text: # engine/base.py
  # Copyright (C) 2005-2014 the SQLAlchemy authors and contributors
  # <see AUTHORS file>
  #
  # This module is part of SQLAlchemy and is released under
  # the MIT License: http://www.opensource.org/licenses/mit-license.php
  from __future__ import with_statement
  
  """Defines :class:`.Connection` and :class:`.Engine`.
  
  """
  
  
  import sys
  from .. import exc, util, log, interfaces
  from ..sql import util as sql_util
  from .interfaces import Connectable, ExceptionContext
  from .util import _distill_params
  import contextlib
  
  
  class Connection(Connectable):
      """Provides high-level functionality for a wrapped DB-API connection.
  
      Provides execution support for string-based SQL statements as well as
      :class:`.ClauseElement`, :class:`.Compiled` and :class:`.DefaultGenerator`
      objects. Provides a :meth:`begin` method to return :class:`.Transaction`
      objects.
  
      The Connection object is **not** thread-safe.  While a Connection can be
      shared among threads using properly synchronized access, it is still
      possible that the underlying DBAPI connection may not support shared
      access between threads.  Check the DBAPI documentation for details.
  
      The Connection object represents a single dbapi connection checked out
      from the connection pool. In this state, the connection pool has no affect
      upon the connection, including its expiration or timeout state. For the
      connection pool to properly manage connections, connections should be
      returned to the connection pool (i.e. ``connection.close()``) whenever the
      connection is not in use.
  
      .. index::
        single: thread safety; Connection
  
      """
  
      def __init__(self, engine, connection=None, close_with_result=False,
                   _branch=False, _execution_options=None,
                   _dispatch=None,
                   _has_events=None):
          """Construct a new Connection.
  
          The constructor here is not public and is only called only by an
          :class:`.Engine`. See :meth:`.Engine.connect` and
          :meth:`.Engine.contextual_connect` methods.
  
          """
          self.engine = engine
          self.dialect = engine.dialect
          self.__connection = connection or engine.raw_connection()
          self.__transaction = None
          self.should_close_with_result = close_with_result
          self.__savepoint_seq = 0
          self.__branch = _branch
          self.__invalid = False
          self.__can_reconnect = True
          if _dispatch:
              self.dispatch = _dispatch
          elif _has_events is None:
              # if _has_events is sent explicitly as False,
              # then don't join the dispatch of the engine; we don't
              # want to handle any of the engine's events in that case.
              self.dispatch = self.dispatch._join(engine.dispatch)
          self._has_events = _has_events or (
              _has_events is None and engine._has_events)
  
          self._echo = self.engine._should_log_info()
          if _execution_options:
              self._execution_options =\
                  engine._execution_options.union(_execution_options)
          else:
              self._execution_options = engine._execution_options
  
          if self._has_events or self.engine._has_events:
              self.dispatch.engine_connect(self, _branch)
  
      def _branch(self):
          """Return a new Connection which references this Connection's
          engine and connection; but does not have close_with_result enabled,
          and also whose close() method does nothing.
  
          This is used to execute "sub" statements within a single execution,
          usually an INSERT statement.
          """
  
          return self.engine._connection_cls(
              self.engine,
              self.__connection,
              _branch=True,
              _has_events=self._has_events,
              _dispatch=self.dispatch)
  
      def _clone(self):
          """Create a shallow copy of this Connection.
  
          """
          c = self.__class__.__new__(self.__class__)
          c.__dict__ = self.__dict__.copy()
          return c
  
      def __enter__(self):
          return self
  
      def __exit__(self, type, value, traceback):
          self.close()
  
      def execution_options(self, **opt):
          """ Set non-SQL options for the connection which take effect
          during execution.
  
          The method returns a copy of this :class:`.Connection` which references
          the same underlying DBAPI connection, but also defines the given
          execution options which will take effect for a call to
          :meth:`execute`. As the new :class:`.Connection` references the same
          underlying resource, it's usually a good idea to ensure that the copies
          will be discarded immediately, which is implicit if used as in::
  
              result = connection.execution_options(stream_results=True).\\
                                  execute(stmt)
  
          Note that any key/value can be passed to
          :meth:`.Connection.execution_options`, and it will be stored in the
          ``_execution_options`` dictionary of the :class:`.Connection`.   It
          is suitable for usage by end-user schemes to communicate with
          event listeners, for example.
  
          The keywords that are currently recognized by SQLAlchemy itself
          include all those listed under :meth:`.Executable.execution_options`,
          as well as others that are specific to :class:`.Connection`.
  
          :param autocommit: Available on: Connection, statement.
            When True, a COMMIT will be invoked after execution
            when executed in 'autocommit' mode, i.e. when an explicit
            transaction is not begun on the connection. Note that DBAPI
            connections by default are always in a transaction - SQLAlchemy uses
            rules applied to different kinds of statements to determine if
            COMMIT will be invoked in order to provide its "autocommit" feature.
            Typically, all INSERT/UPDATE/DELETE statements as well as
            CREATE/DROP statements have autocommit behavior enabled; SELECT
            constructs do not. Use this option when invoking a SELECT or other
            specific SQL construct where COMMIT is desired (typically when
            calling stored procedures and such), and an explicit
            transaction is not in progress.
  
          :param compiled_cache: Available on: Connection.
            A dictionary where :class:`.Compiled` objects
            will be cached when the :class:`.Connection` compiles a clause
            expression into a :class:`.Compiled` object.
            It is the user's responsibility to
            manage the size of this dictionary, which will have keys
            corresponding to the dialect, clause element, the column
            names within the VALUES or SET clause of an INSERT or UPDATE,
            as well as the "batch" mode for an INSERT or UPDATE statement.
            The format of this dictionary is not guaranteed to stay the
            same in future releases.
  
            Note that the ORM makes use of its own "compiled" caches for
            some operations, including flush operations.  The caching
            used by the ORM internally supersedes a cache dictionary
            specified here.
  
          :param isolation_level: Available on: Connection.
            Set the transaction isolation level for
            the lifespan of this connection.   Valid values include
            those string values accepted by the ``isolation_level``
            parameter passed to :func:`.create_engine`, and are
            database specific, including those for :ref:`sqlite_toplevel`,
            :ref:`postgresql_toplevel` - see those dialect's documentation
            for further info.
  
            Note that this option necessarily affects the underlying
            DBAPI connection for the lifespan of the originating
            :class:`.Connection`, and is not per-execution. This
            setting is not removed until the underlying DBAPI connection
            is returned to the connection pool, i.e.
            the :meth:`.Connection.close` method is called.
  
          :param no_parameters: When ``True``, if the final parameter
            list or dictionary is totally empty, will invoke the
            statement on the cursor as ``cursor.execute(statement)``,
            not passing the parameter collection at all.
            Some DBAPIs such as psycopg2 and mysql-python consider
            percent signs as significant only when parameters are
            present; this option allows code to generate SQL
            containing percent signs (and possibly other characters)
            that is neutral regarding whether it's executed by the DBAPI
            or piped into a script that's later invoked by
            command line tools.
  
            .. versionadded:: 0.7.6
  
          :param stream_results: Available on: Connection, statement.
            Indicate to the dialect that results should be
            "streamed" and not pre-buffered, if possible.  This is a limitation
            of many DBAPIs.  The flag is currently understood only by the
            psycopg2 dialect.
  
