asottile-archive / flake8-2020

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

ARCHIVED

3.10 is old news now and I don't plan on updating it further

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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]

flake8-2020's Issues

Bug in test?

Should it test version[0] and not sys.version[0]?

https://github.com/asottile/flake8-2020/blob/f60d4d6b35ec50d6554fc6035c6d9470f628f089/tests/flake8_2020_test.py#L109

version_info.major >= 3 and version_info.minor >= 5

This is used in three places in scipy, and I've seen this sort of thing in other repos too:

if sys.version_info.major >= 3 and sys.version_info.minor >= 5:
    from math import gcd
else:
    from fractions import gcd

The if is incorrectly False for 4.0 - 4.4, 5.0 - 5.4, etc.

To fix:

-if sys.version_info.major >= 3 and sys.version_info.minor >= 5:
+if sys.version_info >= (3, 5):
     from math import gcd
 else:
     from fractions import gcd

Can this easily be detected?

Versioning is not following SemVer

Not sure whether this can be properly fixed now, but at least I'm going post is as feedback item.

I do expect libraries to follow at least some kind of semver-alike versioning. If there are breaking changes, I do expect major version to be bumped. However it's not the case with flake8-2020, we got an error when trying to update 1.6.1 -> 1.7.0:

flake8-2020 1.7.0 depends on flake8>=5

We do use flake8 less than 5, because there are other plugins (namely, wemake-python-styleguide) which require older flake8. And changing to the newer flake8 is a major version bump. This was really unexpected when flake8-2020 got a major change breaking backward compatibility without bumping its major version.

Could you please check and maybe at least somehow improve the release process to avoid such issues in the future? Ideally would be to fix current releases, but I understand this is unlikely to happen....

Treat `PY3` just as `six.PY3`

@hugovk submitted a pull request to urllib3 based on this plugin: urllib3/urllib3#1669. Thanks! But it failed to catch all issues, as PY3 was sometimes imported directly. See urllib3/urllib3#2004 for examples.

This may cause false positives, though.

False positive with version slice

the version slice should probably only check for :1 and :3 as there's a few "legitimate" usages of slicing sys.version I've seen in the wild, in particular here's one from cpython that seems to be well defined:

https://github.com/python/cpython/blob/da27d9b9dc44913ffee8f28d9638985eaaa03755/Lib/distutils/msvc9compiler.py#L174-L179

string comparison with `sys.version` (python3.10)

I haven't seen this one in the wild, but I think it's ~reasonably well defined for 3.0 - 3.9 so it's possible:

Python 3.5.7 (default, Mar 30 2019, 01:05:18) 
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version >= '3.5'
True

Python 3.10.0a0 (default, Jul 18 2019, 06:46:14) 
[GCC 7.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version >= '3.5'
False

Small optimisation of YTT202

With this code:

import six
if not six.PY3:
    pass

The plugin correctly gives:

$ flake8 --select YTT 1.py
1.py:2:8: YTT202: `six.PY3` referenced (python4), use `not six.PY2`

Perhaps it could detect the not before six.PY3, and suggest a clean six.PY2 instead?

For example:

$ flake8 --select YTT 1.py
1.py:2:X: YTT20X: `not six.PY3` referenced (python4), use `six.PY2`

That is:

# Bad
if not six.PY3:

# Better (sort of)
if not not six.PY2:

# Best
if six.PY2:

Add check for float(sys.version_info.major)

I got this in a PR today:

python_version = float(sys.version_info.major) + 0.1 * float(
    sys.version_info.minor,
)

flake8-2020 doesn't catch this, would it be a welcome addition?

Add check for platform.python_version_tuple

I have observed code using platform.python_version_tuple to check version numbers, but since it is a tuple of strings it gets tripped up by Python 3.10:

>>> platform.python_version_tuple()
('3', '10', '0a3')
>>> platform.python_version_tuple() > ("3", "3")
False
>>> "10" > "3"
False

>>> platform.python_version_tuple()
('3', '8', '5')
>>> platform.python_version_tuple() > ("3", "3")
True
>>> 
>>> "8" > "3"
True

Would you be interested in having a check for this?

Report a YTT30X code for `sys.version < '3'` (and other 1-character string comparisons)

Originally from #4 (comment)

Right now it's reported as YTT103

Check for platform.python_version[:3] (python3.10)

Very similar to YTT101, but for platform.python_version:

Python 3.10.0a2+ (heads/master:bbeb2d2, Nov 13 2020, 10:34:11)
[GCC 7.5.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import platform
>>> platform.python_version()[:3]
'3.1'