pre-commit hook I use to prevent trailing commas in JSON test files

pre-commit.py

#!/usr/bin/env python

import sys
import os.path
import subprocess
import tempfile
import re
import json
import shutil
import atexit

tempdir = tempfile.mkdtemp()
atexit.register(lambda: shutil.rmtree(tempdir))

diff = subprocess.Popen(['git', 'diff', '--cached', '--name-only',
                         '--diff-filter=ACM'],
                        stdout=subprocess.PIPE)

changed_json_files = [filename for filename in diff.communicate()[0].split("\n")
                      if re.search('\.json$', filename)]

ci_args = ['git', 'checkout-index', '--prefix=%s/' % tempdir, '-f']
ci_args.extend(changed_json_files)
subprocess.check_call(ci_args)

exitstatus = 0
for filename in changed_json_files:
    with open(os.path.join(tempdir, filename)) as jsonfh:
        print "Syntax checking the staged version of %s" % filename
        try:
            json.loads(jsonfh.read())
        except ValueError:
            sys.stderr.write("%s is not a syntactically valid JSON file. Preventing checkin.\n" % filename)
            sys.stderr.write("If you really need to commit a busted .json file, use 'git commit --no-verify'\n")
            exitstatus = 1

exit(exitstatus)