text: # sqlalchemy/events.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
  
  """Core event interfaces."""
  
  from . import event, exc
  from .pool import Pool
  from .engine import Connectable, Engine, Dialect
  from .sql.base import SchemaEventTarget
  
  
  class DDLEvents(event.Events):
      """
      Define event listeners for schema objects,
      that is, :class:`.SchemaItem` and other :class:`.SchemaEventTarget`
      subclasses, including :class:`.MetaData`, :class:`.Table`,
      :class:`.Column`.
  
      :class:`.MetaData` and :class:`.Table` support events
      specifically regarding when CREATE and DROP
      DDL is emitted to the database.
  
      Attachment events are also provided to customize
      behavior whenever a child schema element is associated
      with a parent, such as, when a :class:`.Column` is associated
      with its :class:`.Table`, when a :class:`.ForeignKeyConstraint`
      is associated with a :class:`.Table`, etc.
  
      Example using the ``after_create`` event::
  
          from sqlalchemy import event
          from sqlalchemy import Table, Column, Metadata, Integer
  
          m = MetaData()
          some_table = Table('some_table', m, Column('data', Integer))
  
          def after_create(target, connection, **kw):
              connection.execute("ALTER TABLE %s SET name=foo_%s" %
                                      (target.name, target.name))
  
          event.listen(some_table, "after_create", after_create)
  
      DDL events integrate closely with the
      :class:`.DDL` class and the :class:`.DDLElement` hierarchy
      of DDL clause constructs, which are themselves appropriate
      as listener callables::
  
          from sqlalchemy import DDL
          event.listen(
              some_table,
              "after_create",
              DDL("ALTER TABLE %(table)s SET name=foo_%(table)s")
          )
  
      The methods here define the name of an event as well
      as the names of members that are passed to listener
      functions.
  
      See also:
  
          :ref:`event_toplevel`
  
          :class:`.DDLElement`
  
          :class:`.DDL`
  
          :ref:`schema_ddl_sequences`
  
      """
  
      _target_class_doc = "SomeSchemaClassOrObject"
      _dispatch_target = SchemaEventTarget
  
      def before_create(self, target, connection, **kw):
          r"""Called before CREATE statements are emitted.
  
          :param target: the :class:`.MetaData` or :class:`.Table`
           object which is the target of the event.
          :param connection: the :class:`.Connection` where the
           CREATE statement or statements will be emitted.
          :param \**kw: additional keyword arguments relevant
           to the event.  The contents of this dictionary
           may vary across releases, and include the
           list of tables being generated for a metadata-level
           event, the checkfirst flag, and other
           elements used by internal events.
  
          """
  
      def after_create(self, target, connection, **kw):
          r"""Called after CREATE statements are emitted.
  
          :param target: the :class:`.MetaData` or :class:`.Table`
           object which is the target of the event.
          :param connection: the :class:`.Connection` where the
           CREATE statement or statements have been emitted.
          :param \**kw: additional keyword arguments relevant
           to the event.  The contents of this dictionary
           may vary across releases, and include the
           list of tables being generated for a metadata-level
           event, the checkfirst flag, and other
           elements used by internal events.
  
          """
  
      def before_drop(self, target, connection, **kw):
          r"""Called before DROP statements are emitted.
  
          :param target: the :class:`.MetaData` or :class:`.Table`
           object which is the target of the event.
          :param connection: the :class:`.Connection` where the
           DROP statement or statements will be emitted.
          :param \**kw: additional keyword arguments relevant
           to the event.  The contents of this dictionary
           may vary across releases, and include the
           list of tables being generated for a metadata-level
           event, the checkfirst flag, and other
           elements used by internal events.
  
          """
  
      def after_drop(self, target, connection, **kw):
          r"""Called after DROP statements are emitted.
  
          :param target: the :class:`.MetaData` or :class:`.Table`
           object which is the target of the event.
          :param connection: the :class:`.Connection` where the
           DROP statement or statements have been emitted.
          :param \**kw: additional keyword arguments relevant
           to the event.  The contents of this dictionary
           may vary across releases, and include the
           list of tables being generated for a metadata-level
           event, the checkfirst flag, and other
           elements used by internal events.
  
          """
  
      def before_parent_attach(self, target, parent):
          """Called before a :class:`.SchemaItem` is associated with
          a parent :class:`.SchemaItem`.
  
          :param target: the target object
          :param parent: the parent to which the target is being attached.
  
          :func:`.event.listen` also accepts a modifier for this event:
  
          :param propagate=False: When True, the listener function will
           be established for any copies made of the target object,
           i.e. those copies that are generated when
           :meth:`.Table.tometadata` is used.
  
          """
  
      def after_parent_attach(self, target, parent):
          """Called after a :class:`.SchemaItem` is associated with
          a parent :class:`.SchemaItem`.
  
