api.py 文件源码

def 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)


    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)

    .. versionadded:: 0.9.4 Added ``once=True`` to :func:`.event.listen`
       and :func:`.event.listens_for`.

    """

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