BSK-E0078 error

Self type violations in generics

This rule detects two kinds of Self type violations:

1. **Return type mismatch**: A method (or classmethod) annotated -> Self returns a concrete class constructor call (e.g. return Shape()) instead of self, cls(), or another Self-compatible expression. In a subclass, Self resolves to the subclass type, so returning the parent class constructor is a type error.

2. **Self is not subscriptable**: Self cannot be parameterized (e.g. Selfint). It already captures the full generic specialization of the enclosing class.

from typing import Self

class Shape:
    def method2(self) -> Self:
        return Shape()  # E — should return self, not Shape()

    @classmethod
    def cls_method2(cls) -> Self:
        return Shape()  # E — should return cls(), not Shape()

class Container(Generic[T]):
    def foo(self, other: Self[int]) -> None:  # E — Self is not subscriptable
        pass

How to handle it

Every rule is on by default — strict is the default, not a cage. You can dial BSK-E0078 down per-file or per-path from your editor or pyproject.toml, or fix the code so it type-checks. See the Type System rules and the complete diagnostic reference.

Canonical URL: https://www.basilisk-python.dev/errors/BSK-E0078