# SQLAlchemy 0.8 Documentation

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Release: 0.8.1 | Release Date: April 27, 2013

# Core Events

This section describes the event interfaces provided in SQLAlchemy Core. For an introduction to the event listening API, see Events. ORM events are described in ORM Events.

New in version 0.7: The event system supercedes the previous system of “extension”, “listener”, and “proxy” classes.

## Connection Pool Events

class `sqlalchemy.events.``PoolEvents`

  Available events for `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 `Pool` class and `Pool` instances, `PoolEvents` also accepts `Engine` objects and the `Engine` class as targets, which will be resolved to the `.pool` attribute of the given engine or the `Pool` class:

      engine = create_engine("postgresql://scott:tiger@localhost/test")

      # will associate with engine.pool
      event.listen(engine, 'checkout', my_on_checkout)

  `checkin`(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.)

    Parameters:

    - dbapi_con – A raw DB-API connection
    - con_record – The `_ConnectionRecord` that persistently manages the connection

  `checkout`(dbapi_connection, connection_record, connection_proxy)

    Called when a connection is retrieved from the Pool.

    Parameters:

    - dbapi_con – A raw DB-API connection
    - con_record – The `_ConnectionRecord` that persistently manages the connection
    - con_proxy – The `_ConnectionFairy` which manages the connection for the span of the current checkout.

    If you raise a `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.

  `connect`(dbapi_connection, connection_record)

    Called once for each new DB-API connection or Pool’s `creator()`.

    Parameters:

    - dbapi_con – A newly connected raw DB-API connection (not a SQLAlchemy `Connection` wrapper).
    - con_record – The `_ConnectionRecord` that persistently manages the connection

  `first_connect`(dbapi_connection, connection_record)

    Called exactly once for the first DB-API connection.

    Parameters:

    - dbapi_con – A newly connected raw DB-API connection (not a SQLAlchemy `Connection` wrapper).
    - con_record – The `_ConnectionRecord` that persistently manages the connection

  `reset`(dbapi_con, con_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 `PoolEvents.reset()` event is usually followed by the the `PoolEvents.checkin()` event is called, except in those cases where the connection is discarded immediately after reset.

    Parameters:

    - dbapi_con – A raw DB-API connection
    - con_record – The `_ConnectionRecord` that persistently manages the connection

    New in version 0.8.

    See also

    `ConnectionEvents.rollback()`

    `ConnectionEvents.commit()`

## SQL Execution and Connection Events

class `sqlalchemy.events.``ConnectionEvents`

  Available events for `Connectable`, which includes `Connection` and `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 `Connectable` class or instance, such as an `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 `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)

  The `before_execute()` and `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 `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

  `ConnectionEvents` can be established on any combination of `Engine`, `Connection`, as well as instances of each of those classes. Events across all four scopes will fire off for a given instance of `Connection`. However, for performance reasons, the `Connection` object determines at instantiation time whether or not its parent `Engine` has event listeners established. Event listeners added to the `Engine` class or to an instance of `Engine` after the instantiation of a dependent `Connection` instance will usually not be available on that `Connection` instance. The newly added listeners will instead take effect for `Connection` instances created subsequent to those event listeners being established on the parent `Engine` class or instance.

  Parameters: | retval=False – Applies to the `before_execute()` and `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.

  Changed in version 0.8: `ConnectionEvents` can now be associated with any `Connectable` including `Connection`, in addition to the existing support for `Engine`.

  `after_cursor_execute`(conn, cursor, statement, parameters, context, executemany)

    Intercept low-level cursor execute() events after execution.

    Parameters:

    - conn – `Connection` object
    - 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 `ResultProxy`.
    - statement – string SQL statement
    - 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`.
    - context – `ExecutionContext` object in use. May be `None`.
    - executemany – boolean, if `True`, this is an `executemany()` call, if `False`, this is an `execute()` call.

  `after_execute`(conn, clauseelement, multiparams, params, result)

    Intercept high level execute() events after execute.

    Parameters:

    - conn – `Connection` object
    - clauseelement – SQL expression construct, `Compiled` instance, or string statement passed to `Connection.execute()`.
    - multiparams – Multiple parameter sets, a list of dictionaries.
    - params – Single parameter set, a single dictionary.
    - result – `ResultProxy` generated by the execution.

  `before_cursor_execute`(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 `ConnectionEvents`.

    Parameters:

    - conn – `Connection` object
    - cursor – DBAPI cursor object
    - statement – string SQL statement
    - 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`.
    - context – `ExecutionContext` object in use. May be `None`.
    - executemany – boolean, if `True`, this is an `executemany()` call, if `False`, this is an `execute()` call.

    See also:

    `before_execute()`

    `after_cursor_execute()`

  `before_execute`(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

    Parameters:

    - conn – `Connection` object
    - clauseelement – SQL expression construct, `Compiled` instance, or string statement passed to `Connection.execute()`.
    - multiparams – Multiple parameter sets, a list of dictionaries.
    - params – Single parameter set, a single dictionary.

    See also:

    `before_cursor_execute()`

  `begin`(conn)

    Intercept begin() events.

    Parameters: | conn – `Connection` object

  `begin_twophase`(conn, xid)

    Intercept begin_twophase() events.

