Method before_mapper_configured

lib/sqlalchemy/orm/events.py:957–1009

Called right before a specific mapper is to be configured. The :meth:`.MapperEvents.before_mapper_configured` event is invoked for each mapper that is encountered when the :func:`_orm.configure_mappers` function proceeds through the current list of not-yet-configured

(
        self, mapper: Mapper[_O], class_: Type[_O]
    )

    @event._omit_standard_example
    def before_mapper_configured(
        self, mapper: Mapper[_O], class_: Type[_O]
    ) -> None:
        """Called right before a specific mapper is to be configured.

        The :meth:`.MapperEvents.before_mapper_configured` event is invoked
        for each mapper that is encountered when the
        :func:`_orm.configure_mappers` function proceeds through the current
        list of not-yet-configured mappers.   It is similar to the
        :meth:`.MapperEvents.mapper_configured` event, except that it's invoked
        right before the configuration occurs, rather than afterwards.

        The :meth:`.MapperEvents.before_mapper_configured` event includes
        the special capability where it can force the configure step for a
        specific mapper to be skipped; to use this feature, establish
        the event using the ``retval=True`` parameter and return
        the :attr:`.orm.interfaces.EXT_SKIP` symbol to indicate the mapper
        should be left unconfigured::

            from sqlalchemy import event
            from sqlalchemy.orm import EXT_SKIP
            from sqlalchemy.orm import DeclarativeBase


            class DontConfigureBase(DeclarativeBase):
                pass


            @event.listens_for(
                DontConfigureBase,
                "before_mapper_configured",
                # support return values for the event
                retval=True,
                # propagate the listener to all subclasses of
                # DontConfigureBase
                propagate=True,
            )
            def dont_configure(mapper, cls):
                return EXT_SKIP

        .. seealso::

            :meth:`.MapperEvents.before_configured`

            :meth:`.MapperEvents.after_configured`

            :meth:`.RegistryEvents.before_configured`

            :meth:`.RegistryEvents.after_configured`

            :meth:`.MapperEvents.mapper_configured`

        """

    def mapper_configured(self, mapper: Mapper[_O], class_: Type[_O]) -> None:
        r"""Called when a specific mapper has completed its own configuration