事件

Events

SQLAlchemy 包含一个事件 API，它将各种各样的钩子发布到 SQLAlchemy Core 和 ORM 的内部。

SQLAlchemy includes an event API which publishes a wide variety of hooks into the internals of both SQLAlchemy Core and ORM.

事件注册

Event Registration

订阅事件是通过一个统一的 API 实现的，即 listen() 函数，或可选地通过 listens_for() 装饰器来完成。这些函数接受一个目标对象、一个用于标识要拦截事件的字符串标识符，以及一个用户定义的监听函数。这两个函数还可以接受额外的位置参数和关键字参数，具体支持哪些参数取决于事件的类型，这些事件类型可能会定义替代的接口形式，或基于目标对象说明次级事件目标的处理方式。

事件的名称及其对应监听函数的参数签名来自绑定于某个类的规范方法，该类作为标记类在文档中进行了说明。例如，PoolEvents.connect() 的文档中指出，事件名称为 "connect"，并且用户定义的监听函数应当接收两个位置参数:

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)

使用装饰器 listens_for() 订阅事件的形式如下:

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)

Subscribing to an event occurs through a single API point, the listen() function, or alternatively the listens_for() 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:

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)

To listen with the listens_for() 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)

命名参数样式

Named Argument Styles

某些类型的事件监听函数可以接受不同风格的参数。以 PoolEvents.connect() 为例，该事件会传入 dbapi_connection 和 connection_record 两个参数。我们可以选择使用具名参数的方式接收这些参数，即编写一个接受 **keyword 参数的监听函数，并向 listen() 或 listens_for() 传递 named=True 参数:

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"])

在使用具名参数时，监听函数将以字典方式接收所有事件参数，其键名对应函数签名中指定的参数名。

具名参数模式会将所有参数按名称传入监听函数，因此监听函数中可以直接列出部分参数，只要名称对应即可，其余参数通过 **kw 捕获:

from sqlalchemy.event import listens_for
from sqlalchemy.pool import Pool


@listens_for(Pool, "connect", named=True)
def my_on_connect(dbapi_connection, **kw):
    print("New DBAPI connection:", dbapi_connection)
    print("Connection record:", kw["connection_record"])

如上所示，监听函数中出现 **kw 是向 listens_for() 表明应该使用具名方式传参，而非按位置传参。

There are some varieties of argument styles which can be accepted by listener functions. Taking the example of PoolEvents.connect(), 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 listens_for():

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:

from sqlalchemy.event import listens_for
from sqlalchemy.pool import Pool


@listens_for(Pool, "connect", named=True)
def my_on_connect(dbapi_connection, **kw):
    print("New DBAPI connection:", dbapi_connection)
    print("Connection record:", kw["connection_record"])

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

目标

Targets

listen() 函数在监听目标的选择上非常灵活。它通常可以接受类、这些类的实例，或者与之相关的类或对象，这些对象中能够派生出适当的监听目标。例如，前文提到的 "connect" 事件可以接受 Engine 类或其实例，也可以接受 Pool 类或其实例:

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(user="ed", host="127.0.0.1", dbname="test")


my_pool = QueuePool(connect)
my_engine = create_engine("postgresql+psycopg2://ed@localhost/test")

# 为所有 Pool 实例添加监听器
listen(Pool, "connect", my_on_connect)

# 通过 Engine 类为所有 Pool 实例添加监听器
listen(Engine, "connect", my_on_connect)

# 为 my_pool 添加监听器
listen(my_pool, "connect", my_on_connect)

# 为 my_engine.pool 添加监听器
listen(my_engine, "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(user="ed", host="127.0.0.1", dbname="test")


my_pool = QueuePool(connect)
my_engine = create_engine("postgresql+psycopg2://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)

修饰符

Modifiers

某些事件监听器允许通过 listen() 传入修饰符参数。这些修饰符有时可以改变监听器的调用签名。例如，在 ORM 事件中，某些监听器可以返回值来影响后续处理。默认情况下，监听函数不需要返回值，但通过传入 retval=True 参数，可以启用返回值的支持:

def validate_phone(target, value, oldvalue, initiator):
    """去除电话号码中的非数字字符"""

    return re.sub(r"\D", "", value)


# 在 UserContact.phone 属性上设置监听器，启用返回值处理
listen(UserContact.phone, "set", validate_phone, retval=True)

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"\D", "", value)


# setup listener on UserContact.phone attribute, instructing
# it to use the return value
listen(UserContact.phone, "set", validate_phone, retval=True)

事件和多处理

Events and Multiprocessing

SQLAlchemy 的事件钩子是通过 Python 函数与对象实现的，因此事件的传播是以 Python 函数调用的方式进行的。Python 的多进程模型遵循操作系统多进程的通用理念，例如父进程派生出子进程，因此我们可以用同样的模型来描述 SQLAlchemy 事件系统在多进程环境中的行为。

在父进程中注册的事件钩子，在其之后派生出来的子进程中也会存在，因为子进程启动时会复制父进程中所有已存在的 Python 结构。相反，如果子进程早于事件钩子的注册而存在，那么这些子进程将不会继承新注册的事件钩子。也就是说，父进程对 Python 结构所做的更改不会传播至已经存在的子进程。

至于事件本身，它们是 Python 函数调用，在进程之间无法传播。SQLAlchemy 的事件系统并不实现任何形式的进程间通信。不过，用户可以自行实现支持 Python 多进程消息通信的事件钩子逻辑，但这部分功能需由用户自定义。

SQLAlchemy’s event hooks are implemented with Python functions and objects, so events propagate via Python function calls. Python multiprocessing follows the same way we think about OS multiprocessing, such as a parent process forking a child process, thus we can describe the SQLAlchemy event system’s behavior using the same model.

