PY-PYDA0006 · constructor_model_candidate

Recommend a model for constructor-heavy validated data classes.

This is a deterministic rule for python. Read its implementation.

Definition

Inspect undecorated classes with no base class and exactly one synchronous __init__. Report a class when the constructor has at least the configured number of fixed parameters, stores those parameters on self, validates parameters through an assertion or a conditional ValueError or TypeError, and supplies signature or expression-level defaults. The class must otherwise contain only data identity methods. These combined facts distinguish a manual data schema from a merely long constructor.

Evidence

Each finding records the class range and exact stored, validated, and defaulted parameter names. Measurements expose all four thresholds so a project can tune its recommendation. The value is the number of plain classes clearing all four floors together.

Exceptions

Existing dataclasses, Pydantic and Patos models, inherited framework classes, decorated classes, variadic constructors, and classes with behavioral methods are excluded. Constructors whose parameter names or annotations explicitly denote clients, services, repositories, factories, callbacks, loggers, sessions, transports, or other dependency-injection roles are also excluded. Constructors that visibly acquire files, sockets, locks, queues, pools, sessions, or connections are resource owners. Classes with close, context-manager, connection, or execution methods remain behavioral classes rather than data models. minimum_parameters, minimum_attributes, minimum_validations, and minimum_defaults are the four floors a class has to clear together, which is what separates a manual data schema from a merely long constructor.

Examples

Bad

class AccountInput:
    def __init__(self, name: str, age: int, locale: str = "en") -> None:
        if not name:
            raise ValueError("name is required")
        self.name = name
        self.age = age
        self.locale = locale

Good

class AccountInput(FrozenModel):
    name: NonEmptyName
    age: NonNegativeInt
    locale: str = "en"

class RepositorySession:
    def __init__(self, client: DatabaseClient) -> None:
        self.client = client

    def close(self) -> None:
        self.client.close()

References

• Cites “Pydantic documentation”, models. Open reference
• Cites “Pydantic documentation”, fields and constraints. Open reference
• Cites “The Python Standard Library”, dataclasses. Open reference
• Cites “The Python Standard Library”, typing.Protocol. Open reference