ty: Python Type Checker by Astral

ty (pronounced tee-why) is a static type checker and language server for Python from Astral (creators of Ruff and uv). It performs static analysis on Python code to identify type-related issues before runtime.

Key Features

• Performance: 10-100x faster than mypy and Pyright. Fine-grained incremental analysis provides near-instant feedback in an IDE.
• Diagnostics: Detailed error messages with code-span annotations and rich contextual information.
• Language Server: Full LSP implementation with go-to-definition, find references, completions with auto-import, rename refactoring, inlay hints, hover, and signature help.
• Configurable Rules: Each diagnostic rule can be set to error, warn, or ignore. Supports per-file overrides and suppression comments.
• Gradual Typing: Avoids false positives on untyped code through a “gradual guarantee” approach, making adoption in existing codebases practical.
• Advanced Type System: First-class intersection types, type narrowing (including hasattr narrowing), and reachability analysis based on type inference.
• Notebook Support: Language server features for Jupyter notebooks with cross-cell analysis.

Type System Features

ty implements several type system features beyond what other type checkers provide:

• Redeclarations: Allows reusing the same symbol with a different type within a function scope.
• Intersection Types: First-class support for intersection types (A & B), which enables more precise type narrowing after isinstance checks.
• Reachability Analysis: Uses type inference to detect unreachable branches, which can handle version-specific code paths for libraries like pydantic.
• Fixpoint Iteration: Infers types for symbols that cyclically depend on themselves by iterating until the type converges.

Installation

The quickest way to run ty is with uvx:

uvx ty check

For project use, add ty as a development dependency:

uv add --dev ty
uv run ty check

ty can also be installed globally with uv tool install ty@latest, via pip, pipx, or using standalone installers available for macOS, Linux, and Windows.

Basic Usage

# Check the current project
ty check

# Check specific files
ty check src/main.py

# Check with a specific Python version target
ty check --python-version 3.12

# Get an explanation of a specific rule
ty explain rule <rule-name>

# Show all rules
ty explain rule

Configuration

ty reads configuration from pyproject.toml (under [tool.ty]) or a dedicated ty.toml file. Rule levels and other settings can be configured per-project:

[tool.ty.rules]
unused-ignore-comment = "warn"
possibly-missing-import = "error"

User-level configuration is also supported at ~/.config/ty/ty.toml.

Editor Integration

ty includes official editor support:

• VS Code: Install the ty extension from the VS Code Marketplace.
• Neovim: Configure via nvim-lspconfig or the built-in vim.lsp.config (Neovim 0.11+).
• Zed: Built-in support (can be enabled in settings).
• PyCharm: Native support starting with version 2025.3.

For other editors, run ty server to start the language server.

Comparison to Other Tools

ty is not a drop-in replacement for mypy or Pyright — it makes different design choices and has different default behaviors. The primary differentiators are speed (orders of magnitude faster) and the gradual typing approach that reduces false positives on untyped code. ty aims for conformance with the Python typing specification.

Learn More

• ty: A Complete Guide
• How do mypy, pyright, and ty compare?
• How to try the ty type checker
• How to migrate from mypy to ty
• How to use ty in CI
• How to gradually adopt type checking in an existing Python project
• ty Documentation
• ty Playground
• ty GitHub Repository