Event hooks registered in a parent process will be present in new child processes that are forked from that parent after the hooks have been registered, since the child process starts with a copy of all existing Python structures from the parent when spawned. Child processes that already exist before the hooks are registered will not receive those new event hooks, as changes made to Python structures in a parent process do not propagate to child processes.

For the events themselves, these are Python function calls, which do not have any ability to propagate between processes. SQLAlchemy’s event system does not implement any inter-process communication. It is possible to implement event hooks that use Python inter-process messaging within them, however this would need to be implemented by the user.

事件参考

Event Reference

SQLAlchemy Core 与 SQLAlchemy ORM 都支持丰富的事件钩子：

  • Core 事件 - 详见 Core 事件，涵盖连接池生命周期、SQL 语句执行、事务生命周期、模式的创建与销毁等相关的事件钩子。

  • ORM 事件 - 详见 ORM 事件，涵盖类与属性的检测钩子、对象初始化钩子、属性变更钩子、会话状态、刷新与提交钩子、映射器初始化、对象/结果填充，以及针对单个对象的持久化钩子等内容。

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

  • Core Events - these are described in Core 事件 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 ORM 事件, 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 参考

API Reference

Object Name Description

contains(target, identifier, fn)

Return True if the given target/ident/fn is set up to listen.

listen(target, identifier, fn, *args, **kw)

Register a listener function for the given target.

listens_for(target, identifier, *args, **kw)

Decorate a function as a listener for the given target + identifier.

remove(target, identifier, fn)

Remove an event listener.
function sqlalchemy.event.listen(target: Any, identifier: str, fn: Callable[[...], Any], *args: Any, **kw: Any) → None

Register a listener function for the given target.

The listen() function is part of the primary interface for the SQLAlchemy event system, documented at 事件.

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
)
参数:

  • insert (bool) – The default behavior for event handlers is to append the decorated user defined function to an internal list of registered event listeners upon discovery. If a user registers a function with insert=True, SQLAlchemy will insert (prepend) the function to the internal list upon discovery. This feature is not typically used or recommended by the SQLAlchemy maintainers, but is provided to ensure certain user defined functions can run before others, such as when Changing the sql_mode in MySQL.

  • named (bool) – When using named argument passing, the names listed in the function argument specification will be used as keys in the dictionary. See 命名参数样式.

  • once (bool) – Private/Internal API usage. Deprecated. This parameter would provide that an event function would run only once per given target. It does not however imply automatic de-registration of the listener function; associating an arbitrarily high number of listeners without explicitly removing them will cause memory to grow unbounded even if once=True is specified.

  • propagate (bool) – The propagate kwarg is available when working with ORM instrumentation and mapping events. See MapperEvents and MapperEvents.before_mapper_configured() for examples.

  • retval (bool) –

    This flag applies only to specific event listeners, each of which includes documentation explaining when it should be used. By default, no listener ever requires a return value. However, some listeners do support special behaviors for return values, and include in their documentation that the retval=True flag is necessary for a return value to be processed.

    Event listener suites that make use of listen.retval include ConnectionEvents and AttributeEvents.

备注

The listen() 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.

参见

listens_for()

remove()

function sqlalchemy.event.listens_for(target: Any, identifier: str, *args: Any, **kw: Any) → Callable[[Callable[[...], Any]], Callable[[...], Any]]

Decorate a function as a listener for the given target + identifier.

The listens_for() decorator is part of the primary interface for the SQLAlchemy event system, documented at 事件.

This function generally shares the same kwargs as listen().

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)

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

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

警告

The once 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.

参见

listen() - general description of event listening

function sqlalchemy.event.remove(target: Any, identifier: str, fn: Callable[[...], Any]) → None

Remove an event listener.

The arguments here should match exactly those which were sent to listen(); all the event registration which proceeded as a result of this call will be reverted by calling remove() with the same arguments.

e.g.:

# if a function was registered like this...
@event.listens_for(SomeMappedClass, "before_insert", propagate=True)
def my_listener_function(*arg):
    pass


# ... it's removed like this
event.remove(SomeMappedClass, "before_insert", my_listener_function)

Above, the listener function associated with SomeMappedClass was also propagated to subclasses of SomeMappedClass; the remove() function will revert all of these operations.

备注

The remove() 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 removed 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.

参见

listen()

function sqlalchemy.event.contains(target: Any, identifier: str, fn: Callable[[...], Any]) → bool

Return True if the given target/ident/fn is set up to listen.