          """
          c = self._clone()
          c._execution_options = c._execution_options.union(opt)
          if self._has_events or self.engine._has_events:
              self.dispatch.set_connection_execution_options(c, opt)
          self.dialect.set_connection_execution_options(c, opt)
          return c
  
      @property
      def closed(self):
          """Return True if this connection is closed."""
  
          return '_Connection__connection' not in self.__dict__ \
              and not self.__can_reconnect
  
      @property
      def invalidated(self):
          """Return True if this connection was invalidated."""
  
          return self.__invalid
  
      @property
      def connection(self):
          "The underlying DB-API connection managed by this Connection."
  
          try:
              return self.__connection
          except AttributeError:
              return self._revalidate_connection()
  
      def _revalidate_connection(self):
          if self.__can_reconnect and self.__invalid:
              if self.__transaction is not None:
                  raise exc.InvalidRequestError(
                      "Can't reconnect until invalid "
                      "transaction is rolled back")
              self.__connection = self.engine.raw_connection()
              self.__invalid = False
              return self.__connection
          raise exc.ResourceClosedError("This Connection is closed")
  
      @property
      def _connection_is_valid(self):
          # use getattr() for is_valid to support exceptions raised in
          # dialect initializer, where the connection is not wrapped in
          # _ConnectionFairy
  
          return getattr(self.__connection, 'is_valid', False)
  
      @property
      def _still_open_and_connection_is_valid(self):
          return \
              not self.closed and \
              not self.invalidated and \
              getattr(self.__connection, 'is_valid', False)
  
      @property
      def info(self):
          """Info dictionary associated with the underlying DBAPI connection
          referred to by this :class:`.Connection`, allowing user-defined
          data to be associated with the connection.
  
          The data here will follow along with the DBAPI connection including
          after it is returned to the connection pool and used again
          in subsequent instances of :class:`.Connection`.
  
          """
  
          return self.connection.info
  
      def connect(self):
          """Returns a branched version of this :class:`.Connection`.
  
          The :meth:`.Connection.close` method on the returned
          :class:`.Connection` can be called and this
          :class:`.Connection` will remain open.
  
          This method provides usage symmetry with
          :meth:`.Engine.connect`, including for usage
          with context managers.
  
          """
  
          return self._branch()
  
      def contextual_connect(self, **kwargs):
          """Returns a branched version of this :class:`.Connection`.
  
          The :meth:`.Connection.close` method on the returned
          :class:`.Connection` can be called and this
          :class:`.Connection` will remain open.
  
          This method provides usage symmetry with
          :meth:`.Engine.contextual_connect`, including for usage
          with context managers.
  
          """
  
          return self._branch()
  
      def invalidate(self, exception=None):
          """Invalidate the underlying DBAPI connection associated with
          this :class:`.Connection`.
  
          The underlying DBAPI connection is literally closed (if
          possible), and is discarded.  Its source connection pool will
          typically lazily create a new connection to replace it.
  
          Upon the next use (where "use" typically means using the
          :meth:`.Connection.execute` method or similar),
          this :class:`.Connection` will attempt to
          procure a new DBAPI connection using the services of the
          :class:`.Pool` as a source of connectivty (e.g. a "reconnection").
  
          If a transaction was in progress (e.g. the
          :meth:`.Connection.begin` method has been called) when
          :meth:`.Connection.invalidate` method is called, at the DBAPI
          level all state associated with this transaction is lost, as
          the DBAPI connection is closed.  The :class:`.Connection`
          will not allow a reconnection to proceed until the
          :class:`.Transaction` object is ended, by calling the
          :meth:`.Transaction.rollback` method; until that point, any attempt at
          continuing to use the :class:`.Connection` will raise an
          :class:`~sqlalchemy.exc.InvalidRequestError`.
          This is to prevent applications from accidentally
          continuing an ongoing transactional operations despite the
          fact that the transaction has been lost due to an
          invalidation.
  
          The :meth:`.Connection.invalidate` method, just like auto-invalidation,
          will at the connection pool level invoke the
          :meth:`.PoolEvents.invalidate` event.
  
          .. seealso::
  
              :ref:`pool_connection_invalidation`
  
          """
          if self.invalidated:
              return
  
          if self.closed:
              raise exc.ResourceClosedError("This Connection is closed")
  
          if self._connection_is_valid:
              self.__connection.invalidate(exception)
          del self.__connection
          self.__invalid = True
  
      def detach(self):
          """Detach the underlying DB-API connection from its connection pool.
  
          E.g.::
  
              with engine.connect() as conn:
                  conn.detach()
                  conn.execute("SET search_path TO schema1, schema2")
  
                  # work with connection
  
              # connection is fully closed (since we used "with:", can
              # also call .close())
  
          This :class:`.Connection` instance will remain usable.  When closed
          (or exited from a context manager context as above),
          the DB-API connection will be literally closed and not
          returned to its originating pool.
  
          This method can be used to insulate the rest of an application
          from a modified state on a connection (such as a transaction
          isolation level or similar).
  
          """
  
          self.__connection.detach()
  
      def begin(self):
          """Begin a transaction and return a transaction handle.
  
          The returned object is an instance of :class:`.Transaction`.
          This object represents the "scope" of the transaction,
          which completes when either the :meth:`.Transaction.rollback`
          or :meth:`.Transaction.commit` method is called.
  
          Nested calls to :meth:`.begin` on the same :class:`.Connection`
          will return new :class:`.Transaction` objects that represent
          an emulated transaction within the scope of the enclosing
          transaction, that is::
  
              trans = conn.begin()   # outermost transaction
              trans2 = conn.begin()  # "nested"
              trans2.commit()        # does nothing
              trans.commit()         # actually commits
  
          Calls to :meth:`.Transaction.commit` only have an effect
          when invoked via the outermost :class:`.Transaction` object, though the
          :meth:`.Transaction.rollback` method of any of the
          :class:`.Transaction` objects will roll back the
          transaction.
  
          See also:
  
          :meth:`.Connection.begin_nested` - use a SAVEPOINT
  
          :meth:`.Connection.begin_twophase` - use a two phase /XID transaction
  
          :meth:`.Engine.begin` - context manager available from
          :class:`.Engine`.
  
          """
  
          if self.__transaction is None:
              self.__transaction = RootTransaction(self)
              return self.__transaction
          else:
              return Transaction(self, self.__transaction)
  
      def begin_nested(self):
          """Begin a nested transaction and return a transaction handle.
  
          The returned object is an instance of :class:`.NestedTransaction`.
  
          Nested transactions require SAVEPOINT support in the
          underlying database.  Any transaction in the hierarchy may
          ``commit`` and ``rollback``, however the outermost transaction
          still controls the overall ``commit`` or ``rollback`` of the
          transaction of a whole.
  
          See also :meth:`.Connection.begin`,
          :meth:`.Connection.begin_twophase`.
          """
          if self.__transaction is None:
              self.__transaction = RootTransaction(self)
          else:
              self.__transaction = NestedTransaction(self, self.__transaction)
          return self.__transaction
  
      def begin_twophase(self, xid=None):
          """Begin a two-phase or XA transaction and return a transaction
          handle.
  