          :param target: the target object
          :param parent: the parent to which the target is being attached.
  
          :func:`.event.listen` also accepts a modifier for this event:
  
          :param propagate=False: When True, the listener function will
           be established for any copies made of the target object,
           i.e. those copies that are generated when
           :meth:`.Table.tometadata` is used.
  
          """
  
      def column_reflect(self, inspector, table, column_info):
          """Called for each unit of 'column info' retrieved when
          a :class:`.Table` is being reflected.
  
          The dictionary of column information as returned by the
          dialect is passed, and can be modified.  The dictionary
          is that returned in each element of the list returned
          by :meth:`.reflection.Inspector.get_columns`.
  
          The event is called before any action is taken against
          this dictionary, and the contents can be modified.
          The :class:`.Column` specific arguments ``info``, ``key``,
          and ``quote`` can also be added to the dictionary and
          will be passed to the constructor of :class:`.Column`.
  
          Note that this event is only meaningful if either
          associated with the :class:`.Table` class across the
          board, e.g.::
  
              from sqlalchemy.schema import Table
              from sqlalchemy import event
  
              def listen_for_reflect(inspector, table, column_info):
                  "receive a column_reflect event"
                  # ...
  
              event.listen(
                      Table,
                      'column_reflect',
                      listen_for_reflect)
  
          ...or with a specific :class:`.Table` instance using
          the ``listeners`` argument::
  
              def listen_for_reflect(inspector, table, column_info):
                  "receive a column_reflect event"
                  # ...
  
              t = Table(
                  'sometable',
                  autoload=True,
                  listeners=[
                      ('column_reflect', listen_for_reflect)
                  ])
  
          This because the reflection process initiated by ``autoload=True``
          completes within the scope of the constructor for :class:`.Table`.
  
          """
  
  
  class PoolEvents(event.Events):
      """Available events for :class:`.Pool`.
  
      The methods here define the name of an event as well
      as the names of members that are passed to listener
      functions.
  
      e.g.::
  
          from sqlalchemy import event
  
          def my_on_checkout(dbapi_conn, connection_rec, connection_proxy):
              "handle an on checkout event"
  
          event.listen(Pool, 'checkout', my_on_checkout)
  
      In addition to accepting the :class:`.Pool` class and
      :class:`.Pool` instances, :class:`.PoolEvents` also accepts
      :class:`.Engine` objects and the :class:`.Engine` class as
      targets, which will be resolved to the ``.pool`` attribute of the
      given engine or the :class:`.Pool` class::
  
          engine = create_engine("postgresql://scott:tiger@localhost/test")
  
          # will associate with engine.pool
          event.listen(engine, 'checkout', my_on_checkout)
  
      """
  
      _target_class_doc = "SomeEngineOrPool"
      _dispatch_target = Pool
  
      @classmethod
      def _accept_with(cls, target):
          if isinstance(target, type):
              if issubclass(target, Engine):
                  return Pool
              elif issubclass(target, Pool):
                  return target
          elif isinstance(target, Engine):
              return target.pool
          else:
              return target
  
      def connect(self, dbapi_connection, connection_record):
          """Called at the moment a particular DBAPI connection is first
          created for a given :class:`.Pool`.
  
          This event allows one to capture the point directly after which
          the DBAPI module-level ``.connect()`` method has been used in order
          to produce a new DBAPI connection.
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          """
  
      def first_connect(self, dbapi_connection, connection_record):
          """Called exactly once for the first time a DBAPI connection is
          checked out from a particular :class:`.Pool`.
  
          The rationale for :meth:`.PoolEvents.first_connect` is to determine
          information about a particular series of database connections based
          on the settings used for all connections.  Since a particular
          :class:`.Pool` refers to a single "creator" function (which in terms
          of a :class:`.Engine` refers to the URL and connection options used),
          it is typically valid to make observations about a single connection
          that can be safely assumed to be valid about all subsequent
          connections, such as the database version, the server and client
          encoding settings, collation settings, and many others.
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          """
  
      def checkout(self, dbapi_connection, connection_record, connection_proxy):
          """Called when a connection is retrieved from the Pool.
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          :param connection_proxy: the :class:`._ConnectionFairy` object which
            will proxy the public interface of the DBAPI connection for the
            lifespan of the checkout.
  
          If you raise a :class:`~sqlalchemy.exc.DisconnectionError`, the current
          connection will be disposed and a fresh connection retrieved.
          Processing of all checkout listeners will abort and restart
          using the new connection.
  
          .. seealso:: :meth:`.ConnectionEvents.engine_connect` - a similar event
             which occurs upon creation of a new :class:`.Connection`.
  
