the modular source code checker: pep8, pyflakes and co
Flake8 is a wrapper around these tools:
- Ned Batchelder’s McCabe script
Flake8 runs all the tools by launching the single flake8 script. It displays the warnings in a per-file, merged output.
It also adds a few features:
files that contain this line are skipped:
# flake8: noqa
lines that contain a # noqa comment at the end will not issue warnings.
a Git and a Mercurial hook.
a McCabe complexity checker.
extendable through flake8.extension entry points.
pip install flake8
To run flake8 just invoke it against any directory or Python module:
$ flake8 coolproject coolproject/mod.py:97:1: F401 'shutil' imported but unused coolproject/mod.py:625:17: E225 missing whitespace around operato coolproject/mod.py:729:1: F811 redefinition of function 'readlines' from line 723 coolproject/mod.py:1028:1: F841 local variable 'errors' is assigned to but never used
The outputs of PyFlakes and pep8 (and the optional plugins) are merged and returned.
flake8 offers an extra option: –max-complexity, which will emit a warning if the McCabe complexity of a function is higher than the value. By default it’s deactivated:
$ flake8 --max-complexity 12 coolproject coolproject/mod.py:97:1: F401 'shutil' imported but unused coolproject/mod.py:625:17: E225 missing whitespace around operator coolproject/mod.py:729:1: F811 redefinition of unused 'readlines' from line 723 coolproject/mod.py:939:1: C901 'Checker.check_all' is too complex (12) coolproject/mod.py:1028:1: F841 local variable 'errors' is assigned to but never used coolproject/mod.py:1204:1: C901 'selftest' is too complex (14)
This feature is quite useful to detect over-complex code. According to McCabe, anything that goes beyond 10 is too complex. See https://en.wikipedia.org/wiki/Cyclomatic_complexity.
Frequently Asked Questions
Why does flake8 pin the version of pep8?
Version 1.6 of pep8 doesn’t work properly with flake8. Until pep8 releases a version that works, flake8 pins the version of pep8 so that flake8 will work as a whole.
Is flake8 broken?
Flake8 combines two other projects that are significant on their own: pep8 and PyFlakes. If flake8 is doing something you don’t like, it is quite likely that the problem lies in one of those other projects. You can run them separately to see if they are the cause of your difficulties. We greatly appreciate your efforts to diagnose the source of the problem before reporting bugs against flake8.
Questions or Feedback
If you have questions you’d like to ask the developers, or feedback you’d like to provide, feel free to use the mailing list: firstname.lastname@example.org We would love to hear from you. Additionally, if you have a feature you’d like to suggest, the mailing list would be the best place for it.
2.5.2 - 2016-01-30
- Bug Parse output_file and enable_extensions from config files
- Improvement Raise upper bound on mccabe plugin to allow for version 0.4.0
2.5.1 - 2015-12-08
2.5.0 - 2015-10-26
- Improvement Raise cap on PyFlakes for Python 3.5 support
- Improvement Avoid deprecation warnings when loading extensions (GitLab#59, GitLab#90)
- Improvement Separate logic to enable “off-by-default” extensions (GitLab#67)
- Bug Properly parse options to setuptools Flake8 command (GitLab!41)
- Bug Fix exceptions when output on stdout is truncated before Flake8 finishes writing the output (GitLab#69)
- Bug Fix error on OS X where Flake8 can no longer acquire or create new semaphores (GitLab#74)
2.4.1 - 2015-05-18
2.4.0 - 2015-03-07
Bug Print filenames when using multiprocessing and -q option. (GitLab#31)
Bug Put upper cap on dependencies. The caps for 2.4.0 are:
- pep8 < 1.6 (Related to GitLab#35)
- mccabe < 0.4
- pyflakes < 0.9
See also GitLab#32
Bug Files excluded in a config file were not being excluded when flake8 was run from a git hook. (GitHub#2)
2.3.0 - 2015-01-04
- Feature: Add --output-file option to specify a file to write to instead of stdout.
- Bug Fix interleaving of output while using multiprocessing (GitLab#17)
2.2.5 - 2014-10-19
- Flush standard out when using multiprocessing
- Make the check for “# flake8: noqa” more strict
2.2.4 - 2014-10-09
Fix bugs triggered by turning multiprocessing on by default (again)
Multiprocessing is forcibly disabled in the following cases:
- Passing something in via stdin
- Analyzing a diff
- Using windows
Fix –install-hook when there are no config files present for pep8 or flake8.
