Useless Overload Body - Ruff

What it does

Checks for various @overload-decorated functions that have non-stub bodies.

Why is this bad?

Functions decorated with @overload are ignored at runtime; they are overridden by the implementation function that follows the series of overloads. While it is not illegal to provide a body for an @overload-decorated function, it may indicate a misunderstanding of how the @overload decorator works.

Example

toml

[environment]
python-version = "3.11"

from typing import overload


@overload
def foo(x: int) -> int:
    # will never be executed
    return x + 1  # error


@overload
def foo(x: str) -> str:
    # will never be executed
    return "Oh no, got a string"  # error


def foo(x: int | str) -> int | str:
    raise Exception("unexpected type encountered")

Use instead:

from typing import assert_never, overload


@overload
def foo(x: int) -> int: ...


@overload
def foo(x: str) -> str: ...


def foo(x: int | str) -> int | str:
    if isinstance(x, int):
        return x + 1
    elif isinstance(x, str):
        return "Oh no, got a string"
    else:
        assert_never(x)

References

Python documentation: @overload