listdiff is just a silly script which merge two list and give you the elements which are not in the first list.

listdiff.py

#!/usr/bin/python
# -*- coding: utf8 -*-
#
# Copyright (c) 2008-2012, Juanje Ojeda Croissier <jojeda@emergya.com>
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this program; if not, write to the
# Free Software Foundation, Inc., 59 Temple Place - Suite 330,
# Boston, MA 02111-1307, USA.

# Authors : Juanje Ojeda Croissier <jojeda@emergya.com>

'''
listdiff is just a silly script which merge two list and give you the elements
which are not in the first list.

The script takes two parameters:
$ listdiff.py file1 file2

file1 and file2 must be paths of a real files in each one should be stored a 
list of element to compare. Each element in a line.

A classic scenario where this is useful is the one where you have a command
who give you a list of things from a system and other (for example the list of
packages installed on two different machines) and you like to see which 
elements do you have in the first, but not in the second one.
'''

__author__ = "Juanje Ojeda Croissier <jojeda@emergya.es>"
__copyright__ = "Copyright 2008-2012, Juanje Ojeda Croissier <jojeda@emergya.com>"
__credits__ = __author__
__license__ = "GPL-3"
__version__ = "0.1"


import sys

def file_to_list(filename):
    """file_to_list(filename) -> set
    
    filename: file path with list of elements inside

    Read all lines from a file and create a set (list) which will
    store all the lines. Each one as one item of the list

    Note: 'set' is a python type for a unordered collections of 
    unique elements
    """
    aux_list = []

    lines = open(filename).readlines()
    for line in lines:
        aux_list.append(line.strip())

    return set(aux_list)


def usage():
    #FIXME: Create this function
    print "How to use it"


def error(msg):
    """Show error message and exit from the script

    msg: This is the message to be showed
    """
    print "Errot: %s" % msg
    sys.exit(1)


if __name__ == '__main__':

    if len(sys.argv) != 3:
        error("Need exactly 2 arguments")

    file_one = sys.argv[1]
    file_two = sys.argv[2]

    #FIXME: Check the files does exist in the filesystem
    list_one = file_to_list(file_one)
    list_two = file_to_list(file_two)

    # Create a new list with the items are just in the first list
    new_list = list_one - list_two
    for item in new_list:
        print item

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