Overview

Flake8

Flake8 is a wrapper around these tools:

  • PyFlakes
  • pep8
  • Ned's McCabe 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 contains 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.

Git hook

To use the Git hook on any commit, add a pre-commit file in the .git/hooks directory containing:

#!/usr/bin/python
import sys
from flake8.run import git_hook

COMPLEXITY = 10
STRICT = False

if __name__ == '__main__':
    sys.exit(git_hook(complexity=COMPLEXITY, strict=STRICT))

If strict option is set to True, any warning will block the commit. When strict is set to False or omited, 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 or set it to -1, it's just ignored. If specified, it must be a positive value. 12 is usually a good value.

Also, make sure the file is executable and adapt the shebang line so it point to your python interpreter.

Buildout integration

In order to use Flake8 inside a buildout, edit your buildout.cfg and add this:

[buildout]

parts +=
    ...
    flake8

[flake8]
recipe = zc.recipe.egg
eggs = flake8
       ${buildout:eggs}
entry-points =
    flake8=flake8.run:main

Original projects

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

Warning / Error codes

Below are lists of all warning and error codes flake8 will generate, broken out by component.

pep8:

  • E101: indentation contains mixed spaces and tabs
  • E111: indentation is not a multiple of four
  • E112: expected an indented block
  • E113: unexpected indentation
  • E201: whitespace after char
  • E202: whitespace before char
  • E203: whitespace before char
  • E211: whitespace before text
  • E223: tab / multiple spaces before operator
  • E224: tab / multiple spaces after operator
  • E225: missing whitespace around operator
  • E225: missing whitespace around operator
  • E231: missing whitespace after char
  • E241: multiple spaces after separator
  • E242: tab after separator
  • E251: no spaces around keyword / parameter equals
  • E262: inline comment should start with '# '
  • E301: expected 1 blank line, found 0
  • E302: expected 2 blank lines, found <n>
  • E303: too many blank lines (<n>)
  • E304: blank lines found after function decorator
  • E401: multiple imports on one line
  • E501: line too long (<n> characters)
  • E701: multiple statements on one line (colon)
  • E702: multiple statements on one line (semicolon)
  • W191: indentation contains tabs
  • W291: trailing whitespace
  • W292: no newline at end of file
  • W293: blank line contains whitespace
  • W391: blank line at end of file
  • W601: .has_key() is deprecated, use 'in'
  • W602: deprecated form of raising exception
  • W603: '<>' is deprecated, use '!='
  • W604: backticks are deprecated, use 'repr()'

pyflakes:

  • W402: <module> imported but unused
  • W403: import <module> from line <n> shadowed by loop variable
  • W404: 'from <module> import *' used; unable to detect undefined names
  • W405: future import(s) <name> after other statements
  • W801: redefinition of unused <name> from line <n>
  • W802: undefined name <name>
  • W803: undefined name <name> in __all__
  • W804: local variable <name> (defined in enclosing scope on line <n>) referenced before assignment
  • W805: duplicate argument <name> in function definition
  • W806: redefinition of function <name> from line <n>
  • W806: local variable <name> is assigned to but never used

McCabe:

  • W901: '<function_name>' is too complex ('<complexity_level>')

CHANGES

1.4 - ?

?

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 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
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