--to-ref: how to indicate worktree/index as reference?

[rogalski]

describe your issue

I am currently experimenting with diff-based pre-commit checks.
The general idea is to run some tools on commit, but filter out its' outputs based on diff between HEAD and index.
You can consider flake --diff as good real-life example.

At the same time, I am trying to expose easy reproduction steps for people who (for any reason) were committing without hook installed or with --no-verify enabled. Taking a look at docs, I think that proper approach is to use --from-ref, --to-ref. With GitHub PR flow, from-ref would take value of git merge-base HEAD TARGET_BRANCH and to-ref would just be HEAD. This is exactly what docs recommends, so far so good.

The problem I see is that if some files are staged but not committed, I can have a discrepancy between file system (which is where all linters actually operate) and diff data (which is used to build affected file list and then to postprocess linter outcome).

Of course I can add arbitrary limitation that no delta between --to-ref and index is allowed when --from-ref and --to-ref is used. At the same time, somehow exposing --to-ref=git_index seems to be a desirable feature, consistent with how static analysis tools actually operate on checked-out codebase. Currently this is not possible, as --to-ref has to be a valid revision.

I am most likely missing something important in my understanding of git semantics or in how hooks generalize outside of pre-commit hook. Yet, I still want to discuss possible approaches.

pre-commit --version

pre-commit 2.15.0

.pre-commit-config.yaml

repos:
  - repo: local
    hooks:
      - id: env
        name: env
        entry: env
        language: system
        pass_filenames: false
        verbose: true

~/.cache/pre-commit/pre-commit.log (if present)

No response

[asottile]

You can consider flake --diff as good real-life example.

flake8 --diff is broken by design -- I would not recommend emulating it

pre-commit intentionally operates on a per-file granularity. I would not recommend trying to build diff-based linters unless you're operating specifically on a per-file basis (which is already what pre-commit does)