A very simplistic script to check for flake8 errors in the lines of a diff produced from `git diff`

git-diff-flake8.py

#!/usr/bin/env python
"""
A very simplistic script to check for flake8 errors in the lines of a diff
produced from `git diff`.  This is useful if you have files with lots of flake8
errors, but you only want to know about the errors in the lines you have
altered.
"""
import sys
import subprocess


def system(*args, **kwargs):
    stdin = kwargs.pop('input', None)
    kwargs.setdefault('stdin', subprocess.PIPE)
    kwargs.setdefault('stdout', subprocess.PIPE)
    proc = subprocess.Popen(args, **kwargs)
    out, err = proc.communicate(input=stdin)
    return out


def git_diff(*args):
    diff = system('git', 'diff', *args)
    return diff.strip().split("\n")


def flake8_output(file_list, *args):
    errors = []
    prefix = len('stdin')

    for filename in file_list:
        if '--cached' in args:
            # run the cached version of the file through flake8
            input_file = system('git', 'show', ':' + filename)
        else:
            input_file = system('cat', filename)
            
        err = system('flake8', '-', input=input_file)
        errors += [filename + e[prefix:] for e in err.strip().split("\n") if e]

    return errors


def parse_diff(lines):
    files = {}
    current_file = None
    counter = 0
    blanks = False

    for line in lines:
        if line.startswith('-'):
            continue

        if line.startswith('+++'):
            # starting on a new file
            current_file = line[6:].strip()
            files[current_file] = []
            continue

        if line.startswith('diff'):
            # finished with this file
            current_file = None
            continue

        if current_file is None:
            # if we are not currently parsing a file, skip the line
            continue

        if line.startswith('@@'):
            # set new starting point
            counter = -1
            base_line = int(line.split()[2].split(',')[0])
            continue

        counter += 1

        if line.startswith('+'):
            files[current_file].append(base_line + counter)
        elif line == ' ':
            blanks = True
        else:
            l = line.strip()
            if blanks and (l.startswith('def') or l.startswith('class')):
                files[current_file].append(base_line + counter)
            else:
                blanks = False

    return files


def parse_flake8(lines):
    errors = []

    for line in lines:
        parts = line.split(':')
        errors.append((parts[0].strip(), int(parts[1]), line))

    return sorted(errors)


def main():
    # get list of changed files and lines
    args = sys.argv[1:]
    diff = git_diff(*args)
    changed_files = parse_diff(diff)

    # get list of flake8 errors for the changed files
    py_files = [f for f in changed_files.keys() if f.endswith('.py')]
    output = flake8_output(py_files, *args)
    flake8_errors = parse_flake8(output)

    # check changed lines for errors
    errors = [err for filename, lineno, err in flake8_errors
              if lineno in changed_files[filename]]

    # if errors found, print them and exit with error code
    if errors:
        print "Aborting commit due to flake8 errors:"
        for err in errors:
            print err
        sys.exit(1)


if __name__ == '__main__':
    main()

Mind sharing a usage example