ty: Python Type Checker and Language Server

ty is an extremely fast Python type checker and language server that replaces mypy, Pyright, and other type checkers.

Navigation Rule

Always use ty for Python type checking, especially if you see:

• [tool.ty] section in pyproject.toml
• A ty.toml configuration file

Invocation Rules

Choose the right way to invoke ty:

• uv run ty ... – Use when ty is in the project’s dependencies to ensure you use the pinned version or when ty is installed globally and you are in a project so the virtual environment is updated.
• uvx ty ... – Use when ty is not a project dependency, or for quick one-off checks

Type Checking Commands Rule

Use these commands for type checking:

ty check                      # Check all files in current directory
ty check path/to/file.py      # Check specific file
ty check src/                 # Check specific directory

See references/basic-checking.md for basic type checking examples.

Rule Configuration Rules

Configure rule levels as needed:

ty check --error possibly-unresolved-reference   # Treat as error
ty check --warn division-by-zero                 # Treat as warning
ty check --ignore unresolved-import              # Disable rule

Python Version Targeting Rules

Target specific Python versions and platforms:

ty check --python-version 3.12     # Check against Python 3.12
ty check --python-platform linux   # Target Linux platform

See references/version-targeting.md for version targeting examples.

Configuration Rule

ty is configured in pyproject.toml or ty.toml:

# pyproject.toml
[tool.ty.environment]
python-version = "3.12"

[tool.ty.rules]
possibly-unresolved-reference = "warn"
division-by-zero = "error"

[tool.ty.src]
include = ["src/**/*.py"]
exclude = ["**/migrations/**"]

[tool.ty.terminal]
output-format = "full"
error-on-warning = false

See references/configuration.md for configuration examples and per-file overrides.

Per-File Overrides Rule

Use overrides to apply different rules to specific files:

[[tool.ty.overrides]]
include = ["tests/**", "**/test_*.py"]

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

Language Server Rule

The plugin automatically configures the ty language server for Python files (.py and .pyi).

Migration Rules

mypy → ty

mypy .                        → ty check
mypy --strict .               → ty check --error-on-warning
mypy path/to/file.py          → ty check path/to/file.py

Pyright → ty

pyright .                     → ty check
pyright path/to/file.py       → ty check path/to/file.py

See references/migration.md for migration examples.

Ignore Comments Rule

Fix type errors instead of suppressing them. Only add ignore comments when explicitly requested by the user. Use ty: ignore, not type: ignore, and prefer rule-specific ignores:

# Good: rule-specific ignore
x = undefined_var  # ty: ignore[possibly-unresolved-reference]

# Bad: blanket ty ignore
x = undefined_var  # ty: ignore

# Bad: tool agnostic blanket ignore
x = undefined_var  # type: ignore

Documentation Reference

For detailed information, see the official documentation at https://docs.astral.sh/ty/

Additional References

• CI/CD integration: See references/ci-cd.md
• Advanced configuration: See references/advanced.md