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# sqlalchemy/events.py
# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""Core event interfaces."""
from . import event
from . import exc
from . import util
from .engine import Connectable
from .engine import Dialect
from .engine import Engine
from .pool import Pool
from .sql.base import SchemaEventTarget
class DDLEvents(event.Events):
"""
Define event listeners for schema objects,
that is, :class:`.SchemaItem` and other :class:`.SchemaEventTarget`
subclasses, including :class:`_schema.MetaData`, :class:`_schema.Table`,
:class:`_schema.Column`.
:class:`_schema.MetaData` and :class:`_schema.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 :class:`_schema.Column` is associated
with its :class:`_schema.Table`, when a
:class:`_schema.ForeignKeyConstraint`
is associated with a :class:`_schema.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
:class:`.DDL` class and the :class:`.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 :class:`.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 :meth:`_schema.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 :class:`.DDL` object will also be associated with the
:class:`_schema.Table` object represented by ``new_table``.
.. seealso::
:ref:`event_toplevel`
:class:`.DDLElement`
:class:`.DDL`
:ref:`schema_ddl_sequences`
"""
_target_class_doc = "SomeSchemaClassOrObject"
_dispatch_target = SchemaEventTarget
def before_create(self, target, connection, **kw):
r"""Called before CREATE statements are emitted.
:param target: the :class:`_schema.MetaData` or :class:`_schema.Table`
object which is the target of the event.
:param connection: the :class:`_engine.Connection` where the
CREATE statement or statements will be emitted.
:param \**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.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def after_create(self, target, connection, **kw):
r"""Called after CREATE statements are emitted.
:param target: the :class:`_schema.MetaData` or :class:`_schema.Table`
object which is the target of the event.
:param connection: the :class:`_engine.Connection` where the
CREATE statement or statements have been emitted.
:param \**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.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def before_drop(self, target, connection, **kw):
r"""Called before DROP statements are emitted.
:param target: the :class:`_schema.MetaData` or :class:`_schema.Table`
object which is the target of the event.
:param connection: the :class:`_engine.Connection` where the
DROP statement or statements will be emitted.
:param \**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.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def after_drop(self, target, connection, **kw):
r"""Called after DROP statements are emitted.
:param target: the :class:`_schema.MetaData` or :class:`_schema.Table`
object which is the target of the event.
:param connection: the :class:`_engine.Connection` where the
DROP statement or statements have been emitted.
:param \**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.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def before_parent_attach(self, target, parent):
"""Called before a :class:`.SchemaItem` is associated with
a parent :class:`.SchemaItem`.
:param target: the target object
:param parent: the parent to which the target is being attached.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def after_parent_attach(self, target, parent):
"""Called after a :class:`.SchemaItem` is associated with
a parent :class:`.SchemaItem`.
:param target: the target object
:param parent: the parent to which the target is being attached.
:func:`.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
:meth:`_schema.Table.tometadata` is used.
"""
def _sa_event_column_added_to_pk_constraint(self, const, col):
"""internal event hook used for primary key naming convention
updates.
"""
def column_reflect(self, inspector, table, column_info):
"""Called for each unit of 'column info' retrieved when
a :class:`_schema.Table` is being reflected.
Currently, this event may only be applied to the :class:`_schema.Table`
class directly::
from sqlalchemy import Table
@event.listens_for(Table, 'column_reflect')
def receive_column_reflect(inspector, table, column_info):
# receives for all Table objects that are reflected
Or applied using the
:paramref:`_schema.Table.listeners` parameter::
t1 = Table(
"my_table",
autoload_with=some_engine,
listeners=[
('column_reflect', receive_column_reflect)
]
)
A future release will allow it to be associated with a specific
:class:`_schema.MetaData` object as well.
