Agent skill

ty-skills

Python type checking expertise using ty - the extremely fast type checker by Astral. Use when: (1) Adding type annotations to Python code, (2) Fixing type errors reported by ty, (3) Migrating from mypy/pyright to ty, (4) Configuring ty for projects, (5) Understanding advanced type patterns (generics, protocols, intersection types), (6) Setting up ty in editors (VS Code, Cursor, Neovim, PyCharm).

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npx add-skill https://github.com/jiatastic/open-python-skills/tree/main/skills/ty-skills

SKILL.md

ty-skills

Master Python type checking with ty - the extremely fast type checker written in Rust by Astral (creators of uv and Ruff).

When to Use This Skill

  • Adding type annotations to Python code
  • Fixing type errors and diagnostics from ty
  • Configuring ty rules and severity levels
  • Migrating from mypy or pyright to ty
  • Understanding advanced type patterns (intersection types, protocols, generics)
  • Setting up ty language server in your editor

Quick Start

bash
# Install
uv tool install ty
# or: pip install ty

# Check current directory
ty check

# Check specific files
ty check src/

# Full diagnostics
ty check --output-format full

Configuration

Configure via pyproject.toml:

toml
[tool.ty.environment]
python-version = "3.12"
python = "./.venv"
python-platform = "linux"
root = ["./src"]
extra-paths = ["./typings"]

[tool.ty.rules]
# error: fail CI, warn: report, ignore: disable
possibly-unresolved-reference = "error"
invalid-argument-type = "error"
division-by-zero = "warn"
unused-ignore-comment = "warn"

[tool.ty.src]
include = ["src", "tests"]
exclude = ["src/migrations/"]

# Per-file overrides
[[tool.ty.overrides]]
include = ["tests/**"]

[tool.ty.overrides.rules]
possibly-unresolved-reference = "warn"

Rules Quick Reference

Rule Default Description
possibly-unresolved-reference error Variable might not be defined
invalid-argument-type error Argument type mismatch
incompatible-assignment error Assigned value incompatible
missing-argument error Required argument missing
unsupported-operator error Operator not supported for types
invalid-return-type error Return type mismatch
division-by-zero warn Potential division by zero
unused-ignore-comment warn Suppression not needed
redundant-cast warn Cast has no effect
possibly-unbound-attribute warn Attribute might not exist
index-out-of-bounds warn Index might be out of range

Intersection Types (ty Exclusive)

ty has first-class intersection type support:

python
def output_as_json(obj: Serializable) -> str:
    if isinstance(obj, Versioned):
        reveal_type(obj)  # reveals: Serializable & Versioned
        return str({
            "data": obj.serialize_json(),  # From Serializable
            "version": obj.version          # From Versioned
        })
    return obj.serialize_json()

Suppression Comments

python
# Suppress single rule
x: int = "hello"  # type: ignore[incompatible-assignment]

# Suppress multiple
y = risky()  # type: ignore[possibly-unresolved-reference, invalid-argument-type]

Reference Documents

For detailed information, see:

Document Content
references/ty_rules_reference.md All rules with examples and fixes
references/typing_cheatsheet.md Python typing module quick reference
references/advanced_patterns.md Protocols, generics, type guards, variance
references/migration_guide.md mypy/pyright → ty migration
references/common_errors.md Error solutions with examples
references/editor_setup/ VS Code, Cursor, Neovim setup

Resources

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