          The returned object is an instance of :class:`.TwoPhaseTransaction`,
          which in addition to the methods provided by
          :class:`.Transaction`, also provides a
          :meth:`~.TwoPhaseTransaction.prepare` method.
  
          :param xid: the two phase transaction id.  If not supplied, a
            random id will be generated.
  
          See also :meth:`.Connection.begin`,
          :meth:`.Connection.begin_twophase`.
  
          """
  
          if self.__transaction is not None:
              raise exc.InvalidRequestError(
                  "Cannot start a two phase transaction when a transaction "
                  "is already in progress.")
          if xid is None:
              xid = self.engine.dialect.create_xid()
          self.__transaction = TwoPhaseTransaction(self, xid)
          return self.__transaction
  
      def recover_twophase(self):
          return self.engine.dialect.do_recover_twophase(self)
  
      def rollback_prepared(self, xid, recover=False):
          self.engine.dialect.do_rollback_twophase(self, xid, recover=recover)
  
      def commit_prepared(self, xid, recover=False):
          self.engine.dialect.do_commit_twophase(self, xid, recover=recover)
  
      def in_transaction(self):
          """Return True if a transaction is in progress."""
  
          return self.__transaction is not None
  
      def _begin_impl(self, transaction):
          if self._echo:
              self.engine.logger.info("BEGIN (implicit)")
  
          if self._has_events or self.engine._has_events:
              self.dispatch.begin(self)
  
          try:
              self.engine.dialect.do_begin(self.connection)
              if self.connection._reset_agent is None:
                  self.connection._reset_agent = transaction
          except Exception as e:
              self._handle_dbapi_exception(e, None, None, None, None)
  
      def _rollback_impl(self):
          if self._has_events or self.engine._has_events:
              self.dispatch.rollback(self)
  
          if self._still_open_and_connection_is_valid:
              if self._echo:
                  self.engine.logger.info("ROLLBACK")
              try:
                  self.engine.dialect.do_rollback(self.connection)
              except Exception as e:
                  self._handle_dbapi_exception(e, None, None, None, None)
              finally:
                  if not self.__invalid and \
                          self.connection._reset_agent is self.__transaction:
                      self.connection._reset_agent = None
                  self.__transaction = None
          else:
              self.__transaction = None
  
      def _commit_impl(self, autocommit=False):
          if self._has_events or self.engine._has_events:
              self.dispatch.commit(self)
  
          if self._echo:
              self.engine.logger.info("COMMIT")
          try:
              self.engine.dialect.do_commit(self.connection)
          except Exception as e:
              self._handle_dbapi_exception(e, None, None, None, None)
          finally:
              if not self.__invalid and \
                      self.connection._reset_agent is self.__transaction:
                  self.connection._reset_agent = None
              self.__transaction = None
  
      def _savepoint_impl(self, name=None):
          if self._has_events or self.engine._has_events:
              self.dispatch.savepoint(self, name)
  
          if name is None:
              self.__savepoint_seq += 1
              name = 'sa_savepoint_%s' % self.__savepoint_seq
          if self._still_open_and_connection_is_valid:
              self.engine.dialect.do_savepoint(self, name)
              return name
  
      def _rollback_to_savepoint_impl(self, name, context):
          if self._has_events or self.engine._has_events:
              self.dispatch.rollback_savepoint(self, name, context)
  
          if self._still_open_and_connection_is_valid:
              self.engine.dialect.do_rollback_to_savepoint(self, name)
          self.__transaction = context
  
      def _release_savepoint_impl(self, name, context):
          if self._has_events or self.engine._has_events:
              self.dispatch.release_savepoint(self, name, context)
  
          if self._still_open_and_connection_is_valid:
              self.engine.dialect.do_release_savepoint(self, name)
          self.__transaction = context
  
      def _begin_twophase_impl(self, transaction):
          if self._echo:
              self.engine.logger.info("BEGIN TWOPHASE (implicit)")
          if self._has_events or self.engine._has_events:
              self.dispatch.begin_twophase(self, transaction.xid)
  
          if self._still_open_and_connection_is_valid:
              self.engine.dialect.do_begin_twophase(self, transaction.xid)
  
              if self.connection._reset_agent is None:
                  self.connection._reset_agent = transaction
  
      def _prepare_twophase_impl(self, xid):
          if self._has_events or self.engine._has_events:
              self.dispatch.prepare_twophase(self, xid)
  
          if self._still_open_and_connection_is_valid:
              assert isinstance(self.__transaction, TwoPhaseTransaction)
              self.engine.dialect.do_prepare_twophase(self, xid)
  
      def _rollback_twophase_impl(self, xid, is_prepared):
          if self._has_events or self.engine._has_events:
              self.dispatch.rollback_twophase(self, xid, is_prepared)
  
          if self._still_open_and_connection_is_valid:
              assert isinstance(self.__transaction, TwoPhaseTransaction)
              try:
                  self.engine.dialect.do_rollback_twophase(
                      self, xid, is_prepared)
              finally:
                  if self.connection._reset_agent is self.__transaction:
                      self.connection._reset_agent = None
                  self.__transaction = None
          else:
              self.__transaction = None
  
      def _commit_twophase_impl(self, xid, is_prepared):
          if self._has_events or self.engine._has_events:
              self.dispatch.commit_twophase(self, xid, is_prepared)
  
          if self._still_open_and_connection_is_valid:
              assert isinstance(self.__transaction, TwoPhaseTransaction)
              try:
                  self.engine.dialect.do_commit_twophase(self, xid, is_prepared)
              finally:
                  if self.connection._reset_agent is self.__transaction:
                      self.connection._reset_agent = None
                  self.__transaction = None
          else:
              self.__transaction = None
  
      def _autorollback(self):
          if not self.in_transaction():
              self._rollback_impl()
  
      def close(self):
          """Close this :class:`.Connection`.
  
          This results in a release of the underlying database
          resources, that is, the DBAPI connection referenced
          internally. The DBAPI connection is typically restored
          back to the connection-holding :class:`.Pool` referenced
          by the :class:`.Engine` that produced this
          :class:`.Connection`. Any transactional state present on
          the DBAPI connection is also unconditionally released via
          the DBAPI connection's ``rollback()`` method, regardless
          of any :class:`.Transaction` object that may be
          outstanding with regards to this :class:`.Connection`.
  
          After :meth:`~.Connection.close` is called, the
          :class:`.Connection` is permanently in a closed state,
          and will allow no further operations.
  
          """
          try:
              conn = self.__connection
          except AttributeError:
              pass
          else:
              if not self.__branch:
                  conn.close()
              if conn._reset_agent is self.__transaction:
                  conn._reset_agent = None
  
              # the close() process can end up invalidating us,
              # as the pool will call our transaction as the "reset_agent"
              # for rollback(), which can then cause an invalidation
              if not self.__invalid:
                  del self.__connection
          self.__can_reconnect = False
          self.__transaction = None
  
      def scalar(self, object, *multiparams, **params):
          """Executes and returns the first column of the first row.
  
          The underlying result/cursor is closed after execution.
          """
  
          return self.execute(object, *multiparams, **params).scalar()
  
      def execute(self, object, *multiparams, **params):
          """Executes the a SQL statement construct and returns a
          :class:`.ResultProxy`.
  