          """
  
      def checkin(self, dbapi_connection, connection_record):
          """Called when a connection returns to the pool.
  
          Note that the connection may be closed, and may be None if the
          connection has been invalidated.  ``checkin`` will not be called
          for detached connections.  (They do not return to the pool.)
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          """
  
      def reset(self, dbapi_connection, connection_record):
          """Called before the "reset" action occurs for a pooled connection.
  
          This event represents
          when the ``rollback()`` method is called on the DBAPI connection
          before it is returned to the pool.  The behavior of "reset" can
          be controlled, including disabled, using the ``reset_on_return``
          pool argument.
  
  
          The :meth:`.PoolEvents.reset` event is usually followed by the
          :meth:`.PoolEvents.checkin` event is called, except in those
          cases where the connection is discarded immediately after reset.
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          .. versionadded:: 0.8
  
          .. seealso::
  
              :meth:`.ConnectionEvents.rollback`
  
              :meth:`.ConnectionEvents.commit`
  
          """
  
      def invalidate(self, dbapi_connection, connection_record, exception):
          """Called when a DBAPI connection is to be "invalidated".
  
          This event is called any time the :meth:`._ConnectionRecord.invalidate`
          method is invoked, either from API usage or via "auto-invalidation",
          without the ``soft`` flag.
  
          The event occurs before a final attempt to call ``.close()`` on the
          connection occurs.
  
          :param dbapi_connection: a DBAPI connection.
  
          :param connection_record: the :class:`._ConnectionRecord` managing the
           DBAPI connection.
  
          :param exception: the exception object corresponding to the reason
           for this invalidation, if any.  May be ``None``.
  
          .. versionadded:: 0.9.2 Added support for connection invalidation
             listening.
  
          .. seealso::
  
              :ref:`pool_connection_invalidation`
  
          """
  
      def soft_invalidate(self, dbapi_connection, connection_record, exception):
          """Called when a DBAPI connection is to be "soft invalidated".
  
          This event is called any time the :meth:`._ConnectionRecord.invalidate`
          method is invoked with the ``soft`` flag.
  
          Soft invalidation refers to when the connection record that tracks
          this connection will force a reconnect after the current connection
          is checked in.   It does not actively close the dbapi_connection
          at the point at which it is called.
  
          .. versionadded:: 1.0.3
  
          """
  
      def close(self, dbapi_connection, connection_record):
          """Called when a DBAPI connection is closed.
  
          The event is emitted before the close occurs.
  
          The close of a connection can fail; typically this is because
          the connection is already closed.  If the close operation fails,
          the connection is discarded.
  
          The :meth:`.close` event corresponds to a connection that's still
          associated with the pool. To intercept close events for detached
          connections use :meth:`.close_detached`.
  
          .. versionadded:: 1.1
  
          """
  
      def detach(self, dbapi_connection, connection_record):
          """Called when a DBAPI connection is "detached" from a pool.
  
          This event is emitted after the detach occurs.  The connection
          is no longer associated with the given connection record.
  
          .. versionadded:: 1.1
  
          """
  
      def close_detached(self, dbapi_connection):
          """Called when a detached DBAPI connection is closed.
  
          The event is emitted before the close occurs.
  
          The close of a connection can fail; typically this is because
          the connection is already closed.  If the close operation fails,
          the connection is discarded.
  
          .. versionadded:: 1.1
  
          """
  
  
  class ConnectionEvents(event.Events):
      """Available events for :class:`.Connectable`, which includes
      :class:`.Connection` and :class:`.Engine`.
  
      The methods here define the name of an event as well as the names of
      members that are passed to listener functions.
  
      An event listener can be associated with any :class:`.Connectable`
      class or instance, such as an :class:`.Engine`, e.g.::
  
          from sqlalchemy import event, create_engine
  
          def before_cursor_execute(conn, cursor, statement, parameters, context,
                                                          executemany):
              log.info("Received statement: %s", statement)
  
          engine = create_engine('postgresql://scott:tiger@localhost/test')
          event.listen(engine, "before_cursor_execute", before_cursor_execute)
  
      or with a specific :class:`.Connection`::
  
          with engine.begin() as conn:
              @event.listens_for(conn, 'before_cursor_execute')
              def before_cursor_execute(conn, cursor, statement, parameters,
                                              context, executemany):
                  log.info("Received statement: %s", statement)
  
      When the methods are called with a `statement` parameter, such as in
      :meth:`.after_cursor_execute`, :meth:`.before_cursor_execute` and
      :meth:`.dbapi_error`, the statement is the exact SQL string that was
      prepared for transmission to the DBAPI ``cursor`` in the connection's
      :class:`.Dialect`.
  
