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¶: Define event listening functions for a particular target type.

Connection Pool Events¶

Object Name	Description
PoolEvents	Available events for `Pool`.

class sqlalchemy.events.PoolEvents¶

Available events for Pool.

The methods here define the name of an event as well as the names of members that are passed to listener functions.

e.g.:

from sqlalchemy import event

def my_on_checkout(dbapi_conn, connection_rec, connection_proxy):
    "handle an on checkout event"

event.listen(Pool, 'checkout', my_on_checkout)

In addition to accepting the Pool class and Pool instances, PoolEvents also accepts Engine objects and the Engine class as targets, which will be resolved to the .pool attribute of the given engine or the Pool class:

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

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

Members

checkin(), checkout(), close(), close_detached(), connect(), detach(), first_connect(), invalidate(), reset(), soft_invalidate()

Class signature

class sqlalchemy.events.PoolEvents (sqlalchemy.event.base.Events)

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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.

method sqlalchemy.events.PoolEvents.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

ConnectionEvents.rollback()

ConnectionEvents.commit()

method sqlalchemy.events.PoolEvents.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¶

Object Name	Description
ConnectionEvents	Available events for `Connectable`, which includes `Connection` and `Engine`.
DialectEvents	event interface for execution-replacement functions.

class sqlalchemy.events.ConnectionEvents¶

Available events for Connectable, which includes Connection and Engine.

The methods here define the name of an event as well as the names of members that are passed to listener functions.

An event listener can be associated with any Connectable class or instance, such as an Engine, e.g.:

from sqlalchemy import event, create_engine

def before_cursor_execute(conn, cursor, statement, parameters, context,
                                                executemany):
    log.info("Received statement: %s", statement)

engine = create_engine('postgresql://scott:tiger@localhost/test')
event.listen(engine, "before_cursor_execute", before_cursor_execute)

or with a specific Connection:

with engine.begin() as conn:
    @event.listens_for(conn, 'before_cursor_execute')
    def before_cursor_execute(conn, cursor, statement, parameters,
                                    context, executemany):
        log.info("Received statement: %s", statement)

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.

Class signature

class sqlalchemy.events.ConnectionEvents (sqlalchemy.event.base.Events)

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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()

method sqlalchemy.events.ConnectionEvents.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()

method sqlalchemy.events.ConnectionEvents.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

method sqlalchemy.events.ConnectionEvents.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

method sqlalchemy.events.ConnectionEvents.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

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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: - The ConnectionEvents.dbapi_error() event is deprecated and will be removed in a future release. Please refer to the ConnectionEvents.handle_error() event.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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()

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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.

method sqlalchemy.events.ConnectionEvents.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¶

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.

ConnectionEvents.before_execute()

ConnectionEvents.after_cursor_execute()

ConnectionEvents.after_execute()

New in version 0.9.4.

Members

do_connect(), do_execute(), do_execute_no_params(), do_executemany(), do_setinputsizes()

Class signature

class sqlalchemy.events.DialectEvents (sqlalchemy.event.base.Events)

method sqlalchemy.events.DialectEvents.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.

method sqlalchemy.events.DialectEvents.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.

method sqlalchemy.events.DialectEvents.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.

method sqlalchemy.events.DialectEvents.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.

method sqlalchemy.events.DialectEvents.do_setinputsizes(inputsizes, cursor, statement, parameters, context)¶

Receive the setinputsizes dictionary for possible modification.

Example argument forms:

from sqlalchemy import event

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

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

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

    # ... (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.

Most dialects do not use this method at all; the only built-in dialect which uses this hook is the cx_Oracle dialect. The hook here is made available so as to allow customization of how datatypes are set up with the cx_Oracle DBAPI.

New in version 1.2.9.

Schema Events¶

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

class sqlalchemy.events.DDLEvents¶

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

Members

after_create(), after_drop(), after_parent_attach(), before_create(), before_drop(), before_parent_attach(), column_reflect()

Class signature

class sqlalchemy.events.DDLEvents (sqlalchemy.event.base.Events)

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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.

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.

method sqlalchemy.events.DDLEvents.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 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 text() object as well.

Changed in version 1.1.6: The DDLEvents.column_reflect() event allows a non string FetchedValue, 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.

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.

SQLAlchemy 1.2 Documentation

SQLAlchemy 1.2 Documentation

SQLAlchemy Core

Core Events¶

Connection Pool Events¶

SQL Execution and Connection Events¶

Schema Events¶