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implicit-optional (RUF013)

Fix is sometimes available.

What it does

Checks for the use of implicit Optional in type annotations when the default parameter value is None.

Why is this bad?

Implicit Optional is prohibited by PEP 484. It is confusing and inconsistent with the rest of the type system.

It's recommended to use Optional[T] instead. For Python 3.10 and later, you can also use T | None.

Example

def foo(arg: int = None):
    pass

Use instead:

from typing import Optional


def foo(arg: Optional[int] = None):
    pass

Or, for Python 3.10 and later:

def foo(arg: int | None = None):
    pass

If you want to use the | operator in Python 3.9 and earlier, you can use future imports:

from __future__ import annotations


def foo(arg: int | None = None):
    pass

Limitations

Type aliases are not supported and could result in false negatives. For example, the following code will not be flagged:

Text = str | bytes


def foo(arg: Text = None):
    pass

Options

Fix safety

This fix is always marked as unsafe because it can change the behavior of code that relies on type hints, and it assumes the default value is always appropriate—which might not be the case.