      The :meth:`.before_execute` and :meth:`.before_cursor_execute`
      events can also be established with the ``retval=True`` flag, which
      allows modification of the statement and parameters to be sent
      to the database.  The :meth:`.before_cursor_execute` event is
      particularly useful here to add ad-hoc string transformations, such
      as comments, to all executions::
  
          from sqlalchemy.engine import Engine
          from sqlalchemy import event
  
          @event.listens_for(Engine, "before_cursor_execute", retval=True)
          def comment_sql_calls(conn, cursor, statement, parameters,
                                              context, executemany):
              statement = statement + " -- some comment"
              return statement, parameters
  
      .. note:: :class:`.ConnectionEvents` can be established on any
         combination of :class:`.Engine`, :class:`.Connection`, as well
         as instances of each of those classes.  Events across all
         four scopes will fire off for a given instance of
         :class:`.Connection`.  However, for performance reasons, the
         :class:`.Connection` object determines at instantiation time
         whether or not its parent :class:`.Engine` has event listeners
         established.   Event listeners added to the :class:`.Engine`
         class or to an instance of :class:`.Engine` *after* the instantiation
         of a dependent :class:`.Connection` instance will usually
         *not* be available on that :class:`.Connection` instance.  The newly
         added listeners will instead take effect for :class:`.Connection`
         instances created subsequent to those event listeners being
         established on the parent :class:`.Engine` class or instance.
  
      :param retval=False: Applies to the :meth:`.before_execute` and
        :meth:`.before_cursor_execute` events only.  When True, the
        user-defined event function must have a return value, which
        is a tuple of parameters that replace the given statement
        and parameters.  See those methods for a description of
        specific return arguments.
  
      .. versionchanged:: 0.8 :class:`.ConnectionEvents` can now be associated
         with any :class:`.Connectable` including :class:`.Connection`,
         in addition to the existing support for :class:`.Engine`.
  
      """
  
      _target_class_doc = "SomeEngine"
      _dispatch_target = Connectable
  
      @classmethod
      def _listen(cls, event_key, retval=False):
          target, identifier, fn = \
              event_key.dispatch_target, event_key.identifier, \
              event_key._listen_fn
  
          target._has_events = True
  
          if not retval:
              if identifier == 'before_execute':
                  orig_fn = fn
  
                  def wrap_before_execute(conn, clauseelement,
                                          multiparams, params):
                      orig_fn(conn, clauseelement, multiparams, params)
                      return clauseelement, multiparams, params
                  fn = wrap_before_execute
              elif identifier == 'before_cursor_execute':
                  orig_fn = fn
  
                  def wrap_before_cursor_execute(conn, cursor, statement,
                                                 parameters, context,
                                                 executemany):
                      orig_fn(conn, cursor, statement,
                              parameters, context, executemany)
                      return statement, parameters
                  fn = wrap_before_cursor_execute
          elif retval and \
              identifier not in ('before_execute',
                                 'before_cursor_execute', 'handle_error'):
              raise exc.ArgumentError(
                  "Only the 'before_execute', "
                  "'before_cursor_execute' and 'handle_error' engine "
                  "event listeners accept the 'retval=True' "
                  "argument.")
          event_key.with_wrapper(fn).base_listen()
  
      def before_execute(self, conn, clauseelement, multiparams, params):
          """Intercept high level execute() events, receiving uncompiled
          SQL constructs and other objects prior to rendering into SQL.
  
          This event is good for debugging SQL compilation issues as well
          as early manipulation of the parameters being sent to the database,
          as the parameter lists will be in a consistent format here.
  
          This event can be optionally established with the ``retval=True``
          flag.  The ``clauseelement``, ``multiparams``, and ``params``
          arguments should be returned as a three-tuple in this case::
  
              @event.listens_for(Engine, "before_execute", retval=True)
              def before_execute(conn, conn, clauseelement, multiparams, params):
                  # do something with clauseelement, multiparams, params
                  return clauseelement, multiparams, params
  
          :param conn: :class:`.Connection` object
          :param clauseelement: SQL expression construct, :class:`.Compiled`
           instance, or string statement passed to :meth:`.Connection.execute`.
          :param multiparams: Multiple parameter sets, a list of dictionaries.
          :param params: Single parameter set, a single dictionary.
  
          See also:
  
          :meth:`.before_cursor_execute`
  
          """
  
      def after_execute(self, conn, clauseelement, multiparams, params, result):
          """Intercept high level execute() events after execute.
  
  
          :param conn: :class:`.Connection` object
          :param clauseelement: SQL expression construct, :class:`.Compiled`
           instance, or string statement passed to :meth:`.Connection.execute`.
          :param multiparams: Multiple parameter sets, a list of dictionaries.
          :param params: Single parameter set, a single dictionary.
          :param result: :class:`.ResultProxy` generated by the execution.
  