          :param object: The statement to be executed.  May be
           one of:
  
           * a plain string
           * any :class:`.ClauseElement` construct that is also
             a subclass of :class:`.Executable`, such as a
             :func:`~.expression.select` construct
           * a :class:`.FunctionElement`, such as that generated
             by :attr:`.func`, will be automatically wrapped in
             a SELECT statement, which is then executed.
           * a :class:`.DDLElement` object
           * a :class:`.DefaultGenerator` object
           * a :class:`.Compiled` object
  
          :param \*multiparams/\**params: represent bound parameter
           values to be used in the execution.   Typically,
           the format is either a collection of one or more
           dictionaries passed to \*multiparams::
  
               conn.execute(
                   table.insert(),
                   {"id":1, "value":"v1"},
                   {"id":2, "value":"v2"}
               )
  
           ...or individual key/values interpreted by \**params::
  
               conn.execute(
                   table.insert(), id=1, value="v1"
               )
  
           In the case that a plain SQL string is passed, and the underlying
           DBAPI accepts positional bind parameters, a collection of tuples
           or individual values in \*multiparams may be passed::
  
               conn.execute(
                   "INSERT INTO table (id, value) VALUES (?, ?)",
                   (1, "v1"), (2, "v2")
               )
  
               conn.execute(
                   "INSERT INTO table (id, value) VALUES (?, ?)",
                   1, "v1"
               )
  
           Note above, the usage of a question mark "?" or other
           symbol is contingent upon the "paramstyle" accepted by the DBAPI
           in use, which may be any of "qmark", "named", "pyformat", "format",
           "numeric".   See `pep-249 <http://www.python.org/dev/peps/pep-0249/>`_
           for details on paramstyle.
  
           To execute a textual SQL statement which uses bound parameters in a
           DBAPI-agnostic way, use the :func:`~.expression.text` construct.
  
          """
          if isinstance(object, util.string_types[0]):
              return self._execute_text(object, multiparams, params)
          try:
              meth = object._execute_on_connection
          except AttributeError:
              raise exc.InvalidRequestError(
                  "Unexecutable object type: %s" %
                  type(object))
          else:
              return meth(self, multiparams, params)
  
      def _execute_function(self, func, multiparams, params):
          """Execute a sql.FunctionElement object."""
  
          return self._execute_clauseelement(func.select(),
                                             multiparams, params)
  
      def _execute_default(self, default, multiparams, params):
          """Execute a schema.ColumnDefault object."""
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_execute:
                  default, multiparams, params = \
                      fn(self, default, multiparams, params)
  
          try:
              try:
                  conn = self.__connection
              except AttributeError:
                  conn = self._revalidate_connection()
  
              dialect = self.dialect
              ctx = dialect.execution_ctx_cls._init_default(
                  dialect, self, conn)
          except Exception as e:
              self._handle_dbapi_exception(e, None, None, None, None)
  
          ret = ctx._exec_default(default, None)
          if self.should_close_with_result:
              self.close()
  
          if self._has_events or self.engine._has_events:
              self.dispatch.after_execute(self,
                                          default, multiparams, params, ret)
  
          return ret
  
      def _execute_ddl(self, ddl, multiparams, params):
          """Execute a schema.DDL object."""
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_execute:
                  ddl, multiparams, params = \
                      fn(self, ddl, multiparams, params)
  
          dialect = self.dialect
  
          compiled = ddl.compile(dialect=dialect)
          ret = self._execute_context(
              dialect,
              dialect.execution_ctx_cls._init_ddl,
              compiled,
              None,
              compiled
          )
          if self._has_events or self.engine._has_events:
              self.dispatch.after_execute(self,
                                          ddl, multiparams, params, ret)
          return ret
  
      def _execute_clauseelement(self, elem, multiparams, params):
          """Execute a sql.ClauseElement object."""
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_execute:
                  elem, multiparams, params = \
                      fn(self, elem, multiparams, params)
  
          distilled_params = _distill_params(multiparams, params)
          if distilled_params:
              # note this is usually dict but we support RowProxy
              # as well; but dict.keys() as an iterable is OK
              keys = distilled_params[0].keys()
          else:
              keys = []
  
          dialect = self.dialect
          if 'compiled_cache' in self._execution_options:
              key = dialect, elem, tuple(sorted(keys)), len(distilled_params) > 1
              if key in self._execution_options['compiled_cache']:
                  compiled_sql = self._execution_options['compiled_cache'][key]
              else:
                  compiled_sql = elem.compile(
                      dialect=dialect, column_keys=keys,
                      inline=len(distilled_params) > 1)
                  self._execution_options['compiled_cache'][key] = compiled_sql
          else:
              compiled_sql = elem.compile(
                  dialect=dialect, column_keys=keys,
                  inline=len(distilled_params) > 1)
  
          ret = self._execute_context(
              dialect,
              dialect.execution_ctx_cls._init_compiled,
              compiled_sql,
              distilled_params,
              compiled_sql, distilled_params
          )
          if self._has_events or self.engine._has_events:
              self.dispatch.after_execute(self,
                                          elem, multiparams, params, ret)
          return ret
  
      def _execute_compiled(self, compiled, multiparams, params):
          """Execute a sql.Compiled object."""
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_execute:
                  compiled, multiparams, params = \
                      fn(self, compiled, multiparams, params)
  
          dialect = self.dialect
          parameters = _distill_params(multiparams, params)
          ret = self._execute_context(
              dialect,
              dialect.execution_ctx_cls._init_compiled,
              compiled,
              parameters,
              compiled, parameters
          )
          if self._has_events or self.engine._has_events:
              self.dispatch.after_execute(self,
                                          compiled, multiparams, params, ret)
          return ret
  
      def _execute_text(self, statement, multiparams, params):
          """Execute a string SQL statement."""
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_execute:
                  statement, multiparams, params = \
                      fn(self, statement, multiparams, params)
  
          dialect = self.dialect
          parameters = _distill_params(multiparams, params)
          ret = self._execute_context(
              dialect,
              dialect.execution_ctx_cls._init_statement,
              statement,
              parameters,
              statement, parameters
          )
          if self._has_events or self.engine._has_events:
              self.dispatch.after_execute(self,
                                          statement, multiparams, params, ret)
          return ret
  
      def _execute_context(self, dialect, constructor,
                           statement, parameters,
                           *args):
          """Create an :class:`.ExecutionContext` and execute, returning
          a :class:`.ResultProxy`."""
  
          try:
              try:
                  conn = self.__connection
              except AttributeError:
                  conn = self._revalidate_connection()
  
              context = constructor(dialect, self, conn, *args)
          except Exception as e:
              self._handle_dbapi_exception(e,
                                           util.text_type(statement), parameters,
                                           None, None)
  
          if context.compiled:
              context.pre_exec()
  
          cursor, statement, parameters = context.cursor, \
              context.statement, \
              context.parameters
  
          if not context.executemany:
              parameters = parameters[0]
  
