Core Events — SQLAlchemy 1.1 Documentation

Release: 1.1.18 | Release Date: March 6, 2018

# SQLAlchemy 1.1 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.

class `sqlalchemy.event.base.` `Events`

  Define event listening functions for a particular target type.

## Connection Pool Events

class `sqlalchemy.events.` `PoolEvents`

  Bases: `sqlalchemy.event.base.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://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.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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. connection_record – the `_ConnectionRecord` managing the DBAPI connection.

  `checkout`(dbapi_connection, connection_record, connection_proxy)

    Called when a connection is retrieved from the Pool.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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. connection_record – the `_ConnectionRecord` managing the DBAPI connection. connection_proxy – the `_ConnectionFairy` 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`.

  `close`(dbapi_connection, connection_record)

    Called when a DBAPI connection is closed.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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()`.

    New in version 1.1.

  `close_detached`(dbapi_connection)

    Called when a detached DBAPI connection is closed.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    New in version 1.1.

  `connect`(dbapi_connection, connection_record)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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. connection_record – the `_ConnectionRecord` managing the DBAPI connection.

  `detach`(dbapi_connection, connection_record)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    New in version 1.1.

  `first_connect`(dbapi_connection, connection_record)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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. connection_record – the `_ConnectionRecord` managing the DBAPI connection.

  `invalidate`(dbapi_connection, connection_record, exception)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `_ConnectionRecord.invalidate()` method is invoked, either from API usage or via “auto-invalidation”, without the `soft` flag.

    The event occurs before a final attempt to call `.close()` on the connection occurs.

    Parameters: | dbapi_connection – a DBAPI connection. connection_record – the `_ConnectionRecord` managing the DBAPI connection. exception – the exception object corresponding to the reason for this invalidation, if any. May be `None`.

    New in version 0.9.2: Added support for connection invalidation listening.

    See also

    More on Invalidation

  `reset`(dbapi_connection, connection_record)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @event.listens_for(SomeEngineOrPool, 'reset')
        def receive_reset(dbapi_connection, connection_record):
            "listen for the 'reset' event"

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

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

    Parameters: | dbapi_connection – a DBAPI connection. connection_record – the `_ConnectionRecord` managing the DBAPI connection.

    New in version 0.8.

    See also

    `ConnectionEvents.rollback()`

    `ConnectionEvents.commit()`

  `soft_invalidate`(dbapi_connection, connection_record, exception)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `_ConnectionRecord.invalidate()` method is invoked with the `soft` flag.

    Soft invalidation refers to when the connection record that tracks this connection will force a reconnect after the current connection is checked in. It does not actively close the dbapi_connection at the point at which it is called.

    New in version 1.0.3.

## SQL Execution and Connection Events

class `sqlalchemy.events.` `ConnectionEvents`

  Bases: `sqlalchemy.event.base.Events`

  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)

  When the methods are called with a statement parameter, such as in `after_cursor_execute()`, `before_cursor_execute()` and `dbapi_error()`, 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.

  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.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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) ...

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'after_cursor_execute', named=True)
        def receive_after_cursor_execute(**kw):
            "listen for the 'after_cursor_execute' event"
            conn = kw['conn']
            cursor = kw['cursor']

            # ... (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 `ResultProxy`. 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.

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

    Intercept high level execute() events after execute.

    Example argument forms:

        from sqlalchemy import event

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

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'after_execute', named=True)
        def receive_after_execute(**kw):
            "listen for the 'after_execute' event"
            conn = kw['conn']
            clauseelement = kw['clauseelement']

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

    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.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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) ...

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'before_cursor_execute', named=True)
        def receive_before_cursor_execute(**kw):
            "listen for the 'before_cursor_execute' event"
            conn = kw['conn']
            cursor = kw['cursor']

            # ... (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()`

  `before_execute`(conn, clauseelement, multiparams, params)

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

    Example argument forms:

        from sqlalchemy import event

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

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'before_execute', named=True)
        def receive_before_execute(**kw):
            "listen for the 'before_execute' event"
            conn = kw['conn']
            clauseelement = kw['clauseelement']

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

    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.

    See also:

    `before_cursor_execute()`

  `begin`(conn)

    Intercept begin() events.