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 :meth:`.reflection.Inspector.get_columns`:
* ``name`` - the column's name, is applied to the
:paramref:`_schema.Column.name` parameter
* ``type`` - the type of this column, which should be an instance
of :class:`~sqlalchemy.types.TypeEngine`, is applied to the
:paramref:`_schema.Column.type` parameter
* ``nullable`` - boolean flag if the column is NULL or NOT NULL,
is applied to the :paramref:`_schema.Column.nullable` parameter
* ``default`` - the column's server default value. This is
normally specified as a plain string SQL expression, however the
event can pass a :class:`.FetchedValue`, :class:`.DefaultClause`,
or :func:`_expression.text` object as well. Is applied to the
:paramref:`_schema.Column.server_default` parameter
.. versionchanged:: 1.1.6
The :meth:`.DDLEvents.column_reflect` event allows a non
string :class:`.FetchedValue`,
:func:`_expression.text`, or derived object to be
specified as the value of ``default`` in the column
dictionary.
The event is called before any action is taken against
this dictionary, and the contents can be modified; the following
additional keys may be added to the dictionary to further modify
how the :class:`_schema.Column` is constructed:
* ``key`` - the string key that will be used to access this
:class:`_schema.Column` in the ``.c`` collection; will be applied
to the :paramref:`_schema.Column.key` parameter. Is also used
for ORM mapping. See the section
:ref:`mapper_automated_reflection_schemes` for an example.
* ``quote`` - force or un-force quoting on the column name;
is applied to the :paramref:`_schema.Column.quote` parameter.
* ``info`` - a dictionary of arbitrary data to follow along with
the :class:`_schema.Column`, is applied to the
:paramref:`_schema.Column.info` parameter.
:func:`.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
:meth:`_schema.Table.to_metadata` is used.
"""
class PoolEvents(event.Events):
"""Available events for :class:`_pool.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 :class:`_pool.Pool` class and
:class:`_pool.Pool` instances, :class:`_events.PoolEvents` also accepts
:class:`_engine.Engine` objects and the :class:`_engine.Engine` class as
targets, which will be resolved to the ``.pool`` attribute of the
given engine or the :class:`_pool.Pool` class::
engine = create_engine("postgresql://scott:tiger@localhost/test")
# will associate with engine.pool
event.listen(engine, 'checkout', my_on_checkout)
"""
_target_class_doc = "SomeEngineOrPool"
_dispatch_target = Pool
@classmethod
def _accept_with(cls, target):
if isinstance(target, type):
if issubclass(target, Engine):
return Pool
elif issubclass(target, Pool):
return target
elif isinstance(target, Engine):
return target.pool
else:
return target
def connect(self, dbapi_connection, connection_record):
"""Called at the moment a particular DBAPI connection is first
created for a given :class:`_pool.Pool`.
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.
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
"""
def first_connect(self, dbapi_connection, connection_record):
"""Called exactly once for the first time a DBAPI connection is
checked out from a particular :class:`_pool.Pool`.
The rationale for :meth:`_events.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
:class:`_pool.Pool`
refers to a single "creator" function (which in terms
of a :class:`_engine.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.
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
"""
def checkout(self, dbapi_connection, connection_record, connection_proxy):
"""Called when a connection is retrieved from the Pool.
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
:param connection_proxy: the :class:`._ConnectionFairy` object which
will proxy the public interface of the DBAPI connection for the
lifespan of the checkout.
If you raise a :class:`~sqlalchemy.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.
.. seealso:: :meth:`_events.ConnectionEvents.engine_connect`
- a similar event
which occurs upon creation of a new :class:`_engine.Connection`.
"""
def checkin(self, dbapi_connection, connection_record):
"""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.)
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
"""
def reset(self, dbapi_connection, connection_record):
"""Called before the "reset" action occurs for a pooled connection.
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 :meth:`_events.PoolEvents.reset` event is usually followed by the
:meth:`_events.PoolEvents.checkin` event is called, except in those
cases where the connection is discarded immediately after reset.
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
.. seealso::
:meth:`_events.ConnectionEvents.rollback`
:meth:`_events.ConnectionEvents.commit`
"""
def invalidate(self, dbapi_connection, connection_record, exception):
"""Called when a DBAPI connection is to be "invalidated".
This event is called any time the :meth:`._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.
:param dbapi_connection: a DBAPI connection.
:param connection_record: the :class:`._ConnectionRecord` managing the
DBAPI connection.
:param exception: the exception object corresponding to the reason
for this invalidation, if any. May be ``None``.
