polylith-migrate-convert-type-checker

Development

[Internal sub-skill of `polylith-migrate-orchestrator` (optional, runs only when opted in during `polylith-migrate-discover`). Do not load directly — load `polylith-migrate-orchestrator` first.] Align the project's type checker with the **workspace's** configured tool (whatever it is — mypy, pyright, ty, etc.). Removes project-specific config and consolidates settings into the workspace root.

QUICK START

How to use this skill

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Prompt to paste

I want to install this Agent Skill for this project in Codex.

Source SKILL.md: https://github.com/DavidVujic/python-polylith/blob/HEAD/.agents/skills/polylith/migrate-project/polylith-migrate-convert-type-checker/SKILL.md

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Skill: polylith-migrate-convert-type-checker

Goal

Align the project with the workspace's type checker. This skill is not opinionated about ty — it reads the workspace's configured tool from the root pyproject.toml and aligns the project to that.

When to Skip

Skip this step if the project's TYPE_CHECKER in migration/<PROJECT>/state.md already matches the workspace's type-checking tool.

Inputs

From migration/<PROJECT>/state.md:

PROJECT_DIR
TYPE_CHECKER
RUN_TEST_CMD (optional: RUN_LINT_CMD, RUN_TYPECHECK_CMD)

All inputs from state.md are assumed to satisfy the validation rules in polylith-migrate-discover (### Validation rules). Validate before proceeding.

Steps

0. Identify the workspace's tool

Open the workspace root pyproject.toml and identify the configured type checker using the same detection table as polylith-migrate-discover ([tool.mypy] or mypy.ini → mypy, [tool.pyright] or pyrightconfig.json → pyright, [tool.ty] → ty). Record what you found — every step below refers to "the workspace's type checker" and means this tool, not necessarily ty.

If the workspace has no type checker configured at all, stop and ask the user how to proceed (introduce one? skip type-check alignment? abort the conversion?).

1. Remove Old Configs and Dependencies

Remove the following sections from pyproject.toml:
- [tool.mypy], [[tool.mypy.overrides]], [mypy-*], [tool.pyright], [tool.pytype]
Remove standalone config files: mypy.ini, .mypy.ini, pyrightconfig.json.
Remove old type checker dependencies from pyproject.toml:
- mypy, mypy-extensions, types-* stub packages, pyright, pytype, sqlalchemy-stubs, django-stubs.

2. Clean Up Type Ignore Comments

Remove # type: ignore[<mypy-code>] comments that reference mypy-specific error codes.
Leave comments that suppress real issues and document them in state.md.

3. Adopt Workspace Type-Checking Config

The workspace root pyproject.toml may define the type-checking configuration. The project inherits this config.
Add project-specific overrides if needed.

4. Run the Workspace's Type-Checking Tool

Run the workspace's type-checking tool to assess errors:
- Same or fewer errors: No action needed.
- New errors: Ask the user whether to fix, suppress, or adjust the config.
- Missing stub errors: Silence with per-module ignores in the workspace's type-checking config.

5. Update `state.md`

Set TYPE_CHECKER to the workspace's type-checking tool.
Update RUN_TYPECHECK_CMD to use the workspace's type-checking command.

Verify

The workspace's type-checking tool runs cleanly (or remaining errors are user-approved).
RUN_TEST_CMD succeeds.
If set, RUN_LINT_CMD succeeds.

Common failure modes

Symptom	Likely cause	Remediation
The workspace's checker surfaces type errors the old checker didn't	Stricter type rules expose real bugs that were always there.	Fix the bugs now where cheap (small handful). For systemic gaps, suppress per-module in the workspace's checker config and document the debt in `state.md`. Do not blanket-ignore at file scope.
Stub packages (`types-*`, `django-stubs`, `sqlalchemy-stubs`) were required by the old checker but the new one bundles its own	Dependency carryover.	Remove the stub packages from `pyproject.toml`. Re-run the workspace's checker to confirm it picks up its own stubs.
`# type: ignore[<old-code>]` comments still reference the old checker's error codes	Step 2 missed some.	Strip the `[<code>]` bracket. If the suppression is still needed, leave a bare `# type: ignore` with a comment explaining why; otherwise remove the suppression entirely.

Done When

No old type checker config files remain.
Old type checker dependencies are removed from pyproject.toml.
Stale # type: ignore comments referencing tool-specific codes are removed (or documented if intentionally kept).
TYPE_CHECKER in migration/<PROJECT>/state.md matches the workspace's type-checking tool.
The workspace's type-checking tool runs cleanly or known issues are documented in state.md.
Tests pass via the workspace's tooling.

Commit

After verification passes, commit this phase to the migration branch:

git add -A && git commit -m "migrate(<PROJECT>): phase optional — convert-type-checker"

Substitute <PROJECT> from state.md. This is an optional skill off the numbered main line, so the commit uses the literal phase optional label (no <N>). Do not proceed without a clean commit — the per-phase commit is the rollback point for the next phase's failure-mode tables.