Type Checking with MyPy and Pyright

Type checking is an essential part of maintaining a robust and error-free codebase. MyPy and Pyright are both static type checkers for Python that help you ensure your code adheres to specified type annotations.

Setting Up MyPy

To set up MyPy for your project, follow these steps:

1. Install MyPy:

   uv pip install mypy
   

2. Configure MyPy: check in the myproject.toml file the [tool.mypy] section

3. Run MyPy: To check your code with MyPy, run the following command:

   uvx --with-editable . mypy py_launch_blueprint/
   

   or

   just typecheck
   

Setting Up Pyright

To set up Pyright for your project, follow these steps:

1. Install Pyright:

   uv pip install pyright
   

2. Configure Pyright: check pyrightconfig.json file in the root of the project

3. Run Pyright: To check your code with Pyright, run the following command:

   uvx --with-editable . pyright py_launch_blueprint/
   

Best Practices for Type Checking

• Annotate All Functions: Ensure all functions have type annotations for their parameters and return types.

• Use Type Hints: Utilize Python’s built-in type hints (e.g., List, Dict, Optional) to specify the expected types.

• Avoid Any: Minimize the use of the Any type to maintain strict type checking.

• Leverage TypedDict: Use TypedDict for dictionaries with a fixed set of keys and value types.

• Check Third-Party Libraries: Ensure third-party libraries used in your project have type stubs available.

Common Issues and Solutions

1. Missing Type Annotations:

   • Issue: MyPy/Pyright reports missing type annotations for functions.

   • Solution: Add type annotations to all function parameters and return types.

2. Incompatible Types:

   • Issue: MyPy/Pyright reports incompatible types in assignments or function calls.

   • Solution: Ensure the types of variables and function arguments match the expected types.

3. Ignoring Errors:

   • Issue: MyPy/Pyright reports errors that you want to ignore.

   • Solution: Use # type: ignore comments to suppress specific errors, but use them sparingly.

      # mypy
      reveal_type(x)  # type: ignore
   
      # pyright
      x = something()  # pyright: ignore
   

4. Third-Party Libraries:

   • Issue: MyPy/Pyright reports missing type stubs for third-party libraries.

   • Solution: Install type stubs for the libraries using uv pip install types-<library>.

5. Type checking only specific files:

   • Issue: You want to run MyPy/Pyright on specific files or directories.

   • Solution: Specify the files or directories to check as arguments to the MyPy/Pyright command.

   # mypy
   mypy src/main.py src/utils.py
   
   # pyright
   pyright src/main.py src/utils.py
   

By following these best practices and addressing common issues, you can effectively use MyPy and Pyright to maintain a type-safe and reliable codebase. Read more about mypy