.. versionadded:: 0.9.2 Added support for connection invalidation
listening.
.. seealso::
:ref:`pool_connection_invalidation`
"""
def soft_invalidate(self, dbapi_connection, connection_record, exception):
"""Called when a DBAPI connection is to be "soft invalidated".
This event is called any time the :meth:`._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.
.. versionadded:: 1.0.3
"""
def close(self, dbapi_connection, connection_record):
"""Called when a DBAPI connection is closed.
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 :meth:`.close` event corresponds to a connection that's still
associated with the pool. To intercept close events for detached
connections use :meth:`.close_detached`.
.. versionadded:: 1.1
"""
def detach(self, dbapi_connection, connection_record):
"""Called when a DBAPI connection is "detached" from a pool.
This event is emitted after the detach occurs. The connection
is no longer associated with the given connection record.
.. versionadded:: 1.1
"""
def close_detached(self, dbapi_connection):
"""Called when a detached DBAPI connection is closed.
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.
.. versionadded:: 1.1
"""
class ConnectionEvents(event.Events):
"""Available events for :class:`.Connectable`, which includes
:class:`_engine.Connection` and :class:`_engine.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 :class:`.Connectable`
class or instance, such as an :class:`_engine.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 :class:`_engine.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
:meth:`.after_cursor_execute`, :meth:`.before_cursor_execute` and
:meth:`.dbapi_error`, the statement is the exact SQL string that was
prepared for transmission to the DBAPI ``cursor`` in the connection's
:class:`.Dialect`.
The :meth:`.before_execute` and :meth:`.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 :meth:`.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:: :class:`_events.ConnectionEvents` can be established on any
combination of :class:`_engine.Engine`, :class:`_engine.Connection`,
as well
as instances of each of those classes. Events across all
four scopes will fire off for a given instance of
:class:`_engine.Connection`. However, for performance reasons, the
:class:`_engine.Connection` object determines at instantiation time
whether or not its parent :class:`_engine.Engine` has event listeners
established. Event listeners added to the :class:`_engine.Engine`
class or to an instance of :class:`_engine.Engine`
*after* the instantiation
of a dependent :class:`_engine.Connection` instance will usually
*not* be available on that :class:`_engine.Connection` instance.
The newly
added listeners will instead take effect for
:class:`_engine.Connection`
instances created subsequent to those event listeners being
established on the parent :class:`_engine.Engine` class or instance.
:param retval=False: Applies to the :meth:`.before_execute` and
:meth:`.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.
"""
_target_class_doc = "SomeEngine"
_dispatch_target = Connectable
@classmethod
def _listen(cls, event_key, retval=False):
target, identifier, fn = (
event_key.dispatch_target,
event_key.identifier,
event_key._listen_fn,
)
target._has_events = True
if not retval:
if identifier == "before_execute":
orig_fn = fn
def wrap_before_execute(
conn, clauseelement, multiparams, params
):
orig_fn(conn, clauseelement, multiparams, params)
return clauseelement, multiparams, params
fn = wrap_before_execute
elif identifier == "before_cursor_execute":
orig_fn = fn
def wrap_before_cursor_execute(
conn, cursor, statement, parameters, context, executemany
):
orig_fn(
conn,
cursor,
statement,
parameters,
context,
executemany,
)
return statement, parameters
fn = wrap_before_cursor_execute
elif retval and identifier not in (
"before_execute",
"before_cursor_execute",
"handle_error",
):
raise exc.ArgumentError(
"Only the 'before_execute', "
"'before_cursor_execute' and 'handle_error' engine "
"event listeners accept the 'retval=True' "
"argument."
)
event_key.with_wrapper(fn).base_listen()
def before_execute(self, conn, clauseelement, multiparams, params):
"""Intercept high level execute() events, receiving uncompiled
SQL constructs and other objects prior to rendering into SQL.
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
:param conn: :class:`_engine.Connection` object
:param clauseelement: SQL expression construct, :class:`.Compiled`
instance, or string statement passed to
:meth:`_engine.Connection.execute`.
:param multiparams: Multiple parameter sets, a list of dictionaries.