    Example argument forms:

        from sqlalchemy import event

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

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

    Parameters: | conn – `Connection` object

  `begin_twophase`(conn, xid)

    Intercept begin_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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

  `commit`(conn)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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

  `commit_twophase`(conn, xid, is_prepared)

    Intercept commit_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

  `dbapi_error`(conn, cursor, statement, parameters, context, exception)

    Intercept a raw DBAPI error.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @event.listens_for(SomeEngine, 'dbapi_error')
        def receive_dbapi_error(conn, cursor, statement, parameters, context, exception):
            "listen for the 'dbapi_error' event"

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'dbapi_error', named=True)
        def receive_dbapi_error(**kw):
            "listen for the 'dbapi_error' event"
            conn = kw['conn']
            cursor = kw['cursor']

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

    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.

    Warning

    Code should not modify any state or throw any exceptions here as this will interfere with SQLAlchemy’s cleanup and error handling routines. For exception modification, please refer to the new `ConnectionEvents.handle_error()` event.

    Subsequent to this hook, SQLAlchemy may attempt any number of operations on the connection/cursor, including closing the cursor, rolling back of the transaction in the case of connectionless execution, and disposing of the entire connection pool if a “disconnect” was detected. The exception is then wrapped in a SQLAlchemy DBAPI exception wrapper and re-thrown.

    Parameters: | conn – `Connection` object cursor – DBAPI cursor object 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`. exception – The unwrapped exception emitted directly from the DBAPI. The class here is specific to the DBAPI module in use.

    Deprecated since version 0.9.7: - replaced by `ConnectionEvents.handle_error()`

  `engine_connect`(conn, branch)

    Intercept the creation of a new `Connection`.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @event.listens_for(SomeEngine, 'engine_connect')
        def receive_engine_connect(conn, branch):
            "listen for the 'engine_connect' event"

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

    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. There can also be multiple `Connection` objects generated for the same already-checked-out DBAPI connection, in the case that a “branch” of a `Connection` is produced.

    Parameters: | conn – `Connection` object. branch – if True, this is a “branch” of an existing `Connection`. A branch is generated within the course of a statement execution to invoke supplemental statements, most typically to pre-execute a SELECT of a default value for the purposes of an INSERT statement.

    New in version 0.9.0.

    See also

    Disconnect Handling - Pessimistic - illustrates how to use `ConnectionEvents.engine_connect()` to transparently ensure pooled connections are connected to the database.

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

    `ConnectionEvents.set_connection_execution_options()` - a copy of a `Connection` is also made when the `Connection.execution_options()` method is called.

  `engine_disposed`(engine)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    New in version 1.0.5.

  `handle_error`(exception_context)

    Intercept all exceptions processed by the `Connection`.

    Example argument forms:

        from sqlalchemy import event

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

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

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

    The hook is called while the cursor from the failed operation (if any) is still open and accessible. Special cleanup operations can be called on this cursor; SQLAlchemy will attempt to close this cursor subsequent to this hook being invoked. If the connection is in “autocommit” mode, the transaction also remains open within the scope of this hook; the rollback of the per-statement transaction also occurs after the hook is called.

    For the common case of detecting a “disconnect” situation which is not currently handled by the SQLAlchemy dialect, the `ExceptionContext.is_disconnect` flag can be set to True which will cause the exception to be considered as a disconnect situation, which typically results in the connection pool being invalidated:

        @event.listens_for(Engine, "handle_error")
        def handle_exception(context):
            if isinstance(context.original_exception, pyodbc.Error):
                for code in (
                    '08S01', '01002', '08003',
                    '08007', '08S02', '08001', 'HYT00', 'HY010'):

                    if code in str(context.original_exception):
                        context.is_disconnect = True

    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 `ConnectionEvents.handle_error()` event specifically provides for exceptions to be re-thrown as the ultimate exception raised by the failed statement, stack traces will be misleading if the user-defined event handler itself fails and throws an unexpected exception; the stack trace may not illustrate the actual code line that failed! It is advised to code carefully here and use logging and/or inline debugging if unexpected exceptions are occurring.

    Alternatively, a “chained” style of event handling can be used, by configuring the handler with the `retval=True` modifier and returning the new exception instance from the function. In this case, event handling will continue onto the next handler. The “chained” exception is available using `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.

    New in version 0.9.7: Added the `ConnectionEvents.handle_error()` hook.

    Changed in version 1.1: The `handle_error()` event will now receive all exceptions that inherit from `BaseException`, including `SystemExit` and `KeyboardInterrupt`. The setting for `ExceptionContext.is_disconnect` is `True` in this case and the default for `ExceptionContext.invalidate_pool_on_disconnect` is `False`.

    Changed in version 1.0.0: The `handle_error()` event is now invoked when an `Engine` fails during the initial call to `Engine.connect()`, as well as when a `Connection` object encounters an error during a reconnect operation.

