Better Python Flymake Integration in Emacs

Flymake is a great feature in Emacs for getting warnings and errors in source code as you type it. When I am coding in Python I like to use PEP8 and pyflakes to warn me about stylistic and potential errors. I integrated those tools into Emacs by following the guide at

However, I find it annoying that all output from PEP8 and pyflakes is treated as an error by flymake. Notice in the following image that only the last highlighted line is an error, while the rest of the lines should be treated as warnings.


Wouldn’t it be much better if flymake treated the warning lines as warnings, and highlighted them as such?


We can accomplish this by modifying the output of PEP8 and pyflakes such that flymake correctly recognizes messages as warnings. The following code does just that (and runs both PEP8 and pyflakes on the input):

#!/usr/bin/env python

import commands
import re
import sys

def make_re(*msgs):
    return re.compile('(%s)' % '|'.join(msgs))

pyflakes_ignore = make_re('unable to detect undefined names')
pyflakes_warning = make_re(
    'imported but unused',
    'is assigned to but never used',
    'redefinition of unused',
pep8_ignore = ['E501']
pep8_warning = make_re('.')

def run(cmd, ignore_re, warning_re):
    output = commands.getoutput(cmd)
    for line in output.splitlines():
        if ignore_re and
        elif ': ' in line and
            line = '%s: WARNING %s' % tuple(line.split(': ', 1))
        print line

run('pyflakes %s' % sys.argv[1], pyflakes_ignore, pyflakes_warning)
print '## pyflakes above, pep8 below ##'
pep8_ignore = ' '.join('--ignore=%s' % i for i in pep8_ignore)
run('pep8 %s --repeat %s' % (pep8_ignore, sys.argv[1]), None, pep8_warning)

This code may change over time, as I find different outputs that should be treated as warnings. The latest version of this code is here.

Here is the snippet of elisp to make flymake use the above command in python-mode. Replace /PATH/TO/ to a valid path to the command.

;; Pyflakes for python
(when (load "flymake" t)
  (defun flymake-pychecker-init ()
    (let* ((temp-file (flymake-init-create-temp-buffer-copy
           (local-file (file-relative-name
                        (file-name-directory buffer-file-name))))
      (list "/PATH/TO/" (list local-file))))
  (add-to-list 'flymake-allowed-file-name-masks
               '("\\.py\\'" flymake-pychecker-init)))

Published on the 18th of October 2011