[Sqlalchemy-tickets] Issue #4117: all_orm_descriptors contains deleted attribute (zzzeek/sqlalchemy)

[dima-starosud <iss...@bi...>]

New issue 4117: all_orm_descriptors contains deleted attribute
https://bitbucket.org/zzzeek/sqlalchemy/issues/4117/all_orm_descriptors-contains-deleted

dima-starosud:

This is using sqlalchemy 1.2.0b3.

Please consider following code snippet.

```
#!python

        class A(Base):
            __tablename__ = 'a'
            id = Column(Integer, primary_key=True)
            value = Column(String)

        mapper = inspect(A).mapper
        print('Initial', mapper.all_orm_descriptors.keys())

        A.value_2 = hybrid_property(lambda me: me.value)
        configure_mappers()
        print('Added', mapper.all_orm_descriptors.keys())

        del A.value_2
        configure_mappers()
        print('Deleted', mapper.all_orm_descriptors.keys())

        from sqlalchemy.orm.mapper import _memoized_configured_property
        _memoized_configured_property.expire_instance(mapper)
        print('Expired', mapper.all_orm_descriptors.keys())

```

I think `_memoized_configured_property` is not supposed to be used outside lib code.
Is there *official* way to do this?