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Sarj Standards

Python · correctness

prefer-self-type-annotation

python:prefer-self-type-annotation

Annotate fluent methods and alternate constructors with `Self`.

Code
SARJ078
Default
error
Fix
none
Languages
python

Why

`Self` preserves the concrete subclass type, while naming the enclosing class narrows inherited return types incorrectly.

Fix

Import `Self` from `typing` and use it as the return annotation.

Before / after

Executed by this rule’s unit tests.

Before

Fluent method names its enclosing class

app/builder.py · focus
class Builder:
    def set_name(self, name: str) -> "Builder":
        self.name = name
        return self

After

Fluent method preserves subclass type

app/builder.py · focus
from typing import Self

class Builder:
    def set_name(self, name: str) -> Self:
        self.name = name
        return self

Limits

  • Only methods that directly return `self` or classmethods that directly return `cls(...)` are analyzed.
  • Annotations naming other classes and methods without a return annotation are excluded.