Events

Subscribing to an event occurs through a single API point, the function, or alternatively the decorator. These functions accept a target, a string identifier which identifies the event to be intercepted, and a user-defined listening function. Additional positional and keyword arguments to these two functions may be supported by specific types of events, which may specify alternate interfaces for the given event function, or provide instructions regarding secondary event targets based on the given target.

The name of an event and the argument signature of a corresponding listener function is derived from a class bound specification method, which exists bound to a marker class that’s described in the documentation. For example, the documentation for PoolEvents.connect() indicates that the event name is "connect" and that a user-defined listener function should receive two positional arguments:

To listen with the decorator looks like:

from sqlalchemy.event import listens_for
from sqlalchemy.pool import Pool
@listens_for(Pool, "connect")
def my_on_connect(dbapi_con, connection_record):
    print("New DBAPI connection:", dbapi_con)

Named Argument Styles

There are some varieties of argument styles which can be accepted by listener functions. Taking the example of , this function is documented as receiving dbapi_connection and connection_record arguments. We can opt to receive these arguments by name, by establishing a listener function that accepts **keyword arguments, by passing named=True to either listen() or :

from sqlalchemy.event import listens_for
from sqlalchemy.pool import Pool
@listens_for(Pool, "connect", named=True)
def my_on_connect(**kw):
    print("New DBAPI connection:", kw['dbapi_connection'])

When using named argument passing, the names listed in the function argument specification will be used as keys in the dictionary.

Named style passes all arguments by name regardless of the function signature, so specific arguments may be listed as well, in any order, as long as the names match up:

Above, the presence of **kw tells listens_for() that arguments should be passed to the function by name, rather than positionally.

New in version 0.9.0: Added optional named argument dispatch to event calling.

The function is very flexible regarding targets. It generally accepts classes, instances of those classes, and related classes or objects from which the appropriate target can be derived. For example, the above mentioned "connect" event accepts Engine classes and objects as well as classes and objects:

from sqlalchemy.event import listen
from sqlalchemy.pool import Pool, QueuePool
from sqlalchemy import create_engine
from sqlalchemy.engine import Engine
import psycopg2
def connect():
my_pool = QueuePool(connect)
my_engine = create_engine('postgresql://ed@localhost/test')
# associate listener with all instances of Pool
listen(Pool, 'connect', my_on_connect)
# associate listener with all instances of Pool
# via the Engine class
listen(Engine, 'connect', my_on_connect)
# associate listener with my_pool
listen(my_pool, 'connect', my_on_connect)
# associate listener with my_engine.pool
listen(my_engine, 'connect', my_on_connect)

Modifiers

Some listeners allow modifiers to be passed to . These modifiers sometimes provide alternate calling signatures for listeners. Such as with ORM events, some event listeners can have a return value which modifies the subsequent handling. By default, no listener ever requires a return value, but by passing retval=True this value can be supported:

def validate_phone(target, value, oldvalue, initiator):
    """Strip non-numeric characters from a phone number"""
    return re.sub(r'\D', '', value)
# it to use the return value
listen(UserContact.phone, 'set', validate_phone, retval=True)

Both SQLAlchemy Core and SQLAlchemy ORM feature a wide variety of event hooks:

Core Events - these are described in Core Events and include event hooks specific to connection pool lifecycle, SQL statement execution, transaction lifecycle, and schema creation and teardown.
ORM Events - these are described in , and include event hooks specific to class and attribute instrumentation, object initialization hooks, attribute on-change hooks, session state, flush, and commit hooks, mapper initialization, object/result population, and per-instance persistence hooks.

API Reference

function sqlalchemy.event.``listen(target, identifier, fn, \args, **kw*)

The function is part of the primary interface for the SQLAlchemy event system, documented at Events.

e.g.:

A given function can also be invoked for only the first invocation of the event using the once argument:

def on_config():
    do_config()
event.listen(Mapper, "before_configure", on_config, once=True)

Warning

The argument does not imply automatic de-registration of the listener function after it has been invoked a first time; a listener entry will remain associated with the target object. Associating an arbitrarily high number of listeners without explicitly removing them will cause memory to grow unbounded even if once=True is specified.

Note

The function cannot be called at the same time that the target event is being run. This has implications for thread safety, and also means an event cannot be added from inside the listener function for itself. The list of events to be run are present inside of a mutable collection that can’t be changed during iteration.

Event registration and removal is not intended to be a “high velocity” operation; it is a configurational operation. For systems that need to quickly associate and deassociate with events at high scale, use a mutable structure that is handled from inside of a single listener.

Changed in version 1.0.0: - a collections.deque() object is now used as the container for the list of events, which explicitly disallows collection mutation while the collection is being iterated.