          """
  
      def before_cursor_execute(self, conn, cursor, statement,
                                parameters, context, executemany):
          """Intercept low-level cursor execute() events before execution,
          receiving the string SQL statement and DBAPI-specific parameter list to
          be invoked against a cursor.
  
          This event is a good choice for logging as well as late modifications
          to the SQL string.  It's less ideal for parameter modifications except
          for those which are specific to a target backend.
  
          This event can be optionally established with the ``retval=True``
          flag.  The ``statement`` and ``parameters`` arguments should be
          returned as a two-tuple in this case::
  
              @event.listens_for(Engine, "before_cursor_execute", retval=True)
              def before_cursor_execute(conn, cursor, statement,
                              parameters, context, executemany):
                  # do something with statement, parameters
                  return statement, parameters
  
          See the example at :class:`.ConnectionEvents`.
  
          :param conn: :class:`.Connection` object
          :param cursor: DBAPI cursor object
          :param statement: string SQL statement, as to be passed to the DBAPI
          :param parameters: Dictionary, tuple, or list of parameters being
           passed to the ``execute()`` or ``executemany()`` method of the
           DBAPI ``cursor``.  In some cases may be ``None``.
          :param context: :class:`.ExecutionContext` object in use.  May
           be ``None``.
          :param executemany: boolean, if ``True``, this is an ``executemany()``
           call, if ``False``, this is an ``execute()`` call.
  
          See also:
  
          :meth:`.before_execute`
  
          :meth:`.after_cursor_execute`
  
          """
  
      def after_cursor_execute(self, conn, cursor, statement,
                               parameters, context, executemany):
          """Intercept low-level cursor execute() events after execution.
  
          :param conn: :class:`.Connection` object
          :param cursor: DBAPI cursor object.  Will have results pending
           if the statement was a SELECT, but these should not be consumed
           as they will be needed by the :class:`.ResultProxy`.
          :param statement: string SQL statement, as passed to the DBAPI
          :param parameters: Dictionary, tuple, or list of parameters being
           passed to the ``execute()`` or ``executemany()`` method of the
           DBAPI ``cursor``.  In some cases may be ``None``.
          :param context: :class:`.ExecutionContext` object in use.  May
           be ``None``.
          :param executemany: boolean, if ``True``, this is an ``executemany()``
           call, if ``False``, this is an ``execute()`` call.
  
          """
  
      def dbapi_error(self, conn, cursor, statement, parameters,
                      context, exception):
          """Intercept a raw DBAPI error.
  
          This event is called with the DBAPI exception instance
          received from the DBAPI itself, *before* SQLAlchemy wraps the
          exception with it's own exception wrappers, and before any
          other operations are performed on the DBAPI cursor; the
          existing transaction remains in effect as well as any state
          on the cursor.
  
          The use case here is to inject low-level exception handling
          into an :class:`.Engine`, typically for logging and
          debugging purposes.
  
          .. warning::
  
              Code should **not** modify
              any state or throw any exceptions here as this will
              interfere with SQLAlchemy's cleanup and error handling
              routines.  For exception modification, please refer to the
              new :meth:`.ConnectionEvents.handle_error` event.
  
          Subsequent to this hook, SQLAlchemy may attempt any
          number of operations on the connection/cursor, including
          closing the cursor, rolling back of the transaction in the
          case of connectionless execution, and disposing of the entire
          connection pool if a "disconnect" was detected.   The
          exception is then wrapped in a SQLAlchemy DBAPI exception
          wrapper and re-thrown.
  
          :param conn: :class:`.Connection` object
          :param cursor: DBAPI cursor object
          :param statement: string SQL statement, as passed to the DBAPI
          :param parameters: Dictionary, tuple, or list of parameters being
           passed to the ``execute()`` or ``executemany()`` method of the
           DBAPI ``cursor``.  In some cases may be ``None``.
          :param context: :class:`.ExecutionContext` object in use.  May
           be ``None``.
          :param exception: The **unwrapped** exception emitted directly from the
           DBAPI.  The class here is specific to the DBAPI module in use.
  
          .. deprecated:: 0.9.7 - replaced by
              :meth:`.ConnectionEvents.handle_error`
  
          """
  
      def handle_error(self, exception_context):
          r"""Intercept all exceptions processed by the :class:`.Connection`.
  
          This includes all exceptions emitted by the DBAPI as well as
          within SQLAlchemy's statement invocation process, including
          encoding errors and other statement validation errors.  Other areas
          in which the event is invoked include transaction begin and end,
          result row fetching, cursor creation.
  
          Note that :meth:`.handle_error` may support new kinds of exceptions
          and new calling scenarios at *any time*.  Code which uses this
          event must expect new calling patterns to be present in minor
          releases.
  
