The sqlalchemy hybrid_property can not be set during initialization, invalid keyword argument?

Question

I am faced with an issue where I am trying to convert a string (in db) to a list of decimals.

My code: `

@hybrid_property
def decimals(self) -> list[Decimal]:
    return [Decimal(dec) for dec in self._decimals.replace('d', '').split(';')]


@decimals.setter
def decimals(self, decimals):
    self._decimals = ';'.join(str(dec) for dec in decimals)

`

I get an error 'decimals' is an invalid keyword argument

Stacktrace:

self = <app.models.models.Test_Db object at 0x000001F2A5EAA2F0>
kwargs = {'id': 0, 'connection': 'D', 'fron': 'loop feed', 'hasRail': False, ...}
cls_ = <class 'app.models.models.Test_Db'>, k = 'decimals'

    def _declarative_constructor(self, **kwargs):
        """A simple constructor that allows initialization from kwargs.
    
        Sets attributes on the constructed instance using the names and
        values in ``kwargs``.
    
        Only keys that are present as
        attributes of the instance's class are allowed. These could be,
        for example, any mapped columns or relationships.
        """
        cls_ = type(self)
        for k in kwargs:
            if not hasattr(cls_, k):
>               raise TypeError(
                    "%r is an invalid keyword argument for %s" % (k, cls_.__name__)
                )
E               TypeError: 'decimals' is an invalid keyword argument for Test_Db

..\env\lib\site-packages\sqlalchemy\orm\decl_base.py:1154: TypeError

Answer 1

What you're trying to acheive is functionaly similar to this:

class Bar:
    def __init__(self, _a):
        self._a = _a

    @property
    def a(self):
        return self._a

    @a.setter
    def a(self, a):
        self._a = a

bar = Bar(a=1)  # TypeError: Bar.__init__() got an unexpected keyword argument 'a'

The __init__ function of Bar knows nothing about a, only _a.

Same thing happens when you're trying to __init__ your SQLAlchemy mapped class with a property rather than the mapped attribute.

Demo:

from decimal import Decimal
from sqlalchemy import Column, Integer, String, create_engine, select
from sqlalchemy.ext.hybrid import hybrid_property
from sqlalchemy.orm import Session, declarative_base

Base = declarative_base()

class Foo(Base):
    __tablename__ = "foo"
    id = Column(Integer, primary_key=True)
    _decimals = Column(String, nullable=False)

    @hybrid_property
    def decimals(self) -> list[Decimal]:
        return [
            Decimal(dec) for dec in self._decimals.replace("d", "").split(";")
        ]

    @decimals.setter
    def decimals(self, decimals: list[Decimal]) -> None:
        self._decimals = ";".join(str(d) for d in decimals)


engine = create_engine("sqlite://", future=True, echo=True)

Base.metadata.create_all(engine)

with Session(bind=engine) as session:
    session.add(Foo(_decimals="d1.0;d2.0;d3.0"))
    session.commit()  # OK

with Session(bind=engine) as session:
    session.add(Foo(decimals=[Decimal("4.0"),Decimal("5.0"),Decimal("6.0"))
    session.commit()  # TypeError

with Session(bind=engine) as session:
    result = session.scalars(select(Foo)).first()
    print(result.decimals)  # [Decimal('1.0'), Decimal('2.0'), Decimal('3.0')]