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any-eq-ne-annotation (PYI032)

Added in v0.0.271 · Related issues · View source

Derived from the flake8-pyi linter.

Fix is always available.

What it does

Checks for __eq__ and __ne__ implementations that use typing.Any as the type annotation for their second parameter.

Why is this bad?

The Python documentation recommends the use of object to "indicate that a value could be any type in a typesafe manner". Any, on the other hand, should be seen as an "escape hatch when you need to mix dynamically and statically typed code". Since using Any allows you to write highly unsafe code, you should generally only use Any when the semantics of your code would otherwise be inexpressible to the type checker.

The expectation in Python is that a comparison of two arbitrary objects using == or != should never raise an exception. This contract can be fully expressed in the type system and does not involve requesting unsound behaviour from a type checker. As such, object is a more appropriate annotation than Any for the second parameter of the methods implementing these comparison operators -- __eq__ and __ne__.

Example

from typing import Any

class Foo:
    def __eq__(self, obj: Any) -> bool: ...

Use instead:

class Foo:
    def __eq__(self, obj: object) -> bool: ...

References