Release: 2.0.45 | Release Date: December 9, 2025

# SQLAlchemy 2.0 Documentation

# 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.

Object Name | Description
--- | ---
Events | Define event listening functions for a particular target type.

class sqlalchemy.event.base.Events

  inherits from `sqlalchemy.event._HasEventsDispatch`

  Define event listening functions for a particular target type.

  Member Name | Description
  --- | ---
  dispatch | reference back to the _Dispatch class.

  attribute `sqlalchemy.event.base.Events.`dispatch: _Dispatch[_ET] = <sqlalchemy.event.base.EventsDispatch object>

    reference back to the _Dispatch class.

    Bidirectional against _Dispatch._events

## Connection Pool Events

Object Name | Description
--- | ---
PoolEvents | Available events for `Pool`.
PoolResetState | describes the state of a DBAPI connection as it is being passed to the `PoolEvents.reset()` connection pool event.

class sqlalchemy.events.PoolEvents

  inherits from `sqlalchemy.event.Events`

  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+psycopg2://scott:tiger@localhost/test")

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

  Member Name | Description
  --- | ---
  checkin() | Called when a connection returns to the pool.
  checkout() | Called when a connection is retrieved from the Pool.
  close() | Called when a DBAPI connection is closed.
  close_detached() | Called when a detached DBAPI connection is closed.
  connect() | Called at the moment a particular DBAPI connection is first created for a given `Pool`.
  detach() | Called when a DBAPI connection is “detached” from a pool.
  dispatch | reference back to the _Dispatch class.
  first_connect() | Called exactly once for the first time a DBAPI connection is checked out from a particular `Pool`.
  invalidate() | Called when a DBAPI connection is to be “invalidated”.
  reset() | Called before the “reset” action occurs for a pooled connection.
  soft_invalidate() | Called when a DBAPI connection is to be “soft invalidated”.

  method `sqlalchemy.events.PoolEvents.`checkin(dbapi_connection: DBAPIConnection | None, connection_record: ConnectionPoolEntry) → None

    Called when a connection returns to the pool.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'checkin')
        def receive_checkin(dbapi_connection, connection_record):
            "listen for the 'checkin' event"

            # ... (event handling logic) ...

    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_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

  method `sqlalchemy.events.PoolEvents.`checkout(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry, connection_proxy: PoolProxiedConnection) → None

    Called when a connection is retrieved from the Pool.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'checkout')
        def receive_checkout(dbapi_connection, connection_record, connection_proxy):
            "listen for the 'checkout' event"

            # ... (event handling logic) ...

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

      - connection_proxy – the `PoolProxiedConnection` object which will proxy the public interface of the DBAPI connection for the lifespan of the 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.

    See also

    `ConnectionEvents.engine_connect()` - a similar event which occurs upon creation of a new `Connection`.

  method `sqlalchemy.events.PoolEvents.`close(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry) → None

    Called when a DBAPI connection is closed.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'close')
        def receive_close(dbapi_connection, connection_record):
            "listen for the 'close' event"

            # ... (event handling logic) ...

    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 `close()` event corresponds to a connection that’s still associated with the pool. To intercept close events for detached connections use `close_detached()`.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

  method `sqlalchemy.events.PoolEvents.`close_detached(dbapi_connection: DBAPIConnection) → None

    Called when a detached DBAPI connection is closed.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'close_detached')
        def receive_close_detached(dbapi_connection):
            "listen for the 'close_detached' event"

            # ... (event handling logic) ...

    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.

    Parameters:

      dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

  method `sqlalchemy.events.PoolEvents.`connect(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry) → None

    Called at the moment a particular DBAPI connection is first created for a given `Pool`.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'connect')
        def receive_connect(dbapi_connection, connection_record):
            "listen for the 'connect' event"

            # ... (event handling logic) ...

    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.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

  method `sqlalchemy.events.PoolEvents.`detach(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry) → None

    Called when a DBAPI connection is “detached” from a pool.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'detach')
        def receive_detach(dbapi_connection, connection_record):
            "listen for the 'detach' event"

            # ... (event handling logic) ...

    This event is emitted after the detach occurs. The connection is no longer associated with the given connection record.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

  attribute `sqlalchemy.events.PoolEvents.`dispatch: _Dispatch[_ET] = <sqlalchemy.event.base.PoolEventsDispatch object>

    reference back to the _Dispatch class.

    Bidirectional against _Dispatch._events

  method `sqlalchemy.events.PoolEvents.`first_connect(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry) → None

    Called exactly once for the first time a DBAPI connection is checked out from a particular `Pool`.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'first_connect')
        def receive_first_connect(dbapi_connection, connection_record):
            "listen for the 'first_connect' event"

            # ... (event handling logic) ...

    The rationale for `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 `Pool` refers to a single “creator” function (which in terms of a `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.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

  method `sqlalchemy.events.PoolEvents.`invalidate(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry, exception: BaseException | None) → None

    Called when a DBAPI connection is to be “invalidated”.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'invalidate')
        def receive_invalidate(dbapi_connection, connection_record, exception):
            "listen for the 'invalidate' event"

            # ... (event handling logic) ...

    This event is called any time the `ConnectionPoolEntry.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.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

      - exception – the exception object corresponding to the reason for this invalidation, if any. May be `None`.

