event.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)

    """

    for evt_cls in _registrars[identifier]:
        tgt = evt_cls._accept_with(target)
        if tgt is not None:
            tgt.dispatch._listen(tgt, identifier, fn, *args, **kw)
            return
    raise exc.InvalidRequestError("No such event '%s' for target '%s'" %
                                (identifier, target))