:param params: Single parameter set, a single dictionary.
.. seealso::
:meth:`.before_cursor_execute`
"""
def after_execute(self, conn, clauseelement, multiparams, params, result):
"""Intercept high level execute() events after execute.
:param conn: :class:`_engine.Connection` object
:param clauseelement: SQL expression construct, :class:`.Compiled`
instance, or string statement passed to
:meth:`_engine.Connection.execute`.
:param multiparams: Multiple parameter sets, a list of dictionaries.
:param params: Single parameter set, a single dictionary.
:param result: :class:`_engine.ResultProxy` generated by the execution
.
"""
def before_cursor_execute(
self, 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.
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 :class:`_events.ConnectionEvents`.
:param conn: :class:`_engine.Connection` object
:param cursor: DBAPI cursor object
:param statement: string SQL statement, as to be passed to the DBAPI
:param 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``.
:param context: :class:`.ExecutionContext` object in use. May
be ``None``.
:param executemany: boolean, if ``True``, this is an ``executemany()``
call, if ``False``, this is an ``execute()`` call.
.. seealso::
:meth:`.before_execute`
:meth:`.after_cursor_execute`
"""
def after_cursor_execute(
self, conn, cursor, statement, parameters, context, executemany
):
"""Intercept low-level cursor execute() events after execution.
:param conn: :class:`_engine.Connection` object
:param 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 :class:`_engine.ResultProxy`.
:param statement: string SQL statement, as passed to the DBAPI
:param 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``.
:param context: :class:`.ExecutionContext` object in use. May
be ``None``.
:param executemany: boolean, if ``True``, this is an ``executemany()``
call, if ``False``, this is an ``execute()`` call.
"""
@util.deprecated(
"0.9",
"The :meth:`_events.ConnectionEvents.dbapi_error` "
"event is deprecated and will be removed in a future release. "
"Please refer to the :meth:`_events.ConnectionEvents.handle_error` "
"event.",
)
def dbapi_error(
self, conn, cursor, statement, parameters, context, exception
):
"""Intercept a raw DBAPI error.
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 :class:`_engine.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 :meth:`_events.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.
:param conn: :class:`_engine.Connection` object
:param cursor: DBAPI cursor object
:param statement: string SQL statement, as passed to the DBAPI
:param 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``.
:param context: :class:`.ExecutionContext` object in use. May
be ``None``.
:param exception: The **unwrapped** exception emitted directly from the
DBAPI. The class here is specific to the DBAPI module in use.
"""
def handle_error(self, exception_context):
r"""Intercept all exceptions processed by the
:class:`_engine.Connection`.
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 :meth:`.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
:class:`.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. [1]_.
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.
.. note::
.. [1] The pool "pre_ping" handler enabled using the
:paramref:`_sa.create_engine.pool_pre_ping` parameter does
**not** consult this event before deciding if the "ping"
returned false, as opposed to receiving an unhandled error.
For this use case, the :ref:`legacy recipe based on
engine_connect() may be used
<pool_disconnects_pessimistic_custom>`. A future API allow
more comprehensive customization of the "disconnect"
detection mechanism across all functions.
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
:meth:`_events.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
:attr:`.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
:class:`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.
:param context: an :class:`.ExceptionContext` object. See this
class for details on all available members.
.. versionadded:: 0.9.7 Added the
:meth:`_events.ConnectionEvents.handle_error` hook.
.. versionchanged:: 1.1 The :meth:`.handle_error` event will now
receive all exceptions that inherit from ``BaseException``,
including ``SystemExit`` and ``KeyboardInterrupt``. The setting for
:attr:`.ExceptionContext.is_disconnect` is ``True`` in this case and
the default for
:attr:`.ExceptionContext.invalidate_pool_on_disconnect` is
``False``.
.. versionchanged:: 1.0.0 The :meth:`.handle_error` event is now
invoked when an :class:`_engine.Engine` fails during the initial
call to :meth:`_engine.Engine.connect`, as well as when a
:class:`_engine.Connection` object encounters an error during a
reconnect operation.
.. versionchanged:: 1.0.0 The :meth:`.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 :meth:`.handle_error` needs
to ensure that exceptions thrown in these scenarios are re-raised
without modification.
