query Debian package for a file

def query_pkg(raw_file):
    """
    Query which Debian package raw_file belongs.
    Use dlocate(1) instead of dpkg(1) for fast search
    If dlocate is not installed, fall back to dpkg(1)
    Return None if the query fails.
    """

    with open(os.devnull, "w") as fnull:
        program = dlocate_installed()

        try:
            result = subprocess.check_output(["dlocate", "-S", raw_file],
                                             stderr=fnull)
        except subprocess.CalledProcessError:
            pass

        try:
            return result.split(':')[0]
        except UnboundLocalError:
            pass