SQLAlchemy includes an event API which publishes a wide variety of hooks into the internals of both SQLAlchemy Core and ORM.
New in version 0.7: The system supercedes the previous system of “extension”, “proxy”, and “listener” classes.
Subscribing to an event occurs through a single API point, the listen()
function. This function
accepts a user-defined listening function, a string identifier which identifies the event to be
intercepted, and a target. Additional positional and keyword arguments 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:
from sqlalchemy.event import listen
from sqlalchemy.pool import Pool
def my_on_connect(dbapi_con, connection_record):
print "New DBAPI connection:", dbapi_con
listen(Pool, 'connect', my_on_connect)
The listen()
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 Pool
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():
return psycopg2.connect(username='ed', host='127.0.0.1', dbname='test')
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)
Some listeners allow modifiers to be passed to listen()
. 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'(?![0-9])', '', value)
# setup listener on UserContact.phone attribute, instructing
# 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:
sqlalchemy.event.
listen
(target, identifier, fn, *args, **kw)¶Register a listener function for the given target.
e.g.:
from sqlalchemy import event
from sqlalchemy.schema import UniqueConstraint
def unique_constraint_name(const, table):
const.name = "uq_%s_%s" % (
table.name,
list(const.columns)[0].name
)
event.listen(
UniqueConstraint,
"after_parent_attach",
unique_constraint_name)
sqlalchemy.event.
listens_for
(target, identifier, *args, **kw)¶Decorate a function as a listener for the given target + identifier.
e.g.:
from sqlalchemy import event
from sqlalchemy.schema import UniqueConstraint
@event.listens_for(UniqueConstraint, "after_parent_attach")
def unique_constraint_name(const, table):
const.name = "uq_%s_%s" % (
table.name,
list(const.columns)[0].name
)