    Parameters:

    - conn – `Connection` object
    - xid – two-phase XID identifier

  `commit`(conn)

    Intercept commit() events, as initiated by a `Transaction`.

    Note that the `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 `PoolEvents.reset()` hook.

    Parameters: | conn – `Connection` object

  `commit_twophase`(conn, xid, is_prepared)

    Intercept commit_twophase() events.

    Parameters:

    - conn – `Connection` object
    - xid – two-phase XID identifier
    - is_prepared – boolean, indicates if `TwoPhaseTransaction.prepare()` was called.

  `dbapi_error`(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 `Engine`, typically for logging and debugging purposes. In general, user code should not modify any state or throw any exceptions here as this will interfere with SQLAlchemy’s cleanup and error handling routines.

    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.

    Parameters:

    - conn – `Connection` object
    - cursor – DBAPI cursor object
    - statement – string SQL statement
    - 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`.
    - context – `ExecutionContext` object in use. May be `None`.
    - exception – The unwrapped exception emitted directly from the DBAPI. The class here is specific to the DBAPI module in use.

    New in version 0.7.7.

  `prepare_twophase`(conn, xid)

    Intercept prepare_twophase() events.

    Parameters:

    - conn – `Connection` object
    - xid – two-phase XID identifier

  `release_savepoint`(conn, name, context)

    Intercept release_savepoint() events.

    Parameters:

    - conn – `Connection` object
    - name – specified name used for the savepoint.
    - context – `ExecutionContext` in use. May be `None`.

  `rollback`(conn)

    Intercept rollback() events, as initiated by a `Transaction`.

    Note that the `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 `PoolEvents.reset()` hook.

    Parameters: | conn – `Connection` object

    See also

    `PoolEvents.reset()`

  `rollback_savepoint`(conn, name, context)

    Intercept rollback_savepoint() events.

    Parameters:

    - conn – `Connection` object
    - name – specified name used for the savepoint.
    - context – `ExecutionContext` in use. May be `None`.

  `rollback_twophase`(conn, xid, is_prepared)

    Intercept rollback_twophase() events.

    Parameters:

    - conn – `Connection` object
    - xid – two-phase XID identifier
    - is_prepared – boolean, indicates if `TwoPhaseTransaction.prepare()` was called.

  `savepoint`(conn, name=None)

    Intercept savepoint() events.

    Parameters:

    - conn – `Connection` object
    - name – specified name used for the savepoint.

## Schema Events

class `sqlalchemy.events.``DDLEvents`

  Define event listeners for schema objects, that is, `SchemaItem` and `SchemaEvent` subclasses, including `MetaData`, `Table`, `Column`.

  `MetaData` and `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 `Column` is associated with its `Table`, when a `ForeignKeyConstraint` is associated with a `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 `DDL` class and the `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:

  > Events

  > `DDLElement`

  > `DDL`

  > Controlling DDL Sequences

  `after_create`(target, connection, **kw)

    Called after CREATE statments are emitted.

    Parameters:

    - target – the `MetaData` or `Table` object which is the target of the event.
    - connection – the `Connection` where the CREATE statement or statements have been emitted.
    - **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.

  `after_drop`(target, connection, **kw)

    Called after DROP statments are emitted.

    Parameters:

    - target – the `MetaData` or `Table` object which is the target of the event.
    - connection – the `Connection` where the DROP statement or statements have been emitted.
    - **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.

  `after_parent_attach`(target, parent)

    Called after a `SchemaItem` is associated with a parent `SchemaItem`.

    Parameters:

    - target – the target object
    - parent – the parent to which the target is being attached.

    `event.listen()` also accepts a modifier for this event:

    Parameters: | 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 `Table.tometadata()` is used.

  `before_create`(target, connection, **kw)

    Called before CREATE statments are emitted.

    Parameters:

    - target – the `MetaData` or `Table` object which is the target of the event.
    - connection – the `Connection` where the CREATE statement or statements will be emitted.
    - **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.

  `before_drop`(target, connection, **kw)

    Called before DROP statments are emitted.

    Parameters:

    - target – the `MetaData` or `Table` object which is the target of the event.
    - connection – the `Connection` where the DROP statement or statements will be emitted.
    - **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.

  `before_parent_attach`(target, parent)

    Called before a `SchemaItem` is associated with a parent `SchemaItem`.

    Parameters:

    - target – the target object
    - parent – the parent to which the target is being attached.

    `event.listen()` also accepts a modifier for this event:

    Parameters: | 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 `Table.tometadata()` is used.

  `column_reflect`(inspector, table, column_info)

    Called for each unit of ‘column info’ retrieved when a `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 `reflection.Inspector.get_columns()`.

    The event is called before any action is taken against this dictionary, and the contents can be modified. The `Column` specific arguments `info`, `key`, and `quote` can also be added to the dictionary and will be passed to the constructor of `Column`.

    Note that this event is only meaningful if either associated with the `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 `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 `Table`.

class `sqlalchemy.events.``SchemaEventTarget`

  Base class for elements that are the targets of `DDLEvents` events.

  This includes `SchemaItem` as well as `SchemaType`.