"""
def engine_connect(self, conn, branch):
"""Intercept the creation of a new :class:`_engine.Connection`.
This event is called typically as the direct result of calling
the :meth:`_engine.Engine.connect` method.
It differs from the :meth:`_events.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
:class:`_engine.Connection` wrapper around such a DBAPI connection.
It also differs from the :meth:`_events.PoolEvents.checkout` event
in that it is specific to the :class:`_engine.Connection` object,
not the
DBAPI connection that :meth:`_events.PoolEvents.checkout` deals with,
although
this DBAPI connection is available here via the
:attr:`_engine.Connection.connection` attribute.
But note there can in fact
be multiple :meth:`_events.PoolEvents.checkout`
events within the lifespan
of a single :class:`_engine.Connection` object, if that
:class:`_engine.Connection`
is invalidated and re-established. There can also be multiple
:class:`_engine.Connection`
objects generated for the same already-checked-out
DBAPI connection, in the case that a "branch" of a
:class:`_engine.Connection`
is produced.
:param conn: :class:`_engine.Connection` object.
:param branch: if True, this is a "branch" of an existing
:class:`_engine.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.
.. versionadded:: 0.9.0
.. seealso::
:ref:`pool_disconnects_pessimistic` - illustrates how to use
:meth:`_events.ConnectionEvents.engine_connect`
to transparently ensure pooled connections are connected to the
database.
:meth:`_events.PoolEvents.checkout`
the lower-level pool checkout event
for an individual DBAPI connection
:meth:`_events.ConnectionEvents.set_connection_execution_options`
- a copy
of a :class:`_engine.Connection` is also made when the
:meth:`_engine.Connection.execution_options` method is called.
"""
def set_connection_execution_options(self, conn, opts):
"""Intercept when the :meth:`_engine.Connection.execution_options`
method is called.
This method is called after the new :class:`_engine.Connection`
has been
produced, with the newly updated execution options collection, but
before the :class:`.Dialect` has acted upon any of those new options.
Note that this method is not called when a new
:class:`_engine.Connection`
is produced which is inheriting execution options from its parent
:class:`_engine.Engine`; to intercept this condition, use the
:meth:`_events.ConnectionEvents.engine_connect` event.
:param conn: The newly copied :class:`_engine.Connection` object
:param opts: dictionary of options that were passed to the
:meth:`_engine.Connection.execution_options` method.
.. versionadded:: 0.9.0
.. seealso::
:meth:`_events.ConnectionEvents.set_engine_execution_options`
- event
which is called when :meth:`_engine.Engine.execution_options`
is called.
"""
def set_engine_execution_options(self, engine, opts):
"""Intercept when the :meth:`_engine.Engine.execution_options`
method is called.
The :meth:`_engine.Engine.execution_options` method produces a shallow
copy of the :class:`_engine.Engine` which stores the new options.
That new
:class:`_engine.Engine` is passed here.
A particular application of this
method is to add a :meth:`_events.ConnectionEvents.engine_connect`
event
handler to the given :class:`_engine.Engine`
which will perform some per-
:class:`_engine.Connection` task specific to these execution options.
:param conn: The newly copied :class:`_engine.Engine` object
:param opts: dictionary of options that were passed to the
:meth:`_engine.Connection.execution_options` method.
.. versionadded:: 0.9.0
.. seealso::
:meth:`_events.ConnectionEvents.set_connection_execution_options`
- event
which is called when :meth:`_engine.Connection.execution_options`
is
called.
"""
def engine_disposed(self, engine):
"""Intercept when the :meth:`_engine.Engine.dispose` method is called.
The :meth:`_engine.Engine.dispose` method instructs the engine to
"dispose" of it's connection pool (e.g. :class:`_pool.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
:class:`_engine.Engine` should also be cleaned up,
keeping in mind that the
:class:`_engine.Engine`
can still be used for new requests in which case
it re-acquires connection resources.
.. versionadded:: 1.0.5
"""
def begin(self, conn):
"""Intercept begin() events.