    See also

    More on Invalidation

  method `sqlalchemy.events.PoolEvents.`reset(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry, reset_state: PoolResetState) → None

    Called before the “reset” action occurs for a pooled connection.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'reset')
        def receive_reset(dbapi_connection, connection_record, reset_state):
            "listen for the 'reset' event"

            # ... (event handling logic) ...

        # DEPRECATED calling style (pre-2.0, will be removed in a future release)
        @event.listens_for(SomeEngineOrPool, 'reset')
        def receive_reset(dbapi_connection, connection_record):
            "listen for the 'reset' event"

            # ... (event handling logic) ...

    Changed in version 2.0: The `PoolEvents.reset()` event now accepts the arguments `PoolEvents.reset.dbapi_connection`, `PoolEvents.reset.connection_record`, `PoolEvents.reset.reset_state`. Support for listener functions which accept the previous argument signature(s) listed above as “deprecated” will be removed in a future release.

    This event represents when the `rollback()` method is called on the DBAPI connection before it is returned to the pool or discarded. A custom “reset” strategy may be implemented using this event hook, which may also be combined with disabling the default “reset” behavior using the `Pool.reset_on_return` parameter.

    The primary difference between the `PoolEvents.reset()` and `PoolEvents.checkin()` events are that `PoolEvents.reset()` is called not just for pooled connections that are being returned to the pool, but also for connections that were detached using the `Connection.detach()` method as well as asyncio connections that are being discarded due to garbage collection taking place on connections before the connection was checked in.

    Note that the event is not invoked for connections that were invalidated using `Connection.invalidate()`. These events may be intercepted using the `PoolEvents.soft_invalidate()` and `PoolEvents.invalidate()` event hooks, and all “connection close” events may be intercepted using `PoolEvents.close()`.

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

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

      - reset_state –

        `PoolResetState` instance which provides information about the circumstances under which the connection is being reset.

        Added in version 2.0.

    See also

    Reset On Return

    `ConnectionEvents.rollback()`

    `ConnectionEvents.commit()`

  method `sqlalchemy.events.PoolEvents.`soft_invalidate(dbapi_connection: DBAPIConnection, connection_record: ConnectionPoolEntry, exception: BaseException | None) → None

    Called when a DBAPI connection is to be “soft invalidated”.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngineOrPool, 'soft_invalidate')
        def receive_soft_invalidate(dbapi_connection, connection_record, exception):
            "listen for the 'soft_invalidate' event"

            # ... (event handling logic) ...

    This event is called any time the `ConnectionPoolEntry.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.

    Parameters:

      - dbapi_connection – a DBAPI connection. The `ConnectionPoolEntry.dbapi_connection` attribute.

      - connection_record – the `ConnectionPoolEntry` managing the DBAPI connection.

      - exception – the exception object corresponding to the reason for this invalidation, if any. May be `None`.

class sqlalchemy.events.PoolResetState

  describes the state of a DBAPI connection as it is being passed to the `PoolEvents.reset()` connection pool event.

  Added in version 2.0.0b3.

  Member Name | Description
  --- | ---
  asyncio_safe | Indicates if the reset operation is occurring within a scope where an enclosing event loop is expected to be present for asyncio applications.
  terminate_only | indicates if the connection is to be immediately terminated and not checked in to the pool.
  transaction_was_reset | Indicates if the transaction on the DBAPI connection was already essentially “reset” back by the `Connection` object.

  attribute `sqlalchemy.events.PoolResetState.`asyncio_safe: bool

    Indicates if the reset operation is occurring within a scope where an enclosing event loop is expected to be present for asyncio applications.

    Will be False in the case that the connection is being garbage collected.

  attribute `sqlalchemy.events.PoolResetState.`terminate_only: bool

    indicates if the connection is to be immediately terminated and not checked in to the pool.

    This occurs for connections that were invalidated, as well as asyncio connections that were not cleanly handled by the calling code that are instead being garbage collected. In the latter case, operations can’t be safely run on asyncio connections within garbage collection as there is not necessarily an event loop present.

  attribute `sqlalchemy.events.PoolResetState.`transaction_was_reset: bool

    Indicates if the transaction on the DBAPI connection was already essentially “reset” back by the `Connection` object.

    This boolean is True if the `Connection` had transactional state present upon it, which was then not closed using the `Connection.rollback()` or `Connection.commit()` method; instead, the transaction was closed inline within the `Connection.close()` method so is guaranteed to remain non-present when this event is reached.

## SQL Execution and Connection Events

Object Name | Description
--- | ---
ConnectionEvents | Available events for `Connection` and `Engine`.
DialectEvents | event interface for execution-replacement functions.

class sqlalchemy.events.ConnectionEvents

  inherits from `sqlalchemy.event.Events`

  Available events for `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 `Connection` or `Engine` 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+psycopg2://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)

  When the methods are called with a statement parameter, such as in `after_cursor_execute()` or `before_cursor_execute()`, the statement is the exact SQL string that was prepared for transmission to the DBAPI `cursor` in the connection’s `Dialect`.

  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.

