Type-agnostic configuration-table in sqlalchemy

Question

I'm building my first bigger database-backend with sqlalchemy and need a table to store configurations (key-value pairs). Keys are strings, values can be quite different, and I'd like to keep the variable-type. This is what I came up with:

class Configuration(Base):
 __tablename__ = "configuration"
 def __init__(self, *args, **kw):
 if len(args) > 0:
 kw['name'] = args[0]
 if len(args) > 1:
 kw['value'] = args[1] 
 try:
 json.loads(kw["value"])
 except Exception:
 try:
 kw["_value"] = json.dumps(kw["value"])
 except KeyError:
 pass
 super().__init__(*args, **kw)
 id: Mapped[int] = mapped_column(primary_key=True)
 name: Mapped[str] = mapped_column(String, CheckConstraint("name <> ''", name="check_name_not_empty"), unique=True)
 _value: Mapped[str] = mapped_column(String, CheckConstraint("_value <> ''", name="check_value_not_empty"))
 @property
 def value(self):
 return json.loads(self._value)
 @value.setter
 def value(self, new_value):
 self._value = json.dumps(new_value)

The basic idea is to have a column '_value' that stores the value as a json-string but the user only iteracts with the value-propery so the conversion into the json-format is hidden (as long as the objects can be converted).

The init is not so nice but I wanted to support args and keyword-args.

Answer 1

docstrings

ctor

The __init__ constructor is entirely too automagical. Please write at least one sentence describing what it does, and three doctests illustrating the three different ways to invoke it.

class

It's unclear what class invariant you're trying to preserve. There is a relationship between _value and value. You really need to explicitly state what it is, and offer a motivating example. Maybe you intend for it to typically be a dict[str, Any]? Say so.

RDBMS

columns

My understanding of this is SELECT * shall offer a _value column and no value column. I imagine that Author's Intent is there shall be exactly three columns. But the ctor admits of many additional columns being passed in, and there is no docstring guidance on what the caller should or should not do.

Primary Key

Given that name is UNIQUE, it's unclear what value add the synthetic id PK is offering. If there's some design consideration supporting it, please write that down explicitly.