PY-CLAS0006 · pass_through_inheritance_layer_count

Count project-owned inheritance layers that add only a name or forwarding frame.

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

Definition

Resolve top-level project classes and their direct bases across relative and absolute imports. Report an undecorated single-inheritance subclass when its body is only pass or an ellipsis, or when every body member is an ordinary method that returns the same-named zero-argument super() method with every positional, variadic, keyword-only, and keyword variadic argument unchanged. The existing closed-world single-subclass-base rule owns a pair when the base itself can be removed, so this rule abstains from that exact overlap.

Evidence

Each finding identifies the fully qualified project base, layer kind, complete child range, and every transparently forwarded method. The result counts shallow child layers rather than base classes. The value is the number of shallow child layers rather than the number of bases beneath them.

Exceptions

Decorated classes, class keywords, multiple or external bases, changed arguments, transformed returns, asynchronous adapters, descriptors, class methods, and static methods are excluded. A name ending in one of the contract_suffixes is an intentional type contract, which by default covers a protocol, a port, a mixin, a plugin, a strategy, a backend, a provider, a registry, and an error. A body holding only a docstring is a class that stated why it exists, so it is not a layer nobody meant to add. Keep an otherwise empty class when runtime registration or external consumers rely on its identity and disable this preference for that path.

Examples

Bad

class JsonSerializer(Serializer):
    pass

class NamedParser(Parser):
    def parse(self, text: str) -> Node:
        return super().parse(text)

Good

class JsonSerializer(Serializer):
    def encode(self, value: JsonValue) -> bytes:
        return json.dumps(value).encode()

class StoragePlugin(Protocol):
    def store(self, payload: bytes) -> None: ...

References

• Generalizes Pylint R0901 too-many-ancestors
• Generalizes Pylint W0246 useless-parent-delegation
• Cites “The Python Language Reference”, custom classes. Open reference
• Cites “PEP 544, Protocols”. Open reference
• Cites “A Philosophy of Software Design”, chapters 4 and 7