flake8-2020

flake8 plugin which checks for misuse of sys.version or sys.version_info

this will become a problem when python3.10 or python4.0 exists (presumably during the year 2020).

you might also find an early build of python3.10 useful

installation

pip install flake8-2020

flake8 codes

Code	Description
YTT101	sys.version[:3] referenced (python3.10)
YTT102	sys.version[2] referenced (python3.10)
YTT103	sys.version compared to string (python3.10)
YTT201	sys.version_info[0] == 3 referenced (python4)
YTT202	six.PY3 referenced (python4)
YTT203	sys.version_info[1] compared to integer (python4)
YTT204	sys.version_info.minor compared to integer (python4)
YTT301	sys.version[0] referenced (python10)
YTT302	sys.version compared to string (python10)
YTT303	sys.version[:1] referenced (python10)

rationale

lots of code incorrectly references the sys.version and sys.version_info members. in particular, this will cause some issues when the version of python after python3.9 is released. my current recommendation is 3.10 since I believe it breaks less code, here's a few patterns that will cause issues:

# in python3.10 this will report as '3.1' (should be '3.10')
python_version = sys.version[:3]  # YTT101
# in python3.10 this will report as '1' (should be '10')
py_minor = sys.version[2]
# in python3.10 this will be False (which goes against developer intention)
sys.version >= '3.5'  # YTT103


# correct way to do this
python_version = '{}.{}'.format(*sys.version_info)
py_minor = str(sys.version_info[1])
sys.version_info >= (3, 5)

# in python4 this will report as `False` (and suddenly run python2 code!)
is_py3 = sys.version_info[0] == 3  # YTT201

# in python4 this will report as `False` (six violates YTT201!)
if six.PY3:  # YTT202
    print('python3!')

if sys.version_info[0] >= 3 and sys.version_info[1] >= 5:  # YTT203
    print('py35+')

if sys.version_info.major >= 3 and sys.version_info.minor >= 6:  # YTT204
    print('py36+')

# correct way to do this
is_py3 = sys.version_info >= (3,)

if not six.PY2:
    print('python3!')

if sys.version_info >= (3, 5):
    print('py35+')

if sys.version_info >= (3, 6):
    print('py36+')

# in python10 this will report as '1'
python_major_version = sys.version[0]  # YTT301
# in python10 this will be False
if sys.version >= '3':  # YTT302
    print('python3!')
# in python10 this will be False
if sys.version[:1] >= '3':  # YTT303
    print('python3!')


# correct way to do this
python_major_version = str(sys.version_info[0])

if sys.version_info >= (3,):
    print('python3!')

if sys.version_info >= (3,):
    print('python3!')

as a pre-commit hook

See pre-commit for instructions

Sample .pre-commit-config.yaml:

-   repo: https://github.com/pycqa/flake8
    rev: 3.7.8
    hooks:
    -   id: flake8
        additional_dependencies: [flake8-2020==1.6.1]