import datetime
import hashlib
import os
import os.path
import shutil
import socket
import tarfile

if __name__ == "__main__":

    # base directory, which is one level upper than target directory
    base      = "<BASE_DIRECTORY_OF_YOUR_CHOICE>"
    # backup target directory under base directory
    target    = "<BACK_UP_TARGET_DIRECTORY>"
    # directories to exclude from backup file
    exclude   = ["<DIRECTORY_TO_EXCLUDE>"]

    # backup file name / HOSTNAME_DAY.tar.bz2
    backup    = socket.gethostname() + "_" +\
                datetime.date.today().strftime("%A") +\
                ".tar.bz2"
    # temporary directory & temporary file
    temp_dir  = "/tmp"
    temp_file = os.path.join(temp_dir, backup)
    # destination directory
    dest_dir  = os.path.join(base, "Dropbox/<TARGET_DIRECTORY>")
    dest_file = os.path.join(dest_dir, backup)

    # let's start backup

    # first create a backup file under temporary file
    tar = tarfile.open(temp_file, "w:bz2")

    os.chdir(base)
    for root, dirs, files in os.walk(target):
        for name in files:
            for d in root.split('/'):  
                if d in exclude:
                    break
            else:
                tar.add(os.path.join(root, name))
    
    tar.close()

    # backup file creation has finished
    # now determine if file replacement is required or not
    if os.path.exists(dest_file):
        # dictionary to store md5 checksum of each file
        md5 = {}
        for f in temp_file, dest_file:
            with open(f) as file_to_check:
                data = file_to_check.read()
                md5[f] = hashlib.md5(data).hexdigest()
        # compare checksum value of each file
        # copy temporary file under destination directory
        if md5[temp_file] != md5[dest_file]:
            shutil.copy(temp_file, dest_dir)
    else:
        # somehow backup of same name does not exist, do copy
        shutil.copy(temp_file, dest_dir)