  Member Name | Description
  --- | ---
  after_cursor_execute() | Intercept low-level cursor execute() events after execution.
  after_execute() | Intercept high level execute() events after execute.
  before_cursor_execute() | Intercept low-level cursor execute() events before execution, receiving the string SQL statement and DBAPI-specific parameter list to be invoked against a cursor.
  before_execute() | Intercept high level execute() events, receiving uncompiled SQL constructs and other objects prior to rendering into SQL.
  begin() | Intercept begin() events.
  begin_twophase() | Intercept begin_twophase() events.
  commit() | Intercept commit() events, as initiated by a `Transaction`.
  commit_twophase() | Intercept commit_twophase() events.
  dispatch | reference back to the _Dispatch class.
  engine_connect() | Intercept the creation of a new `Connection`.
  engine_disposed() | Intercept when the `Engine.dispose()` method is called.
  prepare_twophase() | Intercept prepare_twophase() events.
  release_savepoint() | Intercept release_savepoint() events.
  rollback() | Intercept rollback() events, as initiated by a `Transaction`.
  rollback_savepoint() | Intercept rollback_savepoint() events.
  rollback_twophase() | Intercept rollback_twophase() events.
  savepoint() | Intercept savepoint() events.
  set_connection_execution_options() | Intercept when the `Connection.execution_options()` method is called.
  set_engine_execution_options() | Intercept when the `Engine.execution_options()` method is called.

  method `sqlalchemy.events.ConnectionEvents.`after_cursor_execute(conn: Connection, cursor: DBAPICursor, statement: str, parameters: _DBAPIAnyExecuteParams, context: ExecutionContext | None, executemany: bool) → None

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

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'after_cursor_execute')
        def receive_after_cursor_execute(conn, cursor, statement, parameters, context, executemany):
            "listen for the 'after_cursor_execute' event"

            # ... (event handling logic) ...

    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 `CursorResult`.

      - statement – string SQL statement, as passed to the DBAPI

      - 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.

  method `sqlalchemy.events.ConnectionEvents.`after_execute(conn: Connection, clauseelement: Executable, multiparams: _CoreMultiExecuteParams, params: _CoreSingleExecuteParams, execution_options: _ExecuteOptions, result: Result[Any]) → None

    Intercept high level execute() events after execute.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'after_execute')
        def receive_after_execute(conn, clauseelement, multiparams, params, execution_options, result):
            "listen for the 'after_execute' event"

            # ... (event handling logic) ...

        # DEPRECATED calling style (pre-1.4, will be removed in a future release)
        @event.listens_for(SomeEngine, 'after_execute')
        def receive_after_execute(conn, clauseelement, multiparams, params, result):
            "listen for the 'after_execute' event"

            # ... (event handling logic) ...

    Changed in version 1.4: The `ConnectionEvents.after_execute()` event now accepts the arguments `ConnectionEvents.after_execute.conn`, `ConnectionEvents.after_execute.clauseelement`, `ConnectionEvents.after_execute.multiparams`, `ConnectionEvents.after_execute.params`, `ConnectionEvents.after_execute.execution_options`, `ConnectionEvents.after_execute.result`. Support for listener functions which accept the previous argument signature(s) listed above as “deprecated” will be removed in a future release.

    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.

      - execution_options –

        dictionary of execution options passed along with the statement, if any. This is a merge of all options that will be used, including those of the statement, the connection, and those passed in to the method itself for the 2.0 style of execution.

      - result – `CursorResult` generated by the execution.

  method `sqlalchemy.events.ConnectionEvents.`before_cursor_execute(conn: Connection, cursor: DBAPICursor, statement: str, parameters: _DBAPIAnyExecuteParams, context: ExecutionContext | None, executemany: bool) → Tuple[str, _DBAPIAnyExecuteParams] | None

    Intercept low-level cursor execute() events before execution, receiving the string SQL statement and DBAPI-specific parameter list to be invoked against a cursor.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'before_cursor_execute')
        def receive_before_cursor_execute(conn, cursor, statement, parameters, context, executemany):
            "listen for the 'before_cursor_execute' event"

            # ... (event handling logic) ...

    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, as to be passed to the DBAPI

      - 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()`

  method `sqlalchemy.events.ConnectionEvents.`before_execute(conn: Connection, clauseelement: Executable, multiparams: _CoreMultiExecuteParams, params: _CoreSingleExecuteParams, execution_options: _ExecuteOptions) → Tuple[Executable, _CoreMultiExecuteParams, _CoreSingleExecuteParams] | None

    Intercept high level execute() events, receiving uncompiled SQL constructs and other objects prior to rendering into SQL.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'before_execute')
        def receive_before_execute(conn, clauseelement, multiparams, params, execution_options):
            "listen for the 'before_execute' event"

            # ... (event handling logic) ...

        # DEPRECATED calling style (pre-1.4, will be removed in a future release)
        @event.listens_for(SomeEngine, 'before_execute')
        def receive_before_execute(conn, clauseelement, multiparams, params):
            "listen for the 'before_execute' event"

            # ... (event handling logic) ...

    Changed in version 1.4: The `ConnectionEvents.before_execute()` event now accepts the arguments `ConnectionEvents.before_execute.conn`, `ConnectionEvents.before_execute.clauseelement`, `ConnectionEvents.before_execute.multiparams`, `ConnectionEvents.before_execute.params`, `ConnectionEvents.before_execute.execution_options`. Support for listener functions which accept the previous argument signature(s) listed above as “deprecated” will be removed in a future release.

    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, 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.

      - execution_options –

        dictionary of execution options passed along with the statement, if any. This is a merge of all options that will be used, including those of the statement, the connection, and those passed in to the method itself for the 2.0 style of execution.

