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non-self-return-type (PYI034)

Added in v0.0.271 · Related issues · View source

Derived from the flake8-pyi linter.

Fix is sometimes available.

What it does

Checks for methods that are annotated with a fixed return type which should instead be returning Self.

Why is this bad?

If methods that generally return self at runtime are annotated with a fixed return type, and the class is subclassed, type checkers will not be able to infer the correct return type.

For example:

class Shape:
    def set_scale(self, scale: float) -> Shape:
        self.scale = scale
        return self

class Circle(Shape):
    def set_radius(self, radius: float) -> Circle:
        self.radius = radius
        return self

# Type checker infers return type as `Shape`, not `Circle`.
Circle().set_scale(0.5)

# Thus, this expression is invalid, as `Shape` has no attribute `set_radius`.
Circle().set_scale(0.5).set_radius(2.7)

Specifically, this check enforces that the return type of the following methods is Self:

  1. In-place binary-operation dunder methods, like __iadd__, __imul__, etc.
  2. __new__, __enter__, and __aenter__, if those methods return the class name.
  3. __iter__ methods that return Iterator, despite the class inheriting directly from Iterator.
  4. __aiter__ methods that return AsyncIterator, despite the class inheriting directly from AsyncIterator.

Example

class Foo:
    def __new__(cls, *args: Any, **kwargs: Any) -> Foo: ...
    def __enter__(self) -> Foo: ...
    async def __aenter__(self) -> Foo: ...
    def __iadd__(self, other: Foo) -> Foo: ...

Use instead:

from typing_extensions import Self

class Foo:
    def __new__(cls, *args: Any, **kwargs: Any) -> Self: ...
    def __enter__(self) -> Self: ...
    async def __aenter__(self) -> Self: ...
    def __iadd__(self, other: Foo) -> Self: ...

Fix safety

This rule's fix is marked as unsafe as it changes the meaning of your type annotations.

Availability

Because this rule relies on the third-party typing_extensions module for Python versions before 3.11, its diagnostic will not be emitted, and no fix will be offered, if typing_extensions imports have been disabled by the lint.typing-extensions linter option.

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References