runtime-cast-value (TC006)

Derived from the flake8-type-checking linter.

Fix is always available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for unquoted type expressions in typing.cast() calls.

Why is this bad?

This rule helps enforce a consistent style across your codebase.

It's often necessary to quote the first argument passed to cast(), as type expressions can involve forward references, or references to symbols which are only imported in typing.TYPE_CHECKING blocks. This can lead to a visual inconsistency across different cast() calls, where some type expressions are quoted but others are not. By enabling this rule, you ensure that all type expressions passed to cast() are quoted, enforcing stylistic consistency across all of your cast() calls.

In some cases where cast() is used in a hot loop, this rule may also help avoid overhead from repeatedly evaluating complex type expressions at runtime.

Example

from typing import cast

x = cast(dict[str, int], foo)

Use instead:

from typing import cast

x = cast("dict[str, int]", foo)

Fix safety

This fix is safe as long as the type expression doesn't span multiple lines and includes comments on any of the lines apart from the last one.