Fix how the setuptools command parses excludes in config files
Fix how the git hook determines which files to analyze (Thanks Chris Buccella!)
2.2.3 - 2014-08-25
- Actually turn multiprocessing on by default
2.2.2 - 2014-07-04
- Re-enable multiprocessing by default while fixing the issue Windows users were seeing.
2.2.1 - 2014-06-30
- Turn off multiple jobs by default. To enable automatic use of all CPUs, use --jobs=auto. Fixes #155 and #154.
2.2.0 - 2014-06-22
- New option doctests to run Pyflakes checks on doctests too
- New option jobs to launch multiple jobs in parallel
- Turn on using multiple jobs by default using the CPU count
- Add support for python -m flake8 on Python 2.7 and Python 3
- Fix Git and Mercurial hooks: issues #88, #133, #148 and #149
- Fix crashes with Python 3.4 by upgrading dependencies
- Fix traceback when running tests with Python 2.6
- Fix the setuptools command python setup.py flake8 to read the project configuration
2.1.0 - 2013-10-26
- Add FLAKE8_LAZY and FLAKE8_IGNORE environment variable support to git and mercurial hooks
- Force git and mercurial hooks to repsect configuration in setup.cfg
- Only check staged files if that is specified
- Fix hook file permissions
- Fix the git hook on python 3
- Ignore non-python files when running the git hook
- Ignore .tox directories by default
- Flake8 now reports the column number for PyFlakes messages
2.0.0 - 2013-02-23
- Pyflakes errors are prefixed by an F instead of an E
- McCabe complexity warnings are prefixed by a C instead of a W
- Flake8 supports extensions through entry points
- Due to the above support, we require setuptools
- We publish the documentation
- Fixes #13: pep8, pyflakes and mccabe become external dependencies
- Split run.py into main.py, engine.py and hooks.py for better logic
- Expose our parser for our users
- New feature: Install git and hg hooks automagically
- By relying on pyflakes (0.6.1), we also fixed #45 and #35
1.7.0 - 2012-12-21
- Fixes part of #35: Exception for no WITHITEM being an attribute of Checker for Python 3.3
- Support stdin
- Incorporate @phd’s builtins pull request
- Fix the git hook
- Update pep8.py to the latest version
1.6.2 - 2012-11-25
- fixed the NameError: global name ‘message’ is not defined (#46)
1.6.1 - 2012-11-24
- fixed the mercurial hook, a change from a previous patch was not properly applied
- fixed an assumption about warnings/error messages that caused an exception to be thrown when McCabe is used
1.6 - 2012-11-16
- changed the signatures of the check_file function in flake8/run.py, skip_warning in flake8/util.py and the check, checkPath functions in flake8/pyflakes.py.
- fix --exclude and --ignore command flags (#14, #19)
- fix the git hook that wasn’t catching files not already added to the index (#29)
- pre-emptively includes the addition to pep8 to ignore certain lines. Add # nopep8 to the end of a line to ignore it. (#37)
- check_file can now be used without any special prior setup (#21)
- unpacking exceptions will no longer cause an exception (#20)
- fixed crash on non-existent file (#38)
1.5 - 2012-10-13
- fixed the stdin
- make sure mccabe catches the syntax errors as warnings
- pep8 upgrade
- added max_line_length default value
- added Flake8Command and entry points if setuptools is around
- using the setuptools console wrapper when available
1.4 - 2012-07-12
- git_hook: Only check staged changes for compliance
- use pep8 1.2
1.3.1 - 2012-05-19
- fixed support for Python 2.5
1.3 - 2012-03-12
- fixed false W402 warning on exception blocks.
1.2 - 2012-02-12
- added a git hook
- now Python 3 compatible
- mccabe and pyflakes have warning codes like pep8 now
1.1 - 2012-02-14
- fixed the value returned by –version
- allow the flake8: header to be more generic
- fixed the “hg hook raises ‘physical lines’” bug
- allow three argument form of raise
- now uses setuptools if available, for ‘develop’ command
1.0 - 2011-11-29
- Deactivates by default the complexity checker
- Introduces the complexity option in the HG hook and the command line.
0.9 - 2011-11-09
- update pep8 version to 0.6.1
- mccabe check: gracefully handle compile failure
0.8 - 2011-02-27
- fixed hg hook
- discard unexisting files on hook check
0.7 - 2010-02-18
- Fix pep8 initialization when run through Hg
- Make pep8 short options work when run through the command line
- Skip duplicates when controlling files via Hg
0.6 - 2010-02-15
- Fix the McCabe metric on some loops
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