loads/autoflushes may proceed which can affect
              the modified state itself:
              ``session.is_modified(someobject, passive=True)``\ .
              In 0.8 and above, the behavior is corrected and
              this flag is ignored.
  
          A few caveats to this method apply:
  
          * Instances present in the :attr:`.Session.dirty` collection may
            report ``False`` when tested with this method.  This is because
            the object may have received change events via attribute mutation,
            thus placing it in :attr:`.Session.dirty`, but ultimately the state
            is the same as that loaded from the database, resulting in no net
            change here.
          * Scalar attributes may not have recorded the previously set
            value when a new value was applied, if the attribute was not loaded,
            or was expired, at the time the new value was received - in these
            cases, the attribute is assumed to have a change, even if there is
            ultimately no net change against its database value. SQLAlchemy in
            most cases does not need the "old" value when a set event occurs, so
            it skips the expense of a SQL call if the old value isn't present,
            based on the assumption that an UPDATE of the scalar value is
            usually needed, and in those few cases where it isn't, is less
            expensive on average than issuing a defensive SELECT.
  
            The "old" value is fetched unconditionally upon set only if the
            attribute container has the ``active_history`` flag set to ``True``.
            This flag is set typically for primary key attributes and scalar
            object references that are not a simple many-to-one.  To set this
            flag for any arbitrary mapped column, use the ``active_history``
            argument with :func:`.column_property`.
  
          :param instance: mapped instance to be tested for pending changes.
          :param include_collections: Indicates if multivalued collections
           should be included in the operation.  Setting this to ``False`` is a
           way to detect only local-column based properties (i.e. scalar columns
           or many-to-one foreign keys) that would result in an UPDATE for this
           instance upon flush.
          :param passive:
  
           .. versionchanged:: 0.8
               Ignored for backwards compatibility.
               When using SQLAlchemy 0.7 and earlier, this flag should always
               be set to ``True``.
  
          """
          state = object_state(instance)
  
          if not state.modified:
              return False
  
          dict_ = state.dict
  
          for attr in state.manager.attributes:
              if \
                      (
                          not include_collections and
                          hasattr(attr.impl, 'get_collection')
                      ) or not hasattr(attr.impl, 'get_history'):
                  continue
  
              (added, unchanged, deleted) = \
                  attr.impl.get_history(state, dict_,
                                        passive=attributes.NO_CHANGE)
  
              if added or deleted:
                  return True
          else:
              return False
  
      @property
      def is_active(self):
          """True if this :class:`.Session` is in "transaction mode" and
          is not in "partial rollback" state.
  
          The :class:`.Session` in its default mode of ``autocommit=False``
          is essentially always in "transaction mode", in that a
          :class:`.SessionTransaction` is associated with it as soon as
          it is instantiated.  This :class:`.SessionTransaction` is immediately
          replaced with a new one as soon as it is ended, due to a rollback,
          commit, or close operation.
  
          "Transaction mode" does *not* indicate whether
          or not actual database connection resources are in use;  the
          :class:`.SessionTransaction` object coordinates among zero or more
          actual database transactions, and starts out with none, accumulating
          individual DBAPI connections as different data sources are used
          within its scope.   The best way to track when a particular
          :class:`.Session` has actually begun to use DBAPI resources is to
          implement a listener using the :meth:`.SessionEvents.after_begin`
          method, which will deliver both the :class:`.Session` as well as the
          target :class:`.Connection` to a user-defined event listener.
  
          The "partial rollback" state refers to when an "inner" transaction,
          typically used during a flush, encounters an error and emits a
          rollback of the DBAPI connection.  At this point, the
          :class:`.Session` is in "partial rollback" and awaits for the user to
          call :meth:`.Session.rollback`, in order to close out the
          transaction stack.  It is in this "partial rollback" period that the
          :attr:`.is_active` flag returns False.  After the call to
          :meth:`.Session.rollback`, the :class:`.SessionTransaction` is
          replaced with a new one and :attr:`.is_active` returns ``True`` again.
  
          When a :class:`.Session` is used in ``autocommit=True`` mode, the
          :class:`.SessionTransaction` is only instantiated within the scope
          of a flush call, or when :meth:`.Session.begin` is called.  So
          :attr:`.is_active` will always be ``False`` outside of a flush or
          :meth:`.Session.begin` block in this mode, and will be ``True``
          within the :meth:`.Session.begin` block as long as it doesn't enter
          "partial rollback" state.
  
          From all the above, it follows that the only purpose to this flag is
          for application frameworks that wish to detect is a "rollback" is
          necessary within a generic error handling routine, for
          :class:`.Session` objects that would otherwise be in
          "partial rollback" mode.  In a typical integration case, this is also
          not necessary as it is standard practice to emit
          :meth:`.Session.rollback` unconditionally within the outermost
          exception catch.
  