          To support the wide variety of members that correspond to an exception,
          as well as to allow extensibility of the event without backwards
          incompatibility, the sole argument received is an instance of
          :class:`.ExceptionContext`.   This object contains data members
          representing detail about the exception.
  
          Use cases supported by this hook include:
  
          * read-only, low-level exception handling for logging and
            debugging purposes
          * exception re-writing
          * Establishing or disabling whether a connection or the owning
            connection pool is invalidated or expired in response to a
            specific exception.
  
          The hook is called while the cursor from the failed operation
          (if any) is still open and accessible.   Special cleanup operations
          can be called on this cursor; SQLAlchemy will attempt to close
          this cursor subsequent to this hook being invoked.  If the connection
          is in "autocommit" mode, the transaction also remains open within
          the scope of this hook; the rollback of the per-statement transaction
          also occurs after the hook is called.
  
          The user-defined event handler has two options for replacing
          the SQLAlchemy-constructed exception into one that is user
          defined.   It can either raise this new exception directly, in
          which case all further event listeners are bypassed and the
          exception will be raised, after appropriate cleanup as taken
          place::
  
              @event.listens_for(Engine, "handle_error")
              def handle_exception(context):
                  if isinstance(context.original_exception,
                      psycopg2.OperationalError) and \
                      "failed" in str(context.original_exception):
                      raise MySpecialException("failed operation")
  
          .. warning::  Because the :meth:`.ConnectionEvents.handle_error`
             event specifically provides for exceptions to be re-thrown as
             the ultimate exception raised by the failed statement,
             **stack traces will be misleading** if the user-defined event
             handler itself fails and throws an unexpected exception;
             the stack trace may not illustrate the actual code line that
             failed!  It is advised to code carefully here and use
             logging and/or inline debugging if unexpected exceptions are
             occurring.
  
          Alternatively, a "chained" style of event handling can be
          used, by configuring the handler with the ``retval=True``
          modifier and returning the new exception instance from the
          function.  In this case, event handling will continue onto the
          next handler.   The "chained" exception is available using
          :attr:`.ExceptionContext.chained_exception`::
  
              @event.listens_for(Engine, "handle_error", retval=True)
              def handle_exception(context):
                  if context.chained_exception is not None and \
                      "special" in context.chained_exception.message:
                      return MySpecialException("failed",
                          cause=context.chained_exception)
  
          Handlers that return ``None`` may remain within this chain; the
          last non-``None`` return value is the one that continues to be
          passed to the next handler.
  
          When a custom exception is raised or returned, SQLAlchemy raises
          this new exception as-is, it is not wrapped by any SQLAlchemy
          object.  If the exception is not a subclass of
          :class:`sqlalchemy.exc.StatementError`,
          certain features may not be available; currently this includes
          the ORM's feature of adding a detail hint about "autoflush" to
          exceptions raised within the autoflush process.
  
          :param context: an :class:`.ExceptionContext` object.  See this
           class for details on all available members.
  
          .. versionadded:: 0.9.7 Added the
              :meth:`.ConnectionEvents.handle_error` hook.
  
          .. versionchanged:: 1.1 The :meth:`.handle_error` event will now
             receive all exceptions that inherit from ``BaseException``, including
             ``SystemExit`` and ``KeyboardInterrupt``.  The setting for
             :attr:`.ExceptionContext.is_disconnect` is ``True`` in this case
             and the default for :attr:`.ExceptionContext.invalidate_pool_on_disconnect`
             is ``False``.
  
          .. versionchanged:: 1.0.0 The :meth:`.handle_error` event is now
             invoked when an :class:`.Engine` fails during the initial
             call to :meth:`.Engine.connect`, as well as when a
             :class:`.Connection` object encounters an error during a
             reconnect operation.
  
          .. versionchanged:: 1.0.0 The :meth:`.handle_error` event is
             not fired off when a dialect makes use of the
             ``skip_user_error_events`` execution option.   This is used
             by dialects which intend to catch SQLAlchemy-specific exceptions
             within specific operations, such as when the MySQL dialect detects
             a table not present within the ``has_table()`` dialect method.
             Prior to 1.0.0, code which implements :meth:`.handle_error` needs
             to ensure that exceptions thrown in these scenarios are re-raised
             without modification.
  
          """
  
      def engine_connect(self, conn, branch):
          """Intercept the creation of a new :class:`.Connection`.
  
          This event is called typically as the direct result of calling
          the :meth:`.Engine.connect` method.
  
          It differs from the :meth:`.PoolEvents.connect` method, which
          refers to the actual connection to a database at the DBAPI level;
          a DBAPI connection may be pooled and reused for many operations.
          In contrast, this event refers only to the production of a higher level
          :class:`.Connection` wrapper around such a DBAPI connection.
  