    See also

    `before_cursor_execute()`

  method `sqlalchemy.events.ConnectionEvents.`begin(conn: Connection) → None

    Intercept begin() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'begin')
        def receive_begin(conn):
            "listen for the 'begin' event"

            # ... (event handling logic) ...

    Parameters:

      conn – `Connection` object

  method `sqlalchemy.events.ConnectionEvents.`begin_twophase(conn: Connection, xid: Any) → None

    Intercept begin_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'begin_twophase')
        def receive_begin_twophase(conn, xid):
            "listen for the 'begin_twophase' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - xid – two-phase XID identifier

  method `sqlalchemy.events.ConnectionEvents.`commit(conn: Connection) → None

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

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'commit')
        def receive_commit(conn):
            "listen for the 'commit' event"

            # ... (event handling logic) ...

    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

  method `sqlalchemy.events.ConnectionEvents.`commit_twophase(conn: Connection, xid: Any, is_prepared: bool) → None

    Intercept commit_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'commit_twophase')
        def receive_commit_twophase(conn, xid, is_prepared):
            "listen for the 'commit_twophase' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - xid – two-phase XID identifier

      - is_prepared – boolean, indicates if `TwoPhaseTransaction.prepare()` was called.

  attribute `sqlalchemy.events.ConnectionEvents.`dispatch: _Dispatch[_ET] = <sqlalchemy.event.base.ConnectionEventsDispatch object>

    reference back to the _Dispatch class.

    Bidirectional against _Dispatch._events

  method `sqlalchemy.events.ConnectionEvents.`engine_connect(conn: Connection) → None

    Intercept the creation of a new `Connection`.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'engine_connect')
        def receive_engine_connect(conn):
            "listen for the 'engine_connect' event"

            # ... (event handling logic) ...

        # DEPRECATED calling style (pre-2.0, will be removed in a future release)
        @event.listens_for(SomeEngine, 'engine_connect')
        def receive_engine_connect(conn, branch):
            "listen for the 'engine_connect' event"

            # ... (event handling logic) ...

    Changed in version 2.0: The `ConnectionEvents.engine_connect()` event now accepts the arguments `ConnectionEvents.engine_connect.conn`. Support for listener functions which accept the previous argument signature(s) listed above as “deprecated” will be removed in a future release.

    This event is called typically as the direct result of calling the `Engine.connect()` method.

    It differs from the `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 `Connection` wrapper around such a DBAPI connection.

    It also differs from the `PoolEvents.checkout()` event in that it is specific to the `Connection` object, not the DBAPI connection that `PoolEvents.checkout()` deals with, although this DBAPI connection is available here via the `Connection.connection` attribute. But note there can in fact be multiple `PoolEvents.checkout()` events within the lifespan of a single `Connection` object, if that `Connection` is invalidated and re-established.

    Parameters:

      conn – `Connection` object.

    See also

    `PoolEvents.checkout()` the lower-level pool checkout event for an individual DBAPI connection

  method `sqlalchemy.events.ConnectionEvents.`engine_disposed(engine: Engine) → None

    Intercept when the `Engine.dispose()` method is called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'engine_disposed')
        def receive_engine_disposed(engine):
            "listen for the 'engine_disposed' event"

            # ... (event handling logic) ...

    The `Engine.dispose()` method instructs the engine to “dispose” of it’s connection pool (e.g. `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 `Engine` should also be cleaned up, keeping in mind that the `Engine` can still be used for new requests in which case it re-acquires connection resources.

  method `sqlalchemy.events.ConnectionEvents.`prepare_twophase(conn: Connection, xid: Any) → None

    Intercept prepare_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'prepare_twophase')
        def receive_prepare_twophase(conn, xid):
            "listen for the 'prepare_twophase' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - xid – two-phase XID identifier

  method `sqlalchemy.events.ConnectionEvents.`release_savepoint(conn: Connection, name: str, context: None) → None

    Intercept release_savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'release_savepoint')
        def receive_release_savepoint(conn, name, context):
            "listen for the 'release_savepoint' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - name – specified name used for the savepoint.

      - context – not used

  method `sqlalchemy.events.ConnectionEvents.`rollback(conn: Connection) → None

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

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'rollback')
        def receive_rollback(conn):
            "listen for the 'rollback' event"

            # ... (event handling logic) ...

    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()`

  method `sqlalchemy.events.ConnectionEvents.`rollback_savepoint(conn: Connection, name: str, context: None) → None

    Intercept rollback_savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'rollback_savepoint')
        def receive_rollback_savepoint(conn, name, context):
            "listen for the 'rollback_savepoint' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - name – specified name used for the savepoint.

      - context – not used

  method `sqlalchemy.events.ConnectionEvents.`rollback_twophase(conn: Connection, xid: Any, is_prepared: bool) → None

    Intercept rollback_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'rollback_twophase')
        def receive_rollback_twophase(conn, xid, is_prepared):
            "listen for the 'rollback_twophase' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - xid – two-phase XID identifier

      - is_prepared – boolean, indicates if `TwoPhaseTransaction.prepare()` was called.

  method `sqlalchemy.events.ConnectionEvents.`savepoint(conn: Connection, name: str) → None

    Intercept savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'savepoint')
        def receive_savepoint(conn, name):
            "listen for the 'savepoint' event"

            # ... (event handling logic) ...

    Parameters:

      - conn – `Connection` object

      - name – specified name used for the savepoint.

  method `sqlalchemy.events.ConnectionEvents.`set_connection_execution_options(conn: Connection, opts: Dict[str, Any]) → None

    Intercept when the `Connection.execution_options()` method is called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'set_connection_execution_options')
        def receive_set_connection_execution_options(conn, opts):
            "listen for the 'set_connection_execution_options' event"

            # ... (event handling logic) ...

    This method is called after the new `Connection` has been produced, with the newly updated execution options collection, but before the `Dialect` has acted upon any of those new options.