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_cursor_execute:
                  statement, parameters = \
                      fn(self, cursor, statement, parameters,
                         context, context.executemany)
  
          if self._echo:
              self.engine.logger.info(statement)
              self.engine.logger.info(
                  "%r",
                  sql_util._repr_params(parameters, batches=10)
              )
          try:
              if context.executemany:
                  for fn in () if not self.dialect._has_events \
                          else self.dialect.dispatch.do_executemany:
                      if fn(cursor, statement, parameters, context):
                          break
                  else:
                      self.dialect.do_executemany(
                          cursor,
                          statement,
                          parameters,
                          context)
  
              elif not parameters and context.no_parameters:
                  for fn in () if not self.dialect._has_events \
                          else self.dialect.dispatch.do_execute_no_params:
                      if fn(cursor, statement, context):
                          break
                  else:
                      self.dialect.do_execute_no_params(
                          cursor,
                          statement,
                          context)
  
              else:
                  for fn in () if not self.dialect._has_events \
                          else self.dialect.dispatch.do_execute:
                      if fn(cursor, statement, parameters, context):
                          break
                  else:
                      self.dialect.do_execute(
                          cursor,
                          statement,
                          parameters,
                          context)
          except Exception as e:
              self._handle_dbapi_exception(
                  e,
                  statement,
                  parameters,
                  cursor,
                  context)
  
          if self._has_events or self.engine._has_events:
              self.dispatch.after_cursor_execute(self, cursor,
                                                 statement,
                                                 parameters,
                                                 context,
                                                 context.executemany)
  
          if context.compiled:
              context.post_exec()
  
              if context.isinsert and not context.executemany:
                  context.post_insert()
  
          # create a resultproxy, get rowcount/implicit RETURNING
          # rows, close cursor if no further results pending
          result = context.get_result_proxy()
          if context.isinsert:
              if context._is_implicit_returning:
                  context._fetch_implicit_returning(result)
                  result.close(_autoclose_connection=False)
                  result._metadata = None
              elif not context._is_explicit_returning:
                  result.close(_autoclose_connection=False)
                  result._metadata = None
          elif context.isupdate and context._is_implicit_returning:
              context._fetch_implicit_update_returning(result)
              result.close(_autoclose_connection=False)
              result._metadata = None
  
          elif result._metadata is None:
              # no results, get rowcount
              # (which requires open cursor on some drivers
              # such as kintersbasdb, mxodbc),
              result.rowcount
              result.close(_autoclose_connection=False)
  
          if self.__transaction is None and context.should_autocommit:
              self._commit_impl(autocommit=True)
  
          if result.closed and self.should_close_with_result:
              self.close()
  
          return result
  
      def _cursor_execute(self, cursor, statement, parameters, context=None):
          """Execute a statement + params on the given cursor.
  
          Adds appropriate logging and exception handling.
  
          This method is used by DefaultDialect for special-case
          executions, such as for sequences and column defaults.
          The path of statement execution in the majority of cases
          terminates at _execute_context().
  
          """
          if self._has_events or self.engine._has_events:
              for fn in self.dispatch.before_cursor_execute:
                  statement, parameters = \
                      fn(self, cursor, statement, parameters,
                         context,
                         False)
  
          if self._echo:
              self.engine.logger.info(statement)
              self.engine.logger.info("%r", parameters)
          try:
              for fn in () if not self.dialect._has_events \
                      else self.dialect.dispatch.do_execute:
                  if fn(cursor, statement, parameters, context):
                      break
              else:
                  self.dialect.do_execute(
                      cursor,
                      statement,
                      parameters,
                      context)
          except Exception as e:
              self._handle_dbapi_exception(
                  e,
                  statement,
                  parameters,
                  cursor,
                  context)
  
          if self._has_events or self.engine._has_events:
              self.dispatch.after_cursor_execute(self, cursor,
                                                 statement,
                                                 parameters,
                                                 context,
                                                 False)
  
      def _safe_close_cursor(self, cursor):
          """Close the given cursor, catching exceptions
          and turning into log warnings.
  
          """
          try:
              cursor.close()
          except (SystemExit, KeyboardInterrupt):
              raise
          except Exception:
              # log the error through the connection pool's logger.
              self.engine.pool.logger.error(
                  "Error closing cursor", exc_info=True)
  
      _reentrant_error = False
      _is_disconnect = False
  
      def _handle_dbapi_exception(self,
                                  e,
                                  statement,
                                  parameters,
                                  cursor,
                                  context):
          exc_info = sys.exc_info()
  
          if context and context.exception is None:
              context.exception = e
  
          if not self._is_disconnect:
              self._is_disconnect =  \
                  isinstance(e, self.dialect.dbapi.Error) and \
                  not self.closed and \
                  self.dialect.is_disconnect(e, self.__connection, cursor)
              if context:
                  context.is_disconnect = self._is_disconnect
  
          if self._reentrant_error:
              util.raise_from_cause(
                  exc.DBAPIError.instance(statement,
                                          parameters,
                                          e,
                                          self.dialect.dbapi.Error),
                  exc_info
              )
          self._reentrant_error = True
          try:
              # non-DBAPI error - if we already got a context,
              # or there's no string statement, don't wrap it
              should_wrap = isinstance(e, self.dialect.dbapi.Error) or \
                  (statement is not None and context is None)
  
              if should_wrap:
                  sqlalchemy_exception = exc.DBAPIError.instance(
                      statement,
                      parameters,
                      e,
                      self.dialect.dbapi.Error,
                      connection_invalidated=self._is_disconnect)
              else:
                  sqlalchemy_exception = None
  
              newraise = None
  
              if (self._has_events or self.engine._has_events) and \
                  not self._execution_options.get(
                      'skip_user_error_events', False):
                  # legacy dbapi_error event
                  if should_wrap and context:
                      self.dispatch.dbapi_error(self,
                                                cursor,
                                                statement,
                                                parameters,
                                                context,
                                                e)
  
                  # new handle_error event
                  ctx = ExceptionContextImpl(
                      e, sqlalchemy_exception, self, cursor, statement,
                      parameters, context, self._is_disconnect)
  
                  for fn in self.dispatch.handle_error:
                      try:
                          # handler returns an exception;
                          # call next handler in a chain
                          per_fn = fn(ctx)
                          if per_fn is not None:
                              ctx.chained_exception = newraise = per_fn
                      except Exception as _raised:
                          # handler raises an exception - stop processing
                          newraise = _raised
                          break
  
                  if sqlalchemy_exception and \
                          self._is_disconnect != ctx.is_disconnect:
                      sqlalchemy_exception.connection_invalidated = \
                          self._is_disconnect = ctx.is_disconnect
  
              if should_wrap and context:
                  context.handle_dbapi_exception(e)
  
              if not self._is_disconnect:
                  if cursor:
                      self._safe_close_cursor(cursor)
                  self._autorollback()
  
              if newraise:
                  util.raise_from_cause(newraise, exc_info)
              elif should_wrap:
                  util.raise_from_cause(
                      sqlalchemy_exception,
                      exc_info
                  )
              else:
                  util.reraise(*exc_info)
  
          finally:
              del self._reentrant_error
              if self._is_disconnect:
                  del self._is_disconnect
                  dbapi_conn_wrapper = self.connection
                  self.engine.pool._invalidate(dbapi_conn_wrapper, e)
                  self.invalidate(e)
              if self.should_close_with_result:
                  self.close()
  
      def default_schema_name(self):
          return self.engine.dialect.get_default_schema_name(self)
  
      def transaction(self, callable_, *args, **kwargs):
          """Execute the given function within a transaction boundary.
  
