with open(file_json, 'ab+') as f:
    f.seek(-1,2)
    char_tail = f.read(1).decode()
    if char_tail == ']':
        f.seek(-1,2)
    elif char_tail == '\n':
        f.seek(-2,2)
        char_tail = f.read(1)
        if char_tail.decode() == ']':
            f.seek(-2,2)
        else:
            print('*** Unknown format.')
            print('seek(-2,2) read -> {}'.format(char_tail))
            return
    else:
        print('*** Unknown format.')
        print('seek(-1,2) read -> {}'.format(char_tail))
        return
    f.truncate()
    f.write(', '.encode())
    f.write(json.dumps(data, ensure_ascii=False).encode()[1:-1])
    f.write(']'.encode())