:param conn: :class:`_engine.Connection` object
"""
def rollback(self, conn):
"""Intercept rollback() events, as initiated by a
:class:`.Transaction`.
Note that the :class:`_pool.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 :meth:`_events.PoolEvents.reset` hook.
:param conn: :class:`_engine.Connection` object
.. seealso::
:meth:`_events.PoolEvents.reset`
"""
def commit(self, conn):
"""Intercept commit() events, as initiated by a
:class:`.Transaction`.
Note that the :class:`_pool.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 :meth:`_events.PoolEvents.reset` hook.
:param conn: :class:`_engine.Connection` object
"""
def savepoint(self, conn, name):
"""Intercept savepoint() events.
:param conn: :class:`_engine.Connection` object
:param name: specified name used for the savepoint.
"""
def rollback_savepoint(self, conn, name, context):
"""Intercept rollback_savepoint() events.
:param conn: :class:`_engine.Connection` object
:param name: specified name used for the savepoint.
:param context: :class:`.ExecutionContext` in use. May be ``None``.
"""
def release_savepoint(self, conn, name, context):
"""Intercept release_savepoint() events.
:param conn: :class:`_engine.Connection` object
:param name: specified name used for the savepoint.
:param context: :class:`.ExecutionContext` in use. May be ``None``.
"""
def begin_twophase(self, conn, xid):
"""Intercept begin_twophase() events.
:param conn: :class:`_engine.Connection` object
:param xid: two-phase XID identifier
"""
def prepare_twophase(self, conn, xid):
"""Intercept prepare_twophase() events.
:param conn: :class:`_engine.Connection` object
:param xid: two-phase XID identifier
"""
def rollback_twophase(self, conn, xid, is_prepared):
"""Intercept rollback_twophase() events.
:param conn: :class:`_engine.Connection` object
:param xid: two-phase XID identifier
:param is_prepared: boolean, indicates if
:meth:`.TwoPhaseTransaction.prepare` was called.
"""
def commit_twophase(self, conn, xid, is_prepared):
"""Intercept commit_twophase() events.
:param conn: :class:`_engine.Connection` object
:param xid: two-phase XID identifier
:param is_prepared: boolean, indicates if
:meth:`.TwoPhaseTransaction.prepare` was called.
"""
class DialectEvents(event.Events):
"""event interface for execution-replacement functions.
These events allow direct instrumentation and replacement
of key dialect functions which interact with the DBAPI.
.. note::
:class:`.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 :class:`_events.ConnectionEvents` interface.
.. seealso::
:meth:`_events.ConnectionEvents.before_cursor_execute`
:meth:`_events.ConnectionEvents.before_execute`
:meth:`_events.ConnectionEvents.after_cursor_execute`
:meth:`_events.ConnectionEvents.after_execute`
.. versionadded:: 0.9.4
"""
_target_class_doc = "SomeEngine"
_dispatch_target = Dialect
@classmethod
def _listen(cls, event_key, retval=False):
target = event_key.dispatch_target
target._has_events = True
event_key.base_listen()
@classmethod
def _accept_with(cls, target):
if isinstance(target, type):
if issubclass(target, Engine):
return Dialect
elif issubclass(target, Dialect):
return target
elif isinstance(target, Engine):
return target.dialect
else:
return target
def do_connect(self, dialect, conn_rec, cargs, cparams):
"""Receive connection arguments before a connection is made.
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.
.. versionadded:: 1.0.3
.. seealso::
:ref:`custom_dbapi_args`
"""
def do_executemany(self, cursor, statement, parameters, context):
"""Receive a cursor to have executemany() called.
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.
"""
def do_execute_no_params(self, cursor, statement, context):
"""Receive a cursor to have execute() with no parameters called.
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.
"""
def do_execute(self, cursor, statement, parameters, context):
"""Receive a cursor to have execute() called.
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.
"""
def do_setinputsizes(
self, inputsizes, cursor, statement, parameters, context
):
"""Receive the setinputsizes dictionary for possible modification.
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 :class:`.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
:attr:`.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.
.. versionadded:: 1.2.9
.. seealso::
:ref:`cx_oracle_setinputsizes`
"""
pass