          The function is passed this :class:`.Connection`
          as the first argument, followed by the given \*args and \**kwargs,
          e.g.::
  
              def do_something(conn, x, y):
                  conn.execute("some statement", {'x':x, 'y':y})
  
              conn.transaction(do_something, 5, 10)
  
          The operations inside the function are all invoked within the
          context of a single :class:`.Transaction`.
          Upon success, the transaction is committed.  If an
          exception is raised, the transaction is rolled back
          before propagating the exception.
  
          .. note::
  
             The :meth:`.transaction` method is superseded by
             the usage of the Python ``with:`` statement, which can
             be used with :meth:`.Connection.begin`::
  
                 with conn.begin():
                     conn.execute("some statement", {'x':5, 'y':10})
  
             As well as with :meth:`.Engine.begin`::
  
                 with engine.begin() as conn:
                     conn.execute("some statement", {'x':5, 'y':10})
  
          See also:
  
              :meth:`.Engine.begin` - engine-level transactional
              context
  
              :meth:`.Engine.transaction` - engine-level version of
              :meth:`.Connection.transaction`
  
          """
  
          trans = self.begin()
          try:
              ret = self.run_callable(callable_, *args, **kwargs)
              trans.commit()
              return ret
          except:
              with util.safe_reraise():
                  trans.rollback()
  
      def run_callable(self, callable_, *args, **kwargs):
          """Given a callable object or function, execute it, passing
          a :class:`.Connection` as the first argument.
  
          The given \*args and \**kwargs are passed subsequent
          to the :class:`.Connection` argument.
  
          This function, along with :meth:`.Engine.run_callable`,
          allows a function to be run with a :class:`.Connection`
          or :class:`.Engine` object without the need to know
          which one is being dealt with.
  
          """
          return callable_(self, *args, **kwargs)
  
      def _run_visitor(self, visitorcallable, element, **kwargs):
          visitorcallable(self.dialect, self,
                          **kwargs).traverse_single(element)
  
  
  class ExceptionContextImpl(ExceptionContext):
      """Implement the :class:`.ExceptionContext` interface."""
  
      def __init__(self, exception, sqlalchemy_exception,
                   connection, cursor, statement, parameters,
                   context, is_disconnect):
          self.connection = connection
          self.sqlalchemy_exception = sqlalchemy_exception
          self.original_exception = exception
          self.execution_context = context
          self.statement = statement
          self.parameters = parameters
          self.is_disconnect = is_disconnect
  
  
  class Transaction(object):
      """Represent a database transaction in progress.
  
      The :class:`.Transaction` object is procured by
      calling the :meth:`~.Connection.begin` method of
      :class:`.Connection`::
  
          from sqlalchemy import create_engine
          engine = create_engine("postgresql://scott:tiger@localhost/test")
          connection = engine.connect()
          trans = connection.begin()
          connection.execute("insert into x (a, b) values (1, 2)")
          trans.commit()
  
      The object provides :meth:`.rollback` and :meth:`.commit`
      methods in order to control transaction boundaries.  It
      also implements a context manager interface so that
      the Python ``with`` statement can be used with the
      :meth:`.Connection.begin` method::
  
          with connection.begin():
              connection.execute("insert into x (a, b) values (1, 2)")
  
      The Transaction object is **not** threadsafe.
  
      See also:  :meth:`.Connection.begin`, :meth:`.Connection.begin_twophase`,
      :meth:`.Connection.begin_nested`.
  
      .. index::
        single: thread safety; Transaction
      """
  
      def __init__(self, connection, parent):
          self.connection = connection
          self._parent = parent or self
          self.is_active = True
  
      def close(self):
          """Close this :class:`.Transaction`.
  
          If this transaction is the base transaction in a begin/commit
          nesting, the transaction will rollback().  Otherwise, the
          method returns.
  
          This is used to cancel a Transaction without affecting the scope of
          an enclosing transaction.
  
          """
          if not self._parent.is_active:
              return
          if self._parent is self:
              self.rollback()
  
      def rollback(self):
          """Roll back this :class:`.Transaction`.
  
          """
          if not self._parent.is_active:
              return
          self._do_rollback()
          self.is_active = False
  
      def _do_rollback(self):
          self._parent.rollback()
  
      def commit(self):
          """Commit this :class:`.Transaction`."""
  
          if not self._parent.is_active:
              raise exc.InvalidRequestError("This transaction is inactive")
          self._do_commit()
          self.is_active = False
  
      def _do_commit(self):
          pass
  
      def __enter__(self):
          return self
  
      def __exit__(self, type, value, traceback):
          if type is None and self.is_active:
              try:
                  self.commit()
              except:
                  with util.safe_reraise():
                      self.rollback()
          else:
              self.rollback()
  
  
  class RootTransaction(Transaction):
      def __init__(self, connection):
          super(RootTransaction, self).__init__(connection, None)
          self.connection._begin_impl(self)
  
      def _do_rollback(self):
          if self.is_active:
              self.connection._rollback_impl()
  
      def _do_commit(self):
          if self.is_active:
              self.connection._commit_impl()
  
  
  class NestedTransaction(Transaction):
      """Represent a 'nested', or SAVEPOINT transaction.
  
      A new :class:`.NestedTransaction` object may be procured
      using the :meth:`.Connection.begin_nested` method.
  
      The interface is the same as that of :class:`.Transaction`.
  
      """
  
      def __init__(self, connection, parent):
          super(NestedTransaction, self).__init__(connection, parent)
          self._savepoint = self.connection._savepoint_impl()
  
      def _do_rollback(self):
          if self.is_active:
              self.connection._rollback_to_savepoint_impl(
                  self._savepoint, self._parent)
  
      def _do_commit(self):
          if self.is_active:
              self.connection._release_savepoint_impl(
                  self._savepoint, self._parent)
  
  
  class TwoPhaseTransaction(Transaction):
      """Represent a two-phase transaction.
  
      A new :class:`.TwoPhaseTransaction` object may be procured
      using the :meth:`.Connection.begin_twophase` method.
  
      The interface is the same as that of :class:`.Transaction`
      with the addition of the :meth:`prepare` method.
  
      """
  
      def __init__(self, connection, xid):
          super(TwoPhaseTransaction, self).__init__(connection, None)
          self._is_prepared = False
          self.xid = xid
          self.connection._begin_twophase_impl(self)
  
      def prepare(self):
          """Prepare this :class:`.TwoPhaseTransaction`.
  
          After a PREPARE, the transaction can be committed.
  
          """
          if not self._parent.is_active:
              raise exc.InvalidRequestError("This transaction is inactive")
          self.connection._prepare_twophase_impl(self.xid)
          self._is_prepared = True
  
      def _do_rollback(self):
          self.connection._rollback_twophase_impl(self.xid, self._is_prepared)
  
      def _do_commit(self):
          self.connection._commit_twophase_impl(self.xid, self._is_prepared)
  
  
  class Engine(Connectable, log.Identified):
      """
      Connects a :class:`~sqlalchemy.pool.Pool` and
      :class:`~sqlalchemy.engine.interfaces.Dialect` together to provide a
      source of database connectivity and behavior.
  