          To track the transactional state of a :class:`.Session` fully,
          use event listeners, primarily the :meth:`.SessionEvents.after_begin`,
          :meth:`.SessionEvents.after_commit`,
          :meth:`.SessionEvents.after_rollback` and related events.
  
          """
          return self.transaction and self.transaction.is_active
  
      identity_map = None
      """A mapping of object identities to objects themselves.
  
      Iterating through ``Session.identity_map.values()`` provides
      access to the full set of persistent objects (i.e., those
      that have row identity) currently in the session.
  
      .. seealso::
  
          :func:`.identity_key` - helper function to produce the keys used
          in this dictionary.
  
      """
  
      @property
      def _dirty_states(self):
          """The set of all persistent states considered dirty.
  
          This method returns all states that were modified including
          those that were possibly deleted.
  
          """
          return self.identity_map._dirty_states()
  
      @property
      def dirty(self):
          """The set of all persistent instances considered dirty.
  
          E.g.::
  
              some_mapped_object in session.dirty
  
          Instances are considered dirty when they were modified but not
          deleted.
  
          Note that this 'dirty' calculation is 'optimistic'; most
          attribute-setting or collection modification operations will
          mark an instance as 'dirty' and place it in this set, even if
          there is no net change to the attribute's value.  At flush
          time, the value of each attribute is compared to its
          previously saved value, and if there's no net change, no SQL
          operation will occur (this is a more expensive operation so
          it's only done at flush time).
  
          To check if an instance has actionable net changes to its
          attributes, use the :meth:`.Session.is_modified` method.
  
          """
          return util.IdentitySet(
              [state.obj()
               for state in self._dirty_states
               if state not in self._deleted])
  
      @property
      def deleted(self):
          "The set of all instances marked as 'deleted' within this ``Session``"
  
          return util.IdentitySet(list(self._deleted.values()))
  
      @property
      def new(self):
          "The set of all instances marked as 'new' within this ``Session``."
  
          return util.IdentitySet(list(self._new.values()))
  
  
  class sessionmaker(_SessionClassMethods):
      """A configurable :class:`.Session` factory.
  
      The :class:`.sessionmaker` factory generates new
      :class:`.Session` objects when called, creating them given
      the configurational arguments established here.
  
      e.g.::
  
          # global scope
          Session = sessionmaker(autoflush=False)
  
          # later, in a local scope, create and use a session:
          sess = Session()
  
      Any keyword arguments sent to the constructor itself will override the
      "configured" keywords::
  
          Session = sessionmaker()
  
          # bind an individual session to a connection
          sess = Session(bind=connection)
  
      The class also includes a method :meth:`.configure`, which can
      be used to specify additional keyword arguments to the factory, which
      will take effect for subsequent :class:`.Session` objects generated.
      This is usually used to associate one or more :class:`.Engine` objects
      with an existing :class:`.sessionmaker` factory before it is first
      used::
  
          # application starts
          Session = sessionmaker()
  
          # ... later
          engine = create_engine('sqlite:///foo.db')
          Session.configure(bind=engine)
  
          sess = Session()
  
      .. seealso:
  
          :ref:`session_getting` - introductory text on creating
          sessions using :class:`.sessionmaker`.
  
      """
  
      def __init__(self, bind=None, class_=Session, autoflush=True,
                   autocommit=False,
                   expire_on_commit=True,
                   info=None, **kw):
          r"""Construct a new :class:`.sessionmaker`.
  
          All arguments here except for ``class_`` correspond to arguments
          accepted by :class:`.Session` directly.  See the
          :meth:`.Session.__init__` docstring for more details on parameters.
  
          :param bind: a :class:`.Engine` or other :class:`.Connectable` with
           which newly created :class:`.Session` objects will be associated.
          :param class_: class to use in order to create new :class:`.Session`
           objects.  Defaults to :class:`.Session`.
          :param autoflush: The autoflush setting to use with newly created
           :class:`.Session` objects.
          :param autocommit: The autocommit setting to use with newly created
           :class:`.Session` objects.
          :param expire_on_commit=True: the expire_on_commit setting to use
           with newly created :class:`.Session` objects.
          :param info: optional dictionary of information that will be available
           via :attr:`.Session.info`.  Note this dictionary is *updated*, not
           replaced, when the ``info`` parameter is specified to the specific
           :class:`.Session` construction operation.
  
           .. versionadded:: 0.9.0
  
          :param \**kw: all other keyword arguments are passed to the
           constructor of newly created :class:`.Session` objects.
  