    Note that this method is not called when a new `Connection` is produced which is inheriting execution options from its parent `Engine`; to intercept this condition, use the `ConnectionEvents.engine_connect()` event.

    Parameters:

      - conn – The newly copied `Connection` object

      - opts –

        dictionary of options that were passed to the `Connection.execution_options()` method. This dictionary may be modified in place to affect the ultimate options which take effect.

        Added in version 2.0: the `opts` dictionary may be modified in place.

    See also

    `ConnectionEvents.set_engine_execution_options()` - event which is called when `Engine.execution_options()` is called.

  method `sqlalchemy.events.ConnectionEvents.`set_engine_execution_options(engine: Engine, opts: Dict[str, Any]) → None

    Intercept when the `Engine.execution_options()` method is called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'set_engine_execution_options')
        def receive_set_engine_execution_options(engine, opts):
            "listen for the 'set_engine_execution_options' event"

            # ... (event handling logic) ...

    The `Engine.execution_options()` method produces a shallow copy of the `Engine` which stores the new options. That new `Engine` is passed here. A particular application of this method is to add a `ConnectionEvents.engine_connect()` event handler to the given `Engine` which will perform some per- `Connection` task specific to these execution options.

    Parameters:

      - conn – The newly copied `Engine` object

      - opts –

        dictionary of options that were passed to the `Connection.execution_options()` method. This dictionary may be modified in place to affect the ultimate options which take effect.

        Added in version 2.0: the `opts` dictionary may be modified in place.

    See also

    `ConnectionEvents.set_connection_execution_options()` - event which is called when `Connection.execution_options()` is called.

class sqlalchemy.events.DialectEvents

  inherits from `sqlalchemy.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

  `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 `ConnectionEvents` interface.

  See also

  `ConnectionEvents.before_cursor_execute()`

  `ConnectionEvents.before_execute()`

  `ConnectionEvents.after_cursor_execute()`

  `ConnectionEvents.after_execute()`

  Member Name | Description
  --- | ---
  dispatch | reference back to the _Dispatch class.
  do_connect() | Receive connection arguments before a connection is made.
  do_execute() | Receive a cursor to have execute() called.
  do_execute_no_params() | Receive a cursor to have execute() with no parameters called.
  do_executemany() | Receive a cursor to have executemany() called.
  do_setinputsizes() | Receive the setinputsizes dictionary for possible modification.
  handle_error() | Intercept all exceptions processed by the `Dialect`, typically but not limited to those emitted within the scope of a `Connection`.

  attribute `sqlalchemy.events.DialectEvents.`dispatch: _Dispatch[_ET] = <sqlalchemy.event.base.DialectEventsDispatch object>

    reference back to the _Dispatch class.

    Bidirectional against _Dispatch._events

  method `sqlalchemy.events.DialectEvents.`do_connect(dialect: Dialect, conn_rec: ConnectionPoolEntry, cargs: Tuple[Any, ...], cparams: Dict[str, Any]) → DBAPIConnection | None

    Receive connection arguments before a connection is made.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'do_connect')
        def receive_do_connect(dialect, conn_rec, cargs, cparams):
            "listen for the 'do_connect' event"

            # ... (event handling logic) ...

    This event is useful in that it allows the handler to manipulate the cargs and/or cparams collections that control how the DBAPI `connect()` function will be called. `cargs` will always be a Python list that can be mutated in-place, and `cparams` a Python dictionary that may also be mutated:

        e = create_engine("postgresql+psycopg2://user@host/dbname")

        @event.listens_for(e, "do_connect")
        def receive_do_connect(dialect, conn_rec, cargs, cparams):
            cparams["password"] = "some_password"

    The event hook may also be used to override the call to `connect()` entirely, by returning a non-`None` DBAPI connection object:

        e = create_engine("postgresql+psycopg2://user@host/dbname")

        @event.listens_for(e, "do_connect")
        def receive_do_connect(dialect, conn_rec, cargs, cparams):
            return psycopg2.connect(*cargs, **cparams)

    See also

    Custom DBAPI connect() arguments / on-connect routines

  method `sqlalchemy.events.DialectEvents.`do_execute(cursor: DBAPICursor, statement: str, parameters: _DBAPISingleExecuteParams, context: ExecutionContext) → Literal[True] | None

    Receive a cursor to have execute() called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'do_execute')
        def receive_do_execute(cursor, statement, parameters, context):
            "listen for the 'do_execute' event"

            # ... (event handling logic) ...

    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.

  method `sqlalchemy.events.DialectEvents.`do_execute_no_params(cursor: DBAPICursor, statement: str, context: ExecutionContext) → Literal[True] | None

    Receive a cursor to have execute() with no parameters called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'do_execute_no_params')
        def receive_do_execute_no_params(cursor, statement, context):
            "listen for the 'do_execute_no_params' event"

            # ... (event handling logic) ...

    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.

  method `sqlalchemy.events.DialectEvents.`do_executemany(cursor: DBAPICursor, statement: str, parameters: _DBAPIMultiExecuteParams, context: ExecutionContext) → Literal[True] | None

