SQLAlchemy columns with a hybrid property boolean flag to check for presence (non-nullness)

column_flag.py

from sqlalchemy import (
    Column,
    Integer,
    DateTime,
    Text,
    create_engine,
    func)
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.inspection import inspect
from sqlalchemy.orm import Session

Base = declarative_base()


def column_flag(column, **options):
    def target_key(instance):
        target = inspect(instance).mapper.get_property_by_column(column)
        return target.key

    def fget(self):
        return getattr(self, target_key(self)) is not None

    def fset(self, value, default=options.get('default')):
        if not isinstance(value, bool):
            raise TypeError('Flag only accepts boolean values')
        return setattr(self, target_key(self), default if value else None)

    def expr(cls):
        return column.isnot(None)

    return hybrid_property(
        fget=fget,
        fset=fset if 'default' in options else None,
        expr=expr)


class Article(Base):
    __tablename__ = 'article'

    id = Column(Integer, primary_key=True)
    content = Column(Text)
    published_at = Column('publication_date', DateTime)
    is_published = column_flag(published_at, default=func.now())


def main():
    engine = create_engine('sqlite://', echo=True)
    Base.metadata.create_all(bind=engine)

    session = Session(bind=engine)
    art1 = Article(content='First post', published_at=func.now())
    art2 = Article(content='Tentative content')
    session.add_all([art1, art2])
    session.flush()
    count_total = session.query(Article).count()
    count_published = session.query(Article)\
        .filter(Article.is_published)\
        .count()
    print(f'Articles published out of total: {count_published}/{count_total}')

    assert art1.is_published
    assert not art2.is_published

    # Publish article 2
    art2.is_published = True
    session.flush()
    print(f'Article 2 was published at {art2.published_at}')

    # Unpublish article 2
    art2.is_published = False
    session.flush()
    print(f'Article 2 publication date: {art2.published_at}')


if __name__ == '__main__':
    main()