Overriding one sub-setting nulls out other inherited settings

[hotpxl]

Problem description

I'm using extend to extend the base configuration from a parent directory, then override isort.required-imports. But as soon as I override isort.required-imports, other settings under isort section from the parent configuration are lost. I was expecting to only override the sub-setting I specified and inherit the rest.

For reproducing the issue, I have the following tree:

root/
  .ruff.toml
  subdir/
    .ruff.toml
    a.py

root/.ruff.toml has

select = ["I"]

[isort]
required-imports = ["from __future__ import annotations"]
forced-separate = ["my_utils"]

root/subdir/.ruff.toml has

extend = "../.ruff.toml"

[isort]
required-imports = []

root/subdir/a.py has

import my_utils
import other_utils

my_utils.a()
other_utils.b()

When I run ruff check --fix **/**, ruff does not split my_utils and other_utils imports, unless I also add forced-separate = ["my_utils"] to root/subdir/.ruff.toml.

(I can also verify this behavior using --show-settings)

[charliermarsh]

I agree -- we should be overriding on a per-key basis, not a per-section basis.