Static Type Checking with mypy

Intermediate10 min

Add static type checking to your Python project with mypy to catch type errors before runtime and improve code documentation.

Prerequisites

-Python 3.8+ installed
-Basic understanding of Python type hints

Steps

Install mypy

Install the mypy static type checker. It analyzes your code's type annotations without running it.

$ pip install mypy

Run mypy on your project

Check all Python files in the src/ directory for type errors. mypy reads type annotations and reports inconsistencies.

$ mypy src/

Start with a single module and gradually expand. Running mypy on an untyped codebase will produce many errors at first.

Run mypy in strict mode

Enable all optional type checking flags. Strict mode requires type annotations on all functions and disallows implicit Any types.

$ mypy --strict src/

Strict mode is very rigorous. It is best suited for new projects or well-typed codebases. For legacy code, use incremental strictness.

Install type stubs for third-party libraries

Automatically download and install missing type stub packages for third-party libraries like requests, flask, and sqlalchemy.

$ mypy --install-types

Generate a mypy report

Generate an HTML coverage report showing which files and lines are type-checked. Open mypy-report/index.html in your browser to review.

$ mypy src/ --html-report mypy-report

Install lxml first with 'pip install lxml' for HTML report generation to work.

Full Script

full-script.sh

#!/bin/bash
# mypy type checking setup
set -e

pip install mypy

# Add mypy config to pyproject.toml
if [ -f pyproject.toml ]; then
  cat >> pyproject.toml << 'TOMLEOF'

[tool.mypy]
python_version = "3.11"
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true
check_untyped_defs = true

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false
TOMLEOF
  echo "mypy config added to pyproject.toml"
fi

mypy --install-types --non-interactive
mypy src/ || echo "Type errors found. Review and fix the issues above."

echo "mypy setup complete."

FAQ

Discussion

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#!/bin/bash # mypy type checking setup set -e pip install mypy # Add mypy config to pyproject.toml if [ -f pyproject.toml ]; then cat >> pyproject.toml << 'TOMLEOF' [tool.mypy] python_version = "3.11" warn_return_any = true warn_unused_configs = true disallow_untyped_defs = true check_untyped_defs = true [[tool.mypy.overrides]] module = "tests.*" disallow_untyped_defs = false TOMLEOF echo "mypy config added to pyproject.toml" fi mypy --install-types --non-interactive mypy src/ || echo "Type errors found. Review and fix the issues above." echo "mypy setup complete."