This section describes the event interfaces provided in SQLAlchemy Core. The event system in 0.7 is all new and supercedes the previous system of “extension”, “listener”, and “proxy” classes. For an introduction to the event listening API, see Events. ORM events are described in ORM 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"
events.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
events.listen(engine, 'checkout', my_on_checkout)
Called when a connection returns to the pool.
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.)
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Called when a connection is retrieved from the Pool.
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If you raise an exc.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.
Called once for each new DB-API connection or Pool’s creator().
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Called exactly once for the first DB-API connection.
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Available events for Connection.
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, create_engine
def before_execute(conn, clauseelement, multiparams, params):
log.info("Received statement: %s" % clauseelement)
engine = create_engine('postgresql://scott:tiger@localhost/test')
event.listen(engine, "before_execute", before_execute)
Some events allow modifiers to the listen() function.
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. |
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Intercept low-level cursor execute() events.
Intercept high level execute() events.
Intercept low-level cursor execute() events.
Intercept high level execute() events.
Intercept begin() events.
Intercept begin_twophase() events.
Intercept commit() events.
Intercept commit_twophase() events.
Intercept prepare_twophase() events.
Intercept release_savepoint() events.
Intercept rollback() events.
Intercept rollback_savepoint() events.
Intercept rollback_twophase() events.
Intercept savepoint() events.
Define event listeners for schema objects, that is, SchemaItem and SchemaEvent 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.
See also:
Called after CREATE statments are emitted.
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Called after DROP statments are emitted.
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Called after a SchemaItem is associated with a parent SchemaItem.
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event.listen() also accepts a modifier for this event:
Parameters: | propagate=False – 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. |
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Called before CREATE statments are emitted.
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Called before DROP statments are emitted.
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Called before a SchemaItem is associated with a parent SchemaItem.
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event.listen() also accepts a modifier for this event:
Parameters: | propagate=False – 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. |
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Called for each unit of ‘column info’ retrieved when a Table is being reflected.
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().
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(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(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.
Base class for elements that are the targets of DDLEvents events.
This includes SchemaItem as well as SchemaType.