Simple python script for extracting various archive types

Last couple of days I've been compiling lots of software from source and i got very annoyed typing the tar command over and over again so I created simple python script for this purpose. Now I can extract .tar, .tar.gz, .tar.bz2 and .zip archives with just one command. Of course it could be maybe done better with bash and aliases but i wanted to try out python.
Here's the script:unpack script
And the whole code:

#!/usr/bin/env python
#
# simple python script for extracting mostly used types of archives
# this script extracts .tar, .tar.gz, .tar.bz2, .gz and .zip archives
#

import sys                                # required for fetching command line arguments
import os                                 # required for calling commands for archive extracting

def unpack(s):                             # this is definition of depack
  if (s.find('.tar.gz') != -1):           #   function. It takes string
     os.system("tar -xvvzf " + filename)  #   filename as argument.
  elif (s.find('.tar.bz2') != -1):        #   functon than calls
     os.system("tar -xvvjf " + filename)  #   appropriate command according
  elif (s.find('.tar') != -1):            #   to file extension
     os.system("tar -xvvf " + filename)
  elif (s.find('.gz') != -1):
     os.system("gunzip" + filename)
  elif (s.find('.zip') != -1):
     os.system("unzip " + filename)
  else: print "Wrong archive or filename" # other types not supported

try:                                       # this is main program
  filename = sys.argv[1]                  #   first argument right after
  unpack(filename)                        #   'unpack' command goes in the
except IndexError:                         #   filename string
  print "Filename is invalid!"            #   than the depack function is called

# try-except block is used for handling IndexError exception if no argument is passed

Usage:
./unpack [archive filename]