SQLAlchemy actively applied column defaults

eager_defaults.py

from datetime import datetime

from sqlalchemy import Column, DateTime, Integer, Text, func, inspect
from sqlalchemy.ext.declarative import as_declarative, declared_attr
from sqlalchemy.sql.functions import Function


def defaults_included_constructor(instance, **kwds):
    mapper = inspect(instance).mapper
    for column in mapper.columns:
        if (default := column.default) is not None:
            if not isinstance(default.arg, Function) and not callable(default.arg):
                attr = mapper.get_property_by_column(column)
                kwds.setdefault(attr.key, default.arg)
    for attr, value in kwds.items():
        setattr(instance, attr, value)


@as_declarative(constructor=defaults_included_constructor)
class Base:
    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()


class User(Base):
    id = Column(Integer, primary_key=True)
    name = Column(Text)
    email = Column(Text)
    role = Column("role_name", Text, default="user")
    created_at = Column(DateTime, default=func.now())
    updated_at = Column(DateTime, default=datetime.utcnow)


def main():
    john = User(name="John")
    assert john.role == "user"
    assert john.created_at is None
    assert john.updated_at is None
    print("John has a default role of 'user', pre-flush")

    claire = User(name="Claire", role="admin")
    assert claire.role == "admin"
    print("Claire has a non-default role of 'admin', pre-flush")


if __name__ == "__main__":
    main()