      An :class:`.Engine` object is instantiated publicly using the
      :func:`~sqlalchemy.create_engine` function.
  
      See also:
  
      :doc:`/core/engines`
  
      :ref:`connections_toplevel`
  
      """
  
      _execution_options = util.immutabledict()
      _has_events = False
      _connection_cls = Connection
  
      def __init__(self, pool, dialect, url,
                   logging_name=None, echo=None, proxy=None,
                   execution_options=None
                   ):
          self.pool = pool
          self.url = url
          self.dialect = dialect
          self.pool._dialect = dialect
          if logging_name:
              self.logging_name = logging_name
          self.echo = echo
          self.engine = self
          log.instance_logger(self, echoflag=echo)
          if proxy:
              interfaces.ConnectionProxy._adapt_listener(self, proxy)
          if execution_options:
              self.update_execution_options(**execution_options)
  
      def update_execution_options(self, **opt):
          """Update the default execution_options dictionary
          of this :class:`.Engine`.
  
          The given keys/values in \**opt are added to the
          default execution options that will be used for
          all connections.  The initial contents of this dictionary
          can be sent via the ``execution_options`` parameter
          to :func:`.create_engine`.
  
          .. seealso::
  
              :meth:`.Connection.execution_options`
  
              :meth:`.Engine.execution_options`
  
          """
          self._execution_options = \
              self._execution_options.union(opt)
          self.dispatch.set_engine_execution_options(self, opt)
          self.dialect.set_engine_execution_options(self, opt)
  
      def execution_options(self, **opt):
          """Return a new :class:`.Engine` that will provide
          :class:`.Connection` objects with the given execution options.
  
          The returned :class:`.Engine` remains related to the original
          :class:`.Engine` in that it shares the same connection pool and
          other state:
  
          * The :class:`.Pool` used by the new :class:`.Engine` is the
            same instance.  The :meth:`.Engine.dispose` method will replace
            the connection pool instance for the parent engine as well
            as this one.
          * Event listeners are "cascaded" - meaning, the new :class:`.Engine`
            inherits the events of the parent, and new events can be associated
            with the new :class:`.Engine` individually.
          * The logging configuration and logging_name is copied from the parent
            :class:`.Engine`.
  
          The intent of the :meth:`.Engine.execution_options` method is
          to implement "sharding" schemes where multiple :class:`.Engine`
          objects refer to the same connection pool, but are differentiated
          by options that would be consumed by a custom event::
  
              primary_engine = create_engine("mysql://")
              shard1 = primary_engine.execution_options(shard_id="shard1")
              shard2 = primary_engine.execution_options(shard_id="shard2")
  
          Above, the ``shard1`` engine serves as a factory for
          :class:`.Connection` objects that will contain the execution option
          ``shard_id=shard1``, and ``shard2`` will produce :class:`.Connection`
          objects that contain the execution option ``shard_id=shard2``.
  
          An event handler can consume the above execution option to perform
          a schema switch or other operation, given a connection.  Below
          we emit a MySQL ``use`` statement to switch databases, at the same
          time keeping track of which database we've established using the
          :attr:`.Connection.info` dictionary, which gives us a persistent
          storage space that follows the DBAPI connection::
  
              from sqlalchemy import event
              from sqlalchemy.engine import Engine
  
              shards = {"default": "base", shard_1: "db1", "shard_2": "db2"}
  
              @event.listens_for(Engine, "before_cursor_execute")
              def _switch_shard(conn, cursor, stmt,
                      params, context, executemany):
                  shard_id = conn._execution_options.get('shard_id', "default")
                  current_shard = conn.info.get("current_shard", None)
  
                  if current_shard != shard_id:
                      cursor.execute("use %s" % shards[shard_id])
                      conn.info["current_shard"] = shard_id
  
          .. versionadded:: 0.8
  
          .. seealso::
  
              :meth:`.Connection.execution_options` - update execution options
              on a :class:`.Connection` object.
  
              :meth:`.Engine.update_execution_options` - update the execution
              options for a given :class:`.Engine` in place.
  
          """
          return OptionEngine(self, opt)
  
      @property
      def name(self):
          """String name of the :class:`~sqlalchemy.engine.interfaces.Dialect`
          in use by this :class:`Engine`."""
  
          return self.dialect.name
  
      @property
      def driver(self):
          """Driver name of the :class:`~sqlalchemy.engine.interfaces.Dialect`
          in use by this :class:`Engine`."""
  
          return self.dialect.driver
  
      echo = log.echo_property()
  
      def __repr__(self):
          return 'Engine(%r)' % self.url
  
      def dispose(self):
          """Dispose of the connection pool used by this :class:`.Engine`.
  
          A new connection pool is created immediately after the old one has
          been disposed.   This new pool, like all SQLAlchemy connection pools,
          does not make any actual connections to the database until one is
          first requested.
  
          This method has two general use cases:
  
           * When a dropped connection is detected, it is assumed that all
             connections held by the pool are potentially dropped, and
             the entire pool is replaced.
  
           * An application may want to use :meth:`dispose` within a test
             suite that is creating multiple engines.
  
          It is critical to note that :meth:`dispose` does **not** guarantee
          that the application will release all open database connections - only
          those connections that are checked into the pool are closed.
          Connections which remain checked out or have been detached from
          the engine are not affected.
  
          """
          self.pool.dispose()
          self.pool = self.pool.recreate()
  
      def _execute_default(self, default):
          with self.contextual_connect() as conn:
              return conn._execute_default(default, (), {})
  
      @contextlib.contextmanager
      def _optional_conn_ctx_manager(self, connection=None):
          if connection is None:
              with self.contextual_connect() as conn:
                  yield conn
          else:
              yield connection
  
      def _run_visitor(self, visitorcallable, element,
                       connection=None, **kwargs):
          with self._optional_conn_ctx_manager(connection) as conn:
              conn._run_visitor(visitorcallable, element, **kwargs)
  
      class _trans_ctx(object):
          def __init__(self, conn, transaction, close_with_result):
              self.conn = conn
              self.transaction = transaction
              self.close_with_result = close_with_result
  
          def __enter__(self):
              return self.conn
  
          def __exit__(self, type, value, traceback):
              if type is not None:
                  self.transaction.rollback()
              else:
                  self.transaction.commit()
              if not self.close_with_result:
                  self.conn.close()
  
      def begin(self, close_with_result=False):
          """Return a context manager delivering a :class:`.Connection`
          with a :class:`.Transaction` established.
  
          E.g.::
  
              with engine.begin() as conn:
                  conn.execute("insert into table (x, y, z) values (1, 2, 3)")
                  conn.execute("my_special_procedure(5)")
  
          Upon successful operation, the :class:`.Transaction`
          is committed.  If an error is raised, the :class:`.Transaction`
          is rolled back.
  
          The ``close_with_result`` flag is normally ``False``, and indicates
          that the :class:`.Connection` will be closed when the operation
          is complete.   When set to ``True``, it indicates the
          :class:`.Connection` is in "single use" mode, where the
          :class:`.ResultProxy` returned by the first call to
          :meth:`.Connection.execute` will close the :class:`.Connection` when
          that :class:`.ResultProxy` has exhausted all result rows.
  