    Receive a cursor to have executemany() called.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'do_executemany')
        def receive_do_executemany(cursor, statement, parameters, context):
            "listen for the 'do_executemany' event"

            # ... (event handling logic) ...

    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.

  method `sqlalchemy.events.DialectEvents.`do_setinputsizes(inputsizes: Dict[BindParameter[Any], Any], cursor: DBAPICursor, statement: str, parameters: _DBAPIAnyExecuteParams, context: ExecutionContext) → None

    Receive the setinputsizes dictionary for possible modification.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'do_setinputsizes')
        def receive_do_setinputsizes(inputsizes, cursor, statement, parameters, context):
            "listen for the 'do_setinputsizes' event"

            # ... (event handling logic) ...

    This event is emitted in the case where the dialect makes use of the DBAPI `cursor.setinputsizes()` method which passes information about parameter binding for a particular statement. The given `inputsizes` dictionary will contain `BindParameter` objects as keys, linked to DBAPI-specific type objects as values; for parameters that are not bound, they are added to the dictionary with `None` as the value, which means the parameter will not be included in the ultimate setinputsizes call. The event may be used to inspect and/or log the datatypes that are being bound, as well as to modify the dictionary in place. Parameters can be added, modified, or removed from this dictionary. Callers will typically want to inspect the `BindParameter.type` attribute of the given bind objects in order to make decisions about the DBAPI object.

    After the event, the `inputsizes` dictionary is converted into an appropriate datastructure to be passed to `cursor.setinputsizes`; either a list for a positional bound parameter execution style, or a dictionary of string parameter keys to DBAPI type objects for a named bound parameter execution style.

    The setinputsizes hook overall is only used for dialects which include the flag `use_setinputsizes=True`. Dialects which use this include python-oracledb, cx_Oracle, pg8000, asyncpg, and pyodbc dialects.

    Note

    For use with pyodbc, the `use_setinputsizes` flag must be passed to the dialect, e.g.:

        create_engine("mssql+pyodbc://...", use_setinputsizes=True)

    See also

    Setinputsizes Support

    Added in version 1.2.9.

    See also

    Fine grained control over cx_Oracle data binding performance with setinputsizes

  method `sqlalchemy.events.DialectEvents.`handle_error(exception_context: ExceptionContext) → BaseException | None

    Intercept all exceptions processed by the `Dialect`, typically but not limited to those emitted within the scope of a `Connection`.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeEngine, 'handle_error')
        def receive_handle_error(exception_context):
            "listen for the 'handle_error' event"

            # ... (event handling logic) ...

    Changed in version 2.0: the `DialectEvents.handle_error()` event is moved to the `DialectEvents` class, moved from the `ConnectionEvents` class, so that it may also participate in the “pre ping” operation configured with the `create_engine.pool_pre_ping` parameter. The event remains registered by using the `Engine` as the event target, however note that using the `Connection` as an event target for `DialectEvents.handle_error()` is no longer supported.

    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 `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 `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

    - Establishing whether a DBAPI connection error message indicates that the database connection needs to be reconnected, including for the “pre_ping” handler used by some dialects

    - Establishing or disabling whether a connection or the owning connection pool is invalidated or expired in response to a specific exception

    - exception re-writing

    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.

    As of SQLAlchemy 2.0, the “pre_ping” handler enabled using the `create_engine.pool_pre_ping` parameter will also participate in the `handle_error()` process, for those dialects that rely upon disconnect codes to detect database liveness. Note that some dialects such as psycopg, psycopg2, and most MySQL dialects make use of a native `ping()` method supplied by the DBAPI which does not make use of disconnect codes.

    Changed in version 2.0.0: The `DialectEvents.handle_error()` event hook participates in connection pool “pre-ping” operations. Within this usage, the `ExceptionContext.engine` attribute will be `None`, however the `Dialect` in use is always available via the `ExceptionContext.dialect` attribute.

    Changed in version 2.0.5: Added `ExceptionContext.is_pre_ping` attribute which will be set to `True` when the `DialectEvents.handle_error()` event hook is triggered within a connection pool pre-ping operation.

    Changed in version 2.0.5: An issue was repaired that allows for the PostgreSQL `psycopg` and `psycopg2` drivers, as well as all MySQL drivers, to properly participate in the `DialectEvents.handle_error()` event hook during connection pool “pre-ping” operations; previously, the implementation was non-working for these drivers.

    A handler function 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 `DialectEvents.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 `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 be used within the chain; when a handler returns `None`, the previous exception instance, if any, is maintained as the current exception that is passed onto 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 `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.

    Parameters:

      context – an `ExceptionContext` object. See this class for details on all available members.

    See also

    Supporting new database error codes for disconnect scenarios

## Schema Events

Object Name | Description
--- | ---
DDLEvents | Define event listeners for schema objects, that is, `SchemaItem` and other `SchemaEventTarget` subclasses, including `MetaData`, `Table`, `Column`, etc.
SchemaEventTarget | Base class for elements that are the targets of `DDLEvents` events.

class sqlalchemy.events.DDLEvents

  inherits from `sqlalchemy.event.Events`

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

  Create / Drop Events

  Events emitted when CREATE and DROP commands are emitted to the database. The event hooks in this category include `DDLEvents.before_create()`, `DDLEvents.after_create()`, `DDLEvents.before_drop()`, and `DDLEvents.after_drop()`.

