Tip #1. Using command line arguments.

Command_Line_Arguments.py

#!/usr/bin/python

"""
$ test.py -h
usage: test.py -i <inputfile> -o <outputfile>

$ test.py -i BMP -o
usage: test.py -i <inputfile> -o <outputfile>

$ test.py -i inputfile
Input file is " inputfile
Output file is "
"""

import sys, getopt

def main(argv):
    inputfile = ''
    outputfile = ''
    try:
        opts, args = getopt.getopt(argv,"hi:o:",["ifile=","ofile="])
    except getopt.GetoptError:
        print 'test.py -i <inputfile> -o <outputfile>'
        sys.exit(2)
    for opt, arg in opts:
        if opt == '-h':
            print 'test.py -i <inputfile> -o <outputfile>'
            sys.exit()
        elif opt in ("-i", "--ifile"):
            inputfile = arg
        elif opt in ("-o", "--ofile"):
            outputfile = arg
    print 'Input file is "', inputfile
    print 'Output file is "', outputfile

if __name__ == "__main__":
    main(sys.argv[1:])