Appendix: Type Checkers#

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Getting Started with `mypy`: Installation and Basic Use#

mypy is a popular static type checker for Python that validates type annotations:

Installation#

pip install mypy

Basic Use#

Run mypy on your script or module to identify type errors:

mypy your_script.py

Example:

# script.py


def greet(name: str) -> str:
    return f"Hello, {name}!"


# Incorrect type:
print(greet(123))  # type: ignore
## Hello, 123!

Running mypy script.py outputs:

script.py:4: error: Argument 1 to "greet" has incompatible type "int"; expected "str"

Advanced Configuration and Fine-tuning of `mypy`#

Customize mypy behavior using a mypy.ini or pyproject.toml file:

Example `mypy.ini`#

[mypy]
ignore_missing_imports = True
strict = True
check_untyped_defs = True

Example `pyproject.toml`#

[tool.mypy]
ignore_missing_imports = true
strict = true

Advanced configuration allows precise control over type-checking behavior and strictness levels.

Exploring `pyright` and IDE Integration (VSCode, PyCharm)#

pyright, developed by Microsoft, offers high-performance static type checking, integrated seamlessly with popular IDEs.

Using `pyright` (CLI)#

Install globally using npm:

npm install -g pyright
pyright your_script.py

VSCode Integration#

pyright powers VSCode's built-in Python type checking, providing immediate feedback:

Install the Python extension in VSCode.
Real-time inline error highlighting and suggestions.

PyCharm Integration#

PyCharm supports built-in type checking:

Provides live error detection, highlighting, and quick-fix suggestions.
Navigate to Preferences > Editor > Inspections to configure type-checking rules.

Incremental Typing Strategies: Gradual Adoption#

Adopt typing gradually, focusing first on critical paths:

Annotate high-risk or frequently changing modules first.
Enable type checking incrementally to avoid overwhelming your team:

[mypy]
files = core/, utils/

Incremental typing balances immediate benefits with manageable adoption efforts.

Integrating Type Checking into Continuous Integration#

Automate type checking within your CI/CD pipeline to enforce consistency and catch errors early:

Example GitHub Actions Workflow#

name: Type Check

on: [ push, pull_request ]

jobs:
  type-check:
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v3
      - uses: actions/setup-python@v3
        with:
          python-version: "3.11"

      - run: pip install mypy
      - run: mypy .

Automated checks ensure ongoing compliance with typing standards and maintain high code quality.

Handling and Resolving Common Errors#

Common type-checking errors and their resolutions:

Incorrect Type Annotation#

# example_2.py
# Error: Incompatible types in assignment
# (expression has type "int", variable has type "str")
name: str = 123  # type: ignore

Missing Imports#

Use ignore_missing_imports or install type stubs:

[mypy]
ignore_missing_imports = True

Or install stubs:

pip install types-requests

Union and Optional Errors#

Resolve ambiguity clearly:

# example_3.py
from typing import Optional


# Error: Incompatible return value type (got "None", expected "int")
def find_index(item: str, items: list[str]) -> Optional[int]:
    try:
        return items.index(item)
    except ValueError:
        return None

Effectively handling and resolving these errors leads to clearer, more reliable, and maintainable codebases.