          """
          kw['bind'] = bind
          kw['autoflush'] = autoflush
          kw['autocommit'] = autocommit
          kw['expire_on_commit'] = expire_on_commit
          if info is not None:
              kw['info'] = info
          self.kw = kw
          # make our own subclass of the given class, so that
          # events can be associated with it specifically.
          self.class_ = type(class_.__name__, (class_,), {})
  
      def __call__(self, **local_kw):
          """Produce a new :class:`.Session` object using the configuration
          established in this :class:`.sessionmaker`.
  
          In Python, the ``__call__`` method is invoked on an object when
          it is "called" in the same way as a function::
  
              Session = sessionmaker()
              session = Session()  # invokes sessionmaker.__call__()
  
          """
          for k, v in self.kw.items():
              if k == 'info' and 'info' in local_kw:
                  d = v.copy()
                  d.update(local_kw['info'])
                  local_kw['info'] = d
              else:
                  local_kw.setdefault(k, v)
          return self.class_(**local_kw)
  
      def configure(self, **new_kw):
          """(Re)configure the arguments for this sessionmaker.
  
          e.g.::
  
              Session = sessionmaker()
  
              Session.configure(bind=create_engine('sqlite://'))
          """
          self.kw.update(new_kw)
  
      def __repr__(self):
          return "%s(class_=%r,%s)" % (
              self.__class__.__name__,
              self.class_.__name__,
              ", ".join("%s=%r" % (k, v) for k, v in self.kw.items())
          )
  
  
  def make_transient(instance):
      """Alter the state of the given instance so that it is :term:`transient`.
  
      .. note::
  
          :func:`.make_transient` is a special-case function for
          advanced use cases only.
  
      The given mapped instance is assumed to be in the :term:`persistent` or
      :term:`detached` state.   The function will remove its association with any
      :class:`.Session` as well as its :attr:`.InstanceState.identity`. The
      effect is that the object will behave as though it were newly constructed,
      except retaining any attribute / collection values that were loaded at the
      time of the call.   The :attr:`.InstanceState.deleted` flag is also reset
      if this object had been deleted as a result of using
      :meth:`.Session.delete`.
  
      .. warning::
  
          :func:`.make_transient` does **not** "unexpire" or otherwise eagerly
          load ORM-mapped attributes that are not currently loaded at the time
          the function is called.   This includes attributes which:
  
          * were expired via :meth:`.Session.expire`
  
          * were expired as the natural effect of committing a session
            transaction, e.g. :meth:`.Session.commit`
  
          * are normally :term:`lazy loaded` but are not currently loaded
  
          * are "deferred" via :ref:`deferred` and are not yet loaded
  
          * were not present in the query which loaded this object, such as that
            which is common in joined table inheritance and other scenarios.
  
          After :func:`.make_transient` is called, unloaded attributes such
          as those above will normally resolve to the value ``None`` when
          accessed, or an empty collection for a collection-oriented attribute.
          As the object is transient and un-associated with any database
          identity, it will no longer retrieve these values.
  
      .. seealso::
  
          :func:`.make_transient_to_detached`
  
      """
      state = attributes.instance_state(instance)
      s = _state_session(state)
      if s:
          s._expunge_states([state])
  
      # remove expired state
      state.expired_attributes.clear()
  
      # remove deferred callables
      if state.callables:
          del state.callables
  
      if state.key:
          del state.key
      if state._deleted:
          del state._deleted
  
  
  def make_transient_to_detached(instance):
      """Make the given transient instance :term:`detached`.
  
      .. note::
  
          :func:`.make_transient_to_detached` is a special-case function for
          advanced use cases only.
  
      All attribute history on the given instance
      will be reset as though the instance were freshly loaded
      from a query.  Missing attributes will be marked as expired.
      The primary key attributes of the object, which are required, will be made
      into the "key" of the instance.
  
      The object can then be added to a session, or merged
      possibly with the load=False flag, at which point it will look
      as if it were loaded that way, without emitting SQL.
  
      This is a special use case function that differs from a normal
      call to :meth:`.Session.merge` in that a given persistent state
      can be manufactured without any SQL calls.
  
      .. versionadded:: 0.9.5
  
      .. seealso::
  
          :func:`.make_transient`
  
      """
      state = attributes.instance_state(instance)
      if state.session_id or state.key:
          raise sa_exc.InvalidRequestError(
              "Given object must be transient")
      state.key = state.mapper._identity_key_from_state(state)
      if state._deleted:
          del state._deleted
      state._commit_all(state.dict)
      state._expire_attributes(state.dict, state.unloaded)
  
  
  def object_session(instance):
      """Return the :class:`.Session` to which the given instance belongs.
  
      This is essentially the same as the :attr:`.InstanceState.session`
      accessor.  See that attribute for details.
  
      """
  
      try:
          state = attributes.instance_state(instance)
      except exc.NO_STATE:
          raise exc.UnmappedInstanceError(instance)
      else:
          return _state_session(state)
  
  
  _new_sessionid = util.counter()
  