  These events are emitted when using schema-level methods such as `MetaData.create_all()` and `MetaData.drop_all()`. Per-object create/drop methods such as `Table.create()`, `Table.drop()`, `Index.create()` are also included, as well as dialect-specific methods such as `ENUM.create()`.

  Added in version 2.0: `DDLEvents` event hooks now take place for non-table objects including constraints, indexes, and dialect-specific schema types.

  Event hooks may be attached directly to a `Table` object or to a `MetaData` collection, as well as to any `SchemaItem` class or object that can be individually created and dropped using a distinct SQL command. Such classes include `Index`, `Sequence`, and dialect-specific classes such as `ENUM`.

  Example using the `DDLEvents.after_create()` event, where a custom event hook will emit an `ALTER TABLE` command on the current connection, after `CREATE TABLE` is emitted:

      from sqlalchemy import create_engine
      from sqlalchemy import event
      from sqlalchemy import Table, Column, Metadata, Integer

      m = MetaData()
      some_table = Table("some_table", m, Column("data", Integer))

      @event.listens_for(some_table, "after_create")
      def after_create(target, connection, **kw):
          connection.execute(
              text("ALTER TABLE %s SET name=foo_%s" % (target.name, target.name))
          )

      some_engine = create_engine("postgresql://scott:tiger@host/test")

      # will emit "CREATE TABLE some_table" as well as the above
      # "ALTER TABLE" statement afterwards
      m.create_all(some_engine)

  Constraint objects such as `ForeignKeyConstraint`, `UniqueConstraint`, `CheckConstraint` may also be subscribed to these events, however they will not normally produce events as these objects are usually rendered inline within an enclosing `CREATE TABLE` statement and implicitly dropped from a `DROP TABLE` statement.

  For the `Index` construct, the event hook will be emitted for `CREATE INDEX`, however SQLAlchemy does not normally emit `DROP INDEX` when dropping tables as this is again implicit within the `DROP TABLE` statement.

  Added in version 2.0: Support for `SchemaItem` objects for create/drop events was expanded from its previous support for `MetaData` and `Table` to also include `Constraint` and all subclasses, `Index`, `Sequence` and some type-related constructs such as `ENUM`.

  Note

  These event hooks are only emitted within the scope of SQLAlchemy’s create/drop methods; they are not necessarily supported by tools such as alembic.

  Attachment Events

  Attachment events are 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. These events include `DDLEvents.before_parent_attach()` and `DDLEvents.after_parent_attach()`.

  Reflection Events

  The `DDLEvents.column_reflect()` event is used to intercept and modify the in-Python definition of database columns when reflection of database tables proceeds.

  Use with Generic DDL

  DDL events integrate closely with the `DDL` class and the `ExecutableDDLElement` 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"),
      )

  Event Propagation to MetaData Copies

  For all `DDLEvent` events, the `propagate=True` keyword argument will ensure that a given event handler is propagated to copies of the object, which are made when using the `Table.to_metadata()` method:

      from sqlalchemy import DDL

      metadata = MetaData()
      some_table = Table("some_table", metadata, Column("data", Integer))

      event.listen(
          some_table,
          "after_create",
          DDL("ALTER TABLE %(table)s SET name=foo_%(table)s"),
          propagate=True,
      )

      new_metadata = MetaData()
      new_table = some_table.to_metadata(new_metadata)

  The above `DDL` object will be associated with the `DDLEvents.after_create()` event for both the `some_table` and the `new_table` `Table` objects.

  See also

  Events

  `ExecutableDDLElement`

  `DDL`

  Controlling DDL Sequences

  Member Name | Description
  --- | ---
  after_create() | Called after CREATE statements are emitted.
  after_drop() | Called after DROP statements are emitted.
  after_parent_attach() | Called after a `SchemaItem` is associated with a parent `SchemaItem`.
  before_create() | Called before CREATE statements are emitted.
  before_drop() | Called before DROP statements are emitted.
  before_parent_attach() | Called before a `SchemaItem` is associated with a parent `SchemaItem`.
  column_reflect() | Called for each unit of ‘column info’ retrieved when a `Table` is being reflected.
  dispatch | reference back to the _Dispatch class.

  method `sqlalchemy.events.DDLEvents.`after_create(target: SchemaEventTarget, connection: Connection, **kw: Any) → None

    Called after CREATE statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'after_create')
        def receive_after_create(target, connection, **kw):
            "listen for the 'after_create' event"

            # ... (event handling logic) ...

    Parameters:

      - target –

        the `SchemaObject`, such as a `MetaData` or `Table` but also including all create/drop objects such as `Index`, `Sequence`, etc., object which is the target of the event.

        Added in version 2.0: Support for all `SchemaItem` objects was added.

      - 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.

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

  method `sqlalchemy.events.DDLEvents.`after_drop(target: SchemaEventTarget, connection: Connection, **kw: Any) → None

    Called after DROP statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'after_drop')
        def receive_after_drop(target, connection, **kw):
            "listen for the 'after_drop' event"

            # ... (event handling logic) ...

    Parameters:

      - target –

        the `SchemaObject`, such as a `MetaData` or `Table` but also including all create/drop objects such as `Index`, `Sequence`, etc., object which is the target of the event.

        Added in version 2.0: Support for all `SchemaItem` objects was added.

      - 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.

