Hello, I have recently been looking at mypy[1], a static type checker for Python. It uses optional type hints[2] to perform type checking. Using mypy has many benefits: - it allows developers to find type-related bugs that could cause a program to crash or display an unexpected behaviour at runtime; - it replaces type information that is usually given in docstrings, and may be outdated; - mypy runs can be integrated into a CI, making sure there are no regressions; - mypy works with both Python 2 and 3; - adding type information may be done incrementally; - library developers may expose type information, helping their users. Type information is available for most of the standard Python library, and for the most popular libraries on PyPI, through typeshed[3]. It is therefore possible to write bad code such as: $ cat test.py import requests requests.get(123) And have mypy warn us that something is wrong: $ mypy test.py test.py:2: error: Argument 1 has incompatible type "int"; expected "Union[str, bytes]" I would like to add type hints to OpenStack, starting with a small project (because it would probably be quick and easy) that is used by a lot of other OpenStack projects (because they would benefit from the type hints as well). Oslo seems like a reasonable choice. I took the liberty of implementing a proof of concept on top of oslo.config[4]. Using this branch, you should be able to: 1) Run "tox -etype" to run mypy on the whole code base 2) Write a badly typed program such as this one: $ cat test.py from oslo_config import cfg common_opts = [ cfg.IntOpt('test', min='3'), cfg.HostAddressOpt('host', version='4'), ] And have mypy show you the type errors: $ mypy test.py test.py:4: error: Argument "min" to "IntOpt" has incompatible type "str"; expected "Optional[int]" test.py:5: error: Argument "version" to "HostAddressOpt" has incompatible type "str"; expected "Optional[int]" Does anyone know about mypy? Would anyone be interested in seeing type hints added to OpenStack? Looking forward to hearing your thoughts, Cyril [1] http://mypy-lang.org/ [2] https://www.python.org/dev/peps/pep-0484/ [3] https://github.com/python/typeshed [4] https://github.com/CyrilRoelandteNovance/oslo.config/tree/type