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.

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.

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

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