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

  method `sqlalchemy.events.DDLEvents.`after_parent_attach(target: SchemaEventTarget, parent: SchemaItem) → None

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

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'after_parent_attach')
        def receive_after_parent_attach(target, parent):
            "listen for the 'after_parent_attach' event"

            # ... (event handling logic) ...

    Parameters:

      - target – the target object

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

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

  method `sqlalchemy.events.DDLEvents.`before_create(target: SchemaEventTarget, connection: Connection, **kw: Any) → None

    Called before CREATE statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'before_create')
        def receive_before_create(target, connection, **kw):
            "listen for the 'before_create' event"

            # ... (event handling logic) ...

    Parameters:

      - target –

        the `SchemaObject`, such as a `MetaData` or `Table` but also including all create/drop objects such as `Index`, `Sequence`, etc., object which is the target of the event.

        Added in version 2.0: Support for all `SchemaItem` objects was added.

      - 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.

    `listen()` accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

    `listen()` accepts the `insert=True` modifier for this event; when True, the listener function will be prepended to the internal list of events upon discovery, and execute before registered listener functions that do not pass this argument.

  method `sqlalchemy.events.DDLEvents.`before_drop(target: SchemaEventTarget, connection: Connection, **kw: Any) → None

    Called before DROP statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'before_drop')
        def receive_before_drop(target, connection, **kw):
            "listen for the 'before_drop' event"

            # ... (event handling logic) ...

    Parameters:

      - target –

        the `SchemaObject`, such as a `MetaData` or `Table` but also including all create/drop objects such as `Index`, `Sequence`, etc., object which is the target of the event.

        Added in version 2.0: Support for all `SchemaItem` objects was added.

      - 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.

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

  method `sqlalchemy.events.DDLEvents.`before_parent_attach(target: SchemaEventTarget, parent: SchemaItem) → None

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

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'before_parent_attach')
        def receive_before_parent_attach(target, parent):
            "listen for the 'before_parent_attach' event"

            # ... (event handling logic) ...

    Parameters:

      - target – the target object

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

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

  method `sqlalchemy.events.DDLEvents.`column_reflect(inspector: Inspector, table: Table, column_info: ReflectedColumn) → None

    Called for each unit of ‘column info’ retrieved when a `Table` is being reflected.

    Example argument forms:

        from sqlalchemy import event

        @event.listens_for(SomeSchemaClassOrObject, 'column_reflect')
        def receive_column_reflect(inspector, table, column_info):
            "listen for the 'column_reflect' event"

            # ... (event handling logic) ...

    This event is most easily used by applying it to a specific `MetaData` instance, where it will take effect for all `Table` objects within that `MetaData` that undergo reflection:

        metadata = MetaData()

        @event.listens_for(metadata, "column_reflect")
        def receive_column_reflect(inspector, table, column_info):
            # receives for all Table objects that are reflected
            # under this MetaData
            ...

        # will use the above event hook
        my_table = Table("my_table", metadata, autoload_with=some_engine)

    Added in version 1.4.0b2: The `DDLEvents.column_reflect()` hook may now be applied to a `MetaData` object as well as the `MetaData` class itself where it will take place for all `Table` objects associated with the targeted `MetaData`.

    It may also be applied to the `Table` class across the board:

        from sqlalchemy import Table

        @event.listens_for(Table, "column_reflect")
        def receive_column_reflect(inspector, table, column_info):
            # receives for all Table objects that are reflected
            ...

    It can also be applied to a specific `Table` at the point that one is being reflected using the `Table.listeners` parameter:

        t1 = Table(
            "my_table",
            autoload_with=some_engine,
            listeners=[("column_reflect", receive_column_reflect)],
        )

    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 `Inspector.get_columns()`:

    > - `name` - the column’s name, is applied to the `Column.name` parameter

    > - `type` - the type of this column, which should be an instance of `TypeEngine`, is applied to the `Column.type` parameter

    > - `nullable` - boolean flag if the column is NULL or NOT NULL, is applied to the `Column.nullable` parameter

    > - `default` - the column’s server default value. This is normally specified as a plain string SQL expression, however the event can pass a `FetchedValue`, `DefaultClause`, or `text()` object as well. Is applied to the `Column.server_default` parameter

    The event is called before any action is taken against this dictionary, and the contents can be modified; the following additional keys may be added to the dictionary to further modify how the `Column` is constructed:

    > - `key` - the string key that will be used to access this `Column` in the `.c` collection; will be applied to the `Column.key` parameter. Is also used for ORM mapping. See the section Automating Column Naming Schemes from Reflected Tables for an example.

    > - `quote` - force or un-force quoting on the column name; is applied to the `Column.quote` parameter.

    > - `info` - a dictionary of arbitrary data to follow along with the `Column`, is applied to the `Column.info` parameter.

    `listen()` also accepts the `propagate=True` modifier for this event; 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.to_metadata()` is used.

    See also

    Automating Column Naming Schemes from Reflected Tables - in the ORM mapping documentation

    Intercepting Column Definitions - in the Automap documentation

    Reflecting with Database-Agnostic Types - in the Reflecting Database Objects documentation

  attribute `sqlalchemy.events.DDLEvents.`dispatch: _Dispatch[_ET] = <sqlalchemy.event.base.DDLEventsDispatch object>

    reference back to the _Dispatch class.

    Bidirectional against _Dispatch._events

class sqlalchemy.events.SchemaEventTarget

  inherits from `sqlalchemy.event.registry.EventTarget`

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

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