Mypy To Ty

/migration-patterns:mypy-to-ty

Migrate from mypy to ty — Astral's new fast Python type checker. Replaces the mirrors-mypy pre-commit hook with a local repo: local hook using uvx ty check, migrates config from [tool.mypy] to [tool.ty], and removes mypy.ini.

When to Use This Skill

Context

• Pre-commit config: !find . -maxdepth 1 -name '.pre-commit-config.yaml'
• pyproject.toml: !find . -maxdepth 1 -name 'pyproject.toml'
• mypy.ini: !find . -maxdepth 1 -name 'mypy.ini'
• uv available: !find . -maxdepth 1 -name 'uv.lock'

Parameters

Execution

Execute this mypy-to-ty migration:

Step 1: Audit current mypy configuration

1. Read .pre-commit-config.yaml and locate any mirrors-mypy or mypy entries.
2. Read pyproject.toml and extract the [tool.mypy] section.
3. Check for mypy.ini.
4. Report findings:

   mypy → ty migration audit
   ==========================
   pre-commit hook:   mirrors-mypy found at rev <rev>
   pyproject.toml:    [tool.mypy] section found (N lines)
   mypy.ini:          FOUND / NOT FOUND
   uv.lock:           EXISTS (uv available) / MISSING
   

   If --check-only, stop here.

Step 2: Update .pre-commit-config.yaml

Remove the mirrors-mypy repo block. Add a repo: local hook for ty:

- repo: local
  hooks:
    - id: ty
      name: ty type check
      entry: uvx ty check
      language: system
      types: [python]
      pass_filenames: false

Note: astral-sh/ty-pre-commit does not yet exist as a hosted pre-commit repo. The repo: local pattern with uvx ty check is the correct approach until an official mirror is published.

If an entry: mypy hook in a non-mirrors-mypy repo exists, replace it similarly.

Step 3: Migrate [tool.mypy] → [tool.ty] in pyproject.toml

Read the [tool.mypy] section and convert each key to the ty equivalent:

Remove the [tool.mypy] section (and any [[tool.mypy.overrides]] sections). Add [tool.ty] with converted keys. Annotate any options that need manual review.

Step 4: Remove mypy.ini

If mypy.ini exists, delete it. Its content should already be in [tool.ty] after Step 3.

Step 5: Remove mypy from dependencies

If mypy appears in pyproject.toml dev/lint dependencies, remove it:

# With uv
uvx --from uv uv remove --dev mypy 2>/dev/null || true

If not using uv, note it for manual removal.

Step 6: Verify migration

Run the new hook to confirm it works:

pre-commit run ty --all-files

If it exits 0, the migration is complete. If it exits non-zero:

• Summarize ty errors
• Suggest addressing them before committing

Step 7: Report

Print a summary of all changes made and list files to commit:

mypy → ty migration complete
=============================
.pre-commit-config.yaml   UPDATED (removed mirrors-mypy, added local ty hook)
pyproject.toml            UPDATED ([tool.mypy] → [tool.ty])
mypy.ini                  DELETED / NOT FOUND

Manual review needed:
  - [[tool.mypy.overrides]] sections were dropped — verify ty handles them
  - Check ty docs for any unsupported mypy options

Files to stage:
  git add .pre-commit-config.yaml pyproject.toml

Agentic Optimizations

Quick Reference

See Also

• /migration-patterns:black-to-ruff-format — Migrate from black to ruff format
• /migration-patterns:flake8-to-ruff — Migrate flake8/isort to ruff
• /configure:repo — End-to-end repo config driver