          It also differs from the :meth:`.PoolEvents.checkout` event
          in that it is specific to the :class:`.Connection` object, not the
          DBAPI connection that :meth:`.PoolEvents.checkout` deals with, although
          this DBAPI connection is available here via the
          :attr:`.Connection.connection` attribute.  But note there can in fact
          be multiple :meth:`.PoolEvents.checkout` events within the lifespan
          of a single :class:`.Connection` object, if that :class:`.Connection`
          is invalidated and re-established.  There can also be multiple
          :class:`.Connection` objects generated for the same already-checked-out
          DBAPI connection, in the case that a "branch" of a :class:`.Connection`
          is produced.
  
          :param conn: :class:`.Connection` object.
          :param branch: if True, this is a "branch" of an existing
           :class:`.Connection`.  A branch is generated within the course
           of a statement execution to invoke supplemental statements, most
           typically to pre-execute a SELECT of a default value for the purposes
           of an INSERT statement.
  
          .. versionadded:: 0.9.0
  
          .. seealso::
  
              :ref:`pool_disconnects_pessimistic` - illustrates how to use
              :meth:`.ConnectionEvents.engine_connect`
              to transparently ensure pooled connections are connected to the
              database.
  
              :meth:`.PoolEvents.checkout` the lower-level pool checkout event
              for an individual DBAPI connection
  
              :meth:`.ConnectionEvents.set_connection_execution_options` - a copy
              of a :class:`.Connection` is also made when the
              :meth:`.Connection.execution_options` method is called.
  
          """
  
      def set_connection_execution_options(self, conn, opts):
          """Intercept when the :meth:`.Connection.execution_options`
          method is called.
  
          This method is called after the new :class:`.Connection` has been
          produced, with the newly updated execution options collection, but
          before the :class:`.Dialect` has acted upon any of those new options.
  
          Note that this method is not called when a new :class:`.Connection`
          is produced which is inheriting execution options from its parent
          :class:`.Engine`; to intercept this condition, use the
          :meth:`.ConnectionEvents.engine_connect` event.
  
          :param conn: The newly copied :class:`.Connection` object
  
          :param opts: dictionary of options that were passed to the
           :meth:`.Connection.execution_options` method.
  
          .. versionadded:: 0.9.0
  
          .. seealso::
  
              :meth:`.ConnectionEvents.set_engine_execution_options` - event
              which is called when :meth:`.Engine.execution_options` is called.
  
  
          """
  
      def set_engine_execution_options(self, engine, opts):
          """Intercept when the :meth:`.Engine.execution_options`
          method is called.
  
          The :meth:`.Engine.execution_options` method produces a shallow
          copy of the :class:`.Engine` which stores the new options.  That new
          :class:`.Engine` is passed here.   A particular application of this
          method is to add a :meth:`.ConnectionEvents.engine_connect` event
          handler to the given :class:`.Engine` which will perform some per-
          :class:`.Connection` task specific to these execution options.
  
          :param conn: The newly copied :class:`.Engine` object
  
          :param opts: dictionary of options that were passed to the
           :meth:`.Connection.execution_options` method.
  
          .. versionadded:: 0.9.0
  
          .. seealso::
  
              :meth:`.ConnectionEvents.set_connection_execution_options` - event
              which is called when :meth:`.Connection.execution_options` is
              called.
  
          """
  
      def engine_disposed(self, engine):
          """Intercept when the :meth:`.Engine.dispose` method is called.
  
          The :meth:`.Engine.dispose` method instructs the engine to
          "dispose" of it's connection pool (e.g. :class:`.Pool`), and
          replaces it with a new one.  Disposing of the old pool has the
          effect that existing checked-in connections are closed.  The new
          pool does not establish any new connections until it is first used.
  
          This event can be used to indicate that resources related to the
          :class:`.Engine` should also be cleaned up, keeping in mind that the
          :class:`.Engine` can still be used for new requests in which case
          it re-acquires connection resources.
  
          .. versionadded:: 1.0.5
  
          """
      def begin(self, conn):
          """Intercept begin() events.
  
          :param conn: :class:`.Connection` object
  
          """
  
      def rollback(self, conn):
          """Intercept rollback() events, as initiated by a
          :class:`.Transaction`.
  
          Note that the :class:`.Pool` also "auto-rolls back"
          a DBAPI connection upon checkin, if the ``reset_on_return``
          flag is set to its default value of ``'rollback'``.
          To intercept this
          rollback, use the :meth:`.PoolEvents.reset` hook.
  
          :param conn: :class:`.Connection` object
  
          .. seealso::
  
              :meth:`.PoolEvents.reset`
  
          """
  
      def commit(self, conn):
          """Intercept commit() events, as initiated by a
          :class:`.Transaction`.
  
