flake8 1.0

code checking using pep8 and pyflakes

Flake8 is a wrapper around these tools:

• PyFlakes

• pep8

• Ned’s MacCabe script

Flake8 runs all tools by launching the single ‘flake8’ script, but ignores pep8 and PyFlakes extended options and just uses defaults. It displays the warnings in a per-file, merged output.

It also adds a few features:

• files that starts with this header are skipped:

  # flake8: noqa

• lines that contains a “# NOQA” comment at the end will not issue a warning.

• a Mercurial hook.

• a McCabe complexity checker.

QuickStart

To run flake8 just invoke it against any directory or Python module:

$ flake8 coolproject
coolproject/mod.py:1027: local variable 'errors' is assigned to but never used
coolproject/mod.py:97: 'shutil' imported but unused
coolproject/mod.py:729: redefinition of function 'readlines' from line 723
coolproject/mod.py:1028: local variable 'errors' is assigned to but never used
coolproject/mod.py:625:17: E225 missing whitespace around operato

The output of PyFlakes and pep8 is merged and returned.

flake8 offers an extra option: –max-complexity, which will emit a warning if the McCabe complexityu of a function is higher that the value. By default it’s deactivated:

$ bin/flake8 --max-complexity 12 flake8
coolproject/mod.py:97: 'shutil' imported but unused
coolproject/mod.py:729: redefinition of function 'readlines' from line 723
coolproject/mod.py:1028: local variable 'errors' is assigned to but never used
coolproject/mod.py:625:17: E225 missing whitespace around operator
coolproject/mod.py:452:1: 'missing_whitespace_around_operator' is too complex (18)
coolproject/mod.py:939:1: 'Checker.check_all' is too complex (12)
coolproject/mod.py:1204:1: '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.

Mercurial hook

To use the Mercurial hook on any commit or qrefresh, change your .hg/rc file like this:

[hooks]
commit = python:flake8.run.hg_hook
qrefresh = python:flake8.run.hg_hook

[flake8]
strict = 0
complexity = 12

If strict option is set to 1, any warning will block the commit. When strict is set to 0, warnings are just displayed in the standard output.

complexity defines the maximum McCabe complexity allowed before a warning is emited. If you don’t specify it it’s just ignored. If specified, must be a positive value. 12 is usually a good value.

Original projects

Flake8 is just a glue project, all the merits go to the creators of the original projects:

• pep8: http://github.com/jcrocholl/pep8/

• PyFlakes: http://divmod.org/trac/wiki/DivmodPyflakes

• McCabe: http://nedbatchelder.com/blog/200803/python_code_complexity_microtool.html

CHANGES

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 intialization when run through Hg

• Make pep8 short options work when run throug the command line

• skip duplicates when controlling files via Hg

0.6 - 2010-02-15

• Fix the McCabe metric on some loops