Release: 2.0.49 | Release Date: April 3, 2026

# SQLAlchemy 2.0 Documentation

# Source code for examples.custom_attributes.active_column_defaults

    """Illustrates use of the :meth:`.AttributeEvents.init_scalar`
    event, in conjunction with Core column defaults to provide
    ORM objects that automatically produce the default value
    when an un-set attribute is accessed.

    """

    import datetime

    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import DateTime
    from sqlalchemy import event
    from sqlalchemy import Integer
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import Session

    def configure_listener(mapper, class_):
        """Establish attribute setters for every default-holding column on the
        given mapper."""

        # iterate through ColumnProperty objects
        for col_attr in mapper.column_attrs:
            # look at the Column mapped by the ColumnProperty
            # (we look at the first column in the less common case
            # of a property mapped to multiple columns at once)
            column = col_attr.columns[0]

            # if the Column has a "default", set up a listener
            if column.default is not None:
                default_listener(col_attr, column.default)

    def default_listener(col_attr, default):
        """Establish a default-setting listener.

        Given a class attribute and a :class:`.DefaultGenerator` instance.
        The default generator should be a :class:`.ColumnDefault` object with a
        plain Python value or callable default; otherwise, the appropriate behavior
        for SQL functions and defaults should be determined here by the
        user integrating this feature.

        """

        @event.listens_for(col_attr, "init_scalar", retval=True, propagate=True)
        def init_scalar(target, value, dict_):
            if default.is_callable:
                # the callable of ColumnDefault always accepts a context
                # argument; we can pass it as None here.
                value = default.arg(None)
            elif default.is_scalar:
                value = default.arg
            else:
                # default is a Sequence, a SQL expression, server
                # side default generator, or other non-Python-evaluable
                # object.  The feature here can't easily support this.   This
                # can be made to return None, rather than raising,
                # or can procure a connection from an Engine
                # or Session and actually run the SQL, if desired.
                raise NotImplementedError(
                    "Can't invoke pre-default for a SQL-level column default"
                )

            # set the value in the given dict_; this won't emit any further
            # attribute set events or create attribute "history", but the value
            # will be used in the INSERT statement
            dict_[col_attr.key] = value

            # return the value as well
            return value

    if __name__ == "__main__":
        Base = declarative_base()

        event.listen(Base, "mapper_configured", configure_listener, propagate=True)

        class Widget(Base):
            __tablename__ = "widget"

            id = Column(Integer, primary_key=True)

            radius = Column(Integer, default=30)
            timestamp = Column(DateTime, default=datetime.datetime.now)

        e = create_engine("sqlite://", echo=True)
        Base.metadata.create_all(e)

        w1 = Widget()

        # not persisted at all, default values are present the moment
        # we access them
        assert w1.radius == 30

        # this line will invoke the datetime.now() function, and establish
        # its return value upon the w1 instance, such that the
        # Column-level default for the "timestamp" column will no longer fire
        # off.
        current_time = w1.timestamp
        assert current_time > datetime.datetime.now() - datetime.timedelta(
            seconds=5
        )

        # persist
        sess = Session(e)
        sess.add(w1)
        sess.commit()

        # data is persisted.  The timestamp is also the one we generated above;
        # e.g. the default wasn't re-invoked later.
        assert sess.query(Widget.radius, Widget.timestamp).first() == (
            30,
            current_time,
        )