          Note that the :class:`.Pool` may also "auto-commit"
          a DBAPI connection upon checkin, if the ``reset_on_return``
          flag is set to the value ``'commit'``.  To intercept this
          commit, use the :meth:`.PoolEvents.reset` hook.
  
          :param conn: :class:`.Connection` object
          """
  
      def savepoint(self, conn, name):
          """Intercept savepoint() events.
  
          :param conn: :class:`.Connection` object
          :param name: specified name used for the savepoint.
  
          """
  
      def rollback_savepoint(self, conn, name, context):
          """Intercept rollback_savepoint() events.
  
          :param conn: :class:`.Connection` object
          :param name: specified name used for the savepoint.
          :param context: :class:`.ExecutionContext` in use.  May be ``None``.
  
          """
  
      def release_savepoint(self, conn, name, context):
          """Intercept release_savepoint() events.
  
          :param conn: :class:`.Connection` object
          :param name: specified name used for the savepoint.
          :param context: :class:`.ExecutionContext` in use.  May be ``None``.
  
          """
  
      def begin_twophase(self, conn, xid):
          """Intercept begin_twophase() events.
  
          :param conn: :class:`.Connection` object
          :param xid: two-phase XID identifier
  
          """
  
      def prepare_twophase(self, conn, xid):
          """Intercept prepare_twophase() events.
  
          :param conn: :class:`.Connection` object
          :param xid: two-phase XID identifier
          """
  
      def rollback_twophase(self, conn, xid, is_prepared):
          """Intercept rollback_twophase() events.
  
          :param conn: :class:`.Connection` object
          :param xid: two-phase XID identifier
          :param is_prepared: boolean, indicates if
           :meth:`.TwoPhaseTransaction.prepare` was called.
  
          """
  
      def commit_twophase(self, conn, xid, is_prepared):
          """Intercept commit_twophase() events.
  
          :param conn: :class:`.Connection` object
          :param xid: two-phase XID identifier
          :param is_prepared: boolean, indicates if
           :meth:`.TwoPhaseTransaction.prepare` was called.
  
          """
  
  
  class DialectEvents(event.Events):
      """event interface for execution-replacement functions.
  
      These events allow direct instrumentation and replacement
      of key dialect functions which interact with the DBAPI.
  
      .. note::
  
          :class:`.DialectEvents` hooks should be considered **semi-public**
          and experimental.
          These hooks are not for general use and are only for those situations
          where intricate re-statement of DBAPI mechanics must be injected onto
          an existing dialect.  For general-use statement-interception events,
          please use the :class:`.ConnectionEvents` interface.
  
      .. seealso::
  
          :meth:`.ConnectionEvents.before_cursor_execute`
  
          :meth:`.ConnectionEvents.before_execute`
  
          :meth:`.ConnectionEvents.after_cursor_execute`
  
          :meth:`.ConnectionEvents.after_execute`
  
  
      .. versionadded:: 0.9.4
  
      """
  
      _target_class_doc = "SomeEngine"
      _dispatch_target = Dialect
  
      @classmethod
      def _listen(cls, event_key, retval=False):
          target, identifier, fn = \
              event_key.dispatch_target, event_key.identifier, event_key.fn
  
          target._has_events = True
          event_key.base_listen()
  
      @classmethod
      def _accept_with(cls, target):
          if isinstance(target, type):
              if issubclass(target, Engine):
                  return Dialect
              elif issubclass(target, Dialect):
                  return target
          elif isinstance(target, Engine):
              return target.dialect
          else:
              return target
  
      def do_connect(self, dialect, conn_rec, cargs, cparams):
          """Receive connection arguments before a connection is made.
  
          Return a DBAPI connection to halt further events from invoking;
          the returned connection will be used.
  
          Alternatively, the event can manipulate the cargs and/or cparams
          collections; cargs will always be a Python list that can be mutated
          in-place and cparams a Python dictionary.  Return None to
          allow control to pass to the next event handler and ultimately
          to allow the dialect to connect normally, given the updated
          arguments.
  
          .. versionadded:: 1.0.3
  
          """
  
      def do_executemany(self, cursor, statement, parameters, context):
          """Receive a cursor to have executemany() called.
  
          Return the value True to halt further events from invoking,
          and to indicate that the cursor execution has already taken
          place within the event handler.
  
          """
  
      def do_execute_no_params(self, cursor, statement, context):
          """Receive a cursor to have execute() with no parameters called.
  
          Return the value True to halt further events from invoking,
          and to indicate that the cursor execution has already taken
          place within the event handler.
  
          """
  
      def do_execute(self, cursor, statement, parameters, context):
          """Receive a cursor to have execute() called.
  
          Return the value True to halt further events from invoking,
          and to indicate that the cursor execution has already taken
          place within the event handler.
  
          """
  