examples.custom_attributes.listen_for_events — SQLAlchemy 2.0 Documentation

Release: 2.0.49 | Release Date: April 3, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.custom_attributes.listen_for_events

    """Illustrates how to attach events to all instrumented attributes
    and listen for change events.

    """

    from sqlalchemy import Column
    from sqlalchemy import event
    from sqlalchemy import ForeignKey
    from sqlalchemy import Integer
    from sqlalchemy import String
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import relationship

    def configure_listener(class_, key, inst):
        def append(instance, value, initiator):
            instance.receive_change_event("append", key, value, None)

        def remove(instance, value, initiator):
            instance.receive_change_event("remove", key, value, None)

        def set_(instance, value, oldvalue, initiator):
            instance.receive_change_event("set", key, value, oldvalue)

        event.listen(inst, "append", append)
        event.listen(inst, "remove", remove)
        event.listen(inst, "set", set_)

    if __name__ == "__main__":

        class Base:
            def receive_change_event(self, verb, key, value, oldvalue):
                s = "Value '%s' %s on attribute '%s', " % (value, verb, key)
                if oldvalue:
                    s += "which replaced the value '%s', " % oldvalue
                s += "on object %s" % self
                print(s)

        Base = declarative_base(cls=Base)

        event.listen(Base, "attribute_instrument", configure_listener)

        class MyMappedClass(Base):
            __tablename__ = "mytable"

            id = Column(Integer, primary_key=True)
            data = Column(String(50))
            related_id = Column(Integer, ForeignKey("related.id"))
            related = relationship("Related", backref="mapped")

            def __str__(self):
                return "MyMappedClass(data=%r)" % self.data

        class Related(Base):
            __tablename__ = "related"

            id = Column(Integer, primary_key=True)
            data = Column(String(50))

            def __str__(self):
                return "Related(data=%r)" % self.data

        # classes are instrumented.  Demonstrate the events !

        m1 = MyMappedClass(data="m1", related=Related(data="r1"))
        m1.data = "m1mod"
        m1.related.mapped.append(MyMappedClass(data="m2"))
        del m1.data