    Changed in version 1.0.0: The `handle_error()` event is not fired off when a dialect makes use of the `skip_user_error_events` execution option. This is used by dialects which intend to catch SQLAlchemy-specific exceptions within specific operations, such as when the MySQL dialect detects a table not present within the `has_table()` dialect method. Prior to 1.0.0, code which implements `handle_error()` needs to ensure that exceptions thrown in these scenarios are re-raised without modification.

  `prepare_twophase`(conn, xid)

    Intercept prepare_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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

  `release_savepoint`(conn, name, context)

    Intercept release_savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 – `ExecutionContext` in use. May be `None`.

  `rollback`(conn)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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()`

  `rollback_savepoint`(conn, name, context)

    Intercept rollback_savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 – `ExecutionContext` in use. May be `None`.

  `rollback_twophase`(conn, xid, is_prepared)

    Intercept rollback_twophase() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

  `savepoint`(conn, name)

    Intercept savepoint() events.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

  `set_connection_execution_options`(conn, opts)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    New in version 0.9.0.

    See also

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

  `set_engine_execution_options`(engine, opts)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    New in version 0.9.0.

    See also

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

class `sqlalchemy.events.` `DialectEvents`

  Bases: `sqlalchemy.event.base.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()`

  New in version 0.9.4.

  `do_connect`(dialect, conn_rec, cargs, cparams)

    Receive connection arguments before a connection is made.

    Example argument forms:

        from sqlalchemy import event

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

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'do_connect', named=True)
        def receive_do_connect(**kw):
            "listen for the 'do_connect' event"
            dialect = kw['dialect']
            conn_rec = kw['conn_rec']

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

    Return a DBAPI connection to halt further events from invoking; the returned connection will be used.

    Alternatively, the event can manipulate the cargs and/or cparams collections; cargs will always be a Python list that can be mutated in-place and cparams a Python dictionary. Return None to allow control to pass to the next event handler and ultimately to allow the dialect to connect normally, given the updated arguments.

    New in version 1.0.3.

  `do_execute`(cursor, statement, parameters, context)

    Receive a cursor to have execute() called.

    Example argument forms:

        from sqlalchemy import event

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

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'do_execute', named=True)
        def receive_do_execute(**kw):
            "listen for the 'do_execute' event"
            cursor = kw['cursor']
            statement = kw['statement']

            # ... (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.

  `do_execute_no_params`(cursor, statement, context)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

  `do_executemany`(cursor, statement, parameters, context)

    Receive a cursor to have executemany() called.

    Example argument forms:

        from sqlalchemy import event

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

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

        # named argument style (new in 0.9)
        @event.listens_for(SomeEngine, 'do_executemany', named=True)
        def receive_do_executemany(**kw):
            "listen for the 'do_executemany' event"
            cursor = kw['cursor']
            statement = kw['statement']

            # ... (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.

## Schema Events

class `sqlalchemy.events.` `DDLEvents`

  Bases: `sqlalchemy.event.base.Events`

  Define event listeners for schema objects, that is, `SchemaItem` and other `SchemaEventTarget` 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.

  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.tometadata()` method:

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

      new_table = some_table.tometadata(new_metadata)

  The above `DDL` object will also be associated with the `Table` object represented by `new_table`.

  See also:

  > Events

  > `DDLElement`

  > `DDL`

  > Controlling DDL Sequences

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

    Called after CREATE statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `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.

    `event.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.tometadata()` is used.

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

    Called after DROP statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `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.

    `event.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.tometadata()` is used.

  `after_parent_attach`(target, parent)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    `event.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.tometadata()` is used.

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

    Called before CREATE statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `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.

    `event.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.tometadata()` is used.

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

    Called before DROP statements are emitted.

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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 `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.

    `event.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.tometadata()` is used.

  `before_parent_attach`(target, parent)

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

    Example argument forms:

        from sqlalchemy import event

        # standard decorator style
        @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.

    `event.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.tometadata()` is used.

  `column_reflect`(inspector, table, column_info)

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

    Example argument forms:

        from sqlalchemy import event

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

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

    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()`:

    > - `name` - the column’s name

    > - `type` - the type of this column, which should be an instance of `TypeEngine`

    > - `nullable` - boolean flag if the column is NULL or NOT NULL

    > - `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 `sql.expression.text()` object as well.

    >   Changed in version 1.1.6: The `DDLEvents.column_reflect()` event allows a non string `FetchedValue`, `sql.expression.text()`, or derived object to be specified as the value of `default` in the column dictionary.

    > - `attrs` - dict containing optional column attributes

    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`.

    `event.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.tometadata()` is used.

class `sqlalchemy.events.` `SchemaEventTarget`

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

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