examples.custom_attributes.custom_management — SQLAlchemy 2.0 Documentation

Release: 2.0.49 | Release Date: April 3, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.custom_attributes.custom_management

    """Illustrates customized class instrumentation, using
    the :mod:`sqlalchemy.ext.instrumentation` extension package.

    In this example, mapped classes are modified to
    store their state in a dictionary attached to an attribute
    named "_goofy_dict", instead of using __dict__.
    this example illustrates how to replace SQLAlchemy's class
    descriptors with a user-defined system.

    """

    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import ForeignKey
    from sqlalchemy import Integer
    from sqlalchemy import MetaData
    from sqlalchemy import Table
    from sqlalchemy import Text
    from sqlalchemy.ext.instrumentation import InstrumentationManager
    from sqlalchemy.orm import registry as _reg
    from sqlalchemy.orm import relationship
    from sqlalchemy.orm import Session
    from sqlalchemy.orm.attributes import del_attribute
    from sqlalchemy.orm.attributes import get_attribute
    from sqlalchemy.orm.attributes import set_attribute
    from sqlalchemy.orm.instrumentation import is_instrumented

    registry = _reg()

    class MyClassState(InstrumentationManager):
        def get_instance_dict(self, class_, instance):
            return instance._goofy_dict

        def initialize_instance_dict(self, class_, instance):
            instance.__dict__["_goofy_dict"] = {}

        def install_state(self, class_, instance, state):
            instance.__dict__["_goofy_dict"]["state"] = state

        def state_getter(self, class_):
            def find(instance):
                return instance.__dict__["_goofy_dict"]["state"]

            return find

    class MyClass:
        __sa_instrumentation_manager__ = MyClassState

        def __init__(self, **kwargs):
            for k in kwargs:
                setattr(self, k, kwargs[k])

        def __getattr__(self, key):
            if is_instrumented(self, key):
                return get_attribute(self, key)
            else:
                try:
                    return self._goofy_dict[key]
                except KeyError:
                    raise AttributeError(key)

        def __setattr__(self, key, value):
            if is_instrumented(self, key):
                set_attribute(self, key, value)
            else:
                self._goofy_dict[key] = value

        def __delattr__(self, key):
            if is_instrumented(self, key):
                del_attribute(self, key)
            else:
                del self._goofy_dict[key]

    if __name__ == "__main__":
        engine = create_engine("sqlite://")
        meta = MetaData()

        table1 = Table(
            "table1",
            meta,
            Column("id", Integer, primary_key=True),
            Column("name", Text),
        )
        table2 = Table(
            "table2",
            meta,
            Column("id", Integer, primary_key=True),
            Column("name", Text),
            Column("t1id", Integer, ForeignKey("table1.id")),
        )
        meta.create_all(engine)

        class A(MyClass):
            pass

        class B(MyClass):
            pass

        registry.map_imperatively(A, table1, properties={"bs": relationship(B)})

        registry.map_imperatively(B, table2)

        a1 = A(name="a1", bs=[B(name="b1"), B(name="b2")])

        assert a1.name == "a1"
        assert a1.bs[0].name == "b1"

        sess = Session(engine)
        sess.add(a1)

        sess.commit()

        a1 = sess.query(A).get(a1.id)

        assert a1.name == "a1"
        assert a1.bs[0].name == "b1"

        a1.bs.remove(a1.bs[0])

        sess.commit()

        a1 = sess.query(A).get(a1.id)
        assert len(a1.bs) == 1