          .. versionadded:: 0.7.6
  
          See also:
  
          :meth:`.Engine.connect` - procure a :class:`.Connection` from
          an :class:`.Engine`.
  
          :meth:`.Connection.begin` - start a :class:`.Transaction`
          for a particular :class:`.Connection`.
  
          """
          conn = self.contextual_connect(close_with_result=close_with_result)
          try:
              trans = conn.begin()
          except:
              with util.safe_reraise():
                  conn.close()
          return Engine._trans_ctx(conn, trans, close_with_result)
  
      def transaction(self, callable_, *args, **kwargs):
          """Execute the given function within a transaction boundary.
  
          The function is passed a :class:`.Connection` newly procured
          from :meth:`.Engine.contextual_connect` as the first argument,
          followed by the given \*args and \**kwargs.
  
          e.g.::
  
              def do_something(conn, x, y):
                  conn.execute("some statement", {'x':x, 'y':y})
  
              engine.transaction(do_something, 5, 10)
  
          The operations inside the function are all invoked within the
          context of a single :class:`.Transaction`.
          Upon success, the transaction is committed.  If an
          exception is raised, the transaction is rolled back
          before propagating the exception.
  
          .. note::
  
             The :meth:`.transaction` method is superseded by
             the usage of the Python ``with:`` statement, which can
             be used with :meth:`.Engine.begin`::
  
                 with engine.begin() as conn:
                     conn.execute("some statement", {'x':5, 'y':10})
  
          See also:
  
              :meth:`.Engine.begin` - engine-level transactional
              context
  
              :meth:`.Connection.transaction` - connection-level version of
              :meth:`.Engine.transaction`
  
          """
  
          with self.contextual_connect() as conn:
              return conn.transaction(callable_, *args, **kwargs)
  
      def run_callable(self, callable_, *args, **kwargs):
          """Given a callable object or function, execute it, passing
          a :class:`.Connection` as the first argument.
  
          The given \*args and \**kwargs are passed subsequent
          to the :class:`.Connection` argument.
  
          This function, along with :meth:`.Connection.run_callable`,
          allows a function to be run with a :class:`.Connection`
          or :class:`.Engine` object without the need to know
          which one is being dealt with.
  
          """
          with self.contextual_connect() as conn:
              return conn.run_callable(callable_, *args, **kwargs)
  
      def execute(self, statement, *multiparams, **params):
          """Executes the given construct and returns a :class:`.ResultProxy`.
  
          The arguments are the same as those used by
          :meth:`.Connection.execute`.
  
          Here, a :class:`.Connection` is acquired using the
          :meth:`~.Engine.contextual_connect` method, and the statement executed
          with that connection. The returned :class:`.ResultProxy` is flagged
          such that when the :class:`.ResultProxy` is exhausted and its
          underlying cursor is closed, the :class:`.Connection` created here
          will also be closed, which allows its associated DBAPI connection
          resource to be returned to the connection pool.
  
          """
  
          connection = self.contextual_connect(close_with_result=True)
          return connection.execute(statement, *multiparams, **params)
  
      def scalar(self, statement, *multiparams, **params):
          return self.execute(statement, *multiparams, **params).scalar()
  
      def _execute_clauseelement(self, elem, multiparams=None, params=None):
          connection = self.contextual_connect(close_with_result=True)
          return connection._execute_clauseelement(elem, multiparams, params)
  
      def _execute_compiled(self, compiled, multiparams, params):
          connection = self.contextual_connect(close_with_result=True)
          return connection._execute_compiled(compiled, multiparams, params)
  
      def connect(self, **kwargs):
          """Return a new :class:`.Connection` object.
  
          The :class:`.Connection` object is a facade that uses a DBAPI
          connection internally in order to communicate with the database.  This
          connection is procured from the connection-holding :class:`.Pool`
          referenced by this :class:`.Engine`. When the
          :meth:`~.Connection.close` method of the :class:`.Connection` object
          is called, the underlying DBAPI connection is then returned to the
          connection pool, where it may be used again in a subsequent call to
          :meth:`~.Engine.connect`.
  
          """
  
          return self._connection_cls(self, **kwargs)
  
      def contextual_connect(self, close_with_result=False, **kwargs):
          """Return a :class:`.Connection` object which may be part of some
          ongoing context.
  
          By default, this method does the same thing as :meth:`.Engine.connect`.
          Subclasses of :class:`.Engine` may override this method
          to provide contextual behavior.
  
          :param close_with_result: When True, the first :class:`.ResultProxy`
            created by the :class:`.Connection` will call the
            :meth:`.Connection.close` method of that connection as soon as any
            pending result rows are exhausted. This is used to supply the
            "connectionless execution" behavior provided by the
            :meth:`.Engine.execute` method.
  
          """
  
          return self._connection_cls(self,
                                      self.pool.connect(),
                                      close_with_result=close_with_result,
                                      **kwargs)
  
      def table_names(self, schema=None, connection=None):
          """Return a list of all table names available in the database.
  
          :param schema: Optional, retrieve names from a non-default schema.
  
          :param connection: Optional, use a specified connection. Default is
            the ``contextual_connect`` for this ``Engine``.
          """
  
          with self._optional_conn_ctx_manager(connection) as conn:
              if not schema:
                  schema = self.dialect.default_schema_name
              return self.dialect.get_table_names(conn, schema)
  
      def has_table(self, table_name, schema=None):
          """Return True if the given backend has a table of the given name.
  
          .. seealso::
  
              :ref:`metadata_reflection_inspector` - detailed schema inspection
              using the :class:`.Inspector` interface.
  
              :class:`.quoted_name` - used to pass quoting information along
              with a schema identifier.
  
          """
          return self.run_callable(self.dialect.has_table, table_name, schema)
  
      def raw_connection(self):
          """Return a "raw" DBAPI connection from the connection pool.
  
          The returned object is a proxied version of the DBAPI
          connection object used by the underlying driver in use.
          The object will have all the same behavior as the real DBAPI
          connection, except that its ``close()`` method will result in the
          connection being returned to the pool, rather than being closed
          for real.
  
          This method provides direct DBAPI connection access for
          special situations.  In most situations, the :class:`.Connection`
          object should be used, which is procured using the
          :meth:`.Engine.connect` method.
  
          """
  
          return self.pool.unique_connection()
  
  
  class OptionEngine(Engine):
      def __init__(self, proxied, execution_options):
          self._proxied = proxied
          self.url = proxied.url
          self.dialect = proxied.dialect
          self.logging_name = proxied.logging_name
          self.echo = proxied.echo
          log.instance_logger(self, echoflag=self.echo)
          self.dispatch = self.dispatch._join(proxied.dispatch)
          self._execution_options = proxied._execution_options
          self.update_execution_options(**execution_options)
  
      def _get_pool(self):
          return self._proxied.pool
  
      def _set_pool(self, pool):
          self._proxied.pool = pool
  
      pool = property(_get_pool, _set_pool)
  
      def _get_has_events(self):
          return self._proxied._has_events or \
              self.__dict__.get('_has_events', False)
  
      def _set_has_events(self, value):
          self.__dict__['_has_events'] = value
  
      _has_events = property(_get_has_events, _set_has_events)
  