Constructor for class optparse.OptionParser(...) has optional named parameter add_help_option , which defaults to True . You will have to explicitly reject default help option and message, if you want to provide your own.

parser = OptionParser(usage, add_help_option=False)

The bug is that your arguments array includes the name of the script as sys.argv[0]. Thus your if is off-by-one:

# python sys.argv[0] sys.argv[1] sys.argv[2] sys.argv[3]
if len(sys.argv) < 4:
        sys.exit(usage)

This should do what you require, based on your example above:

#!/usr/bin/env python
from optparse import OptionParser

def main():
    usage = "Usage: negative_bin_base.py <eksponentti> <siirre> <figu>"
    parser = OptionParser(usage)
    (options, args) = parser.parse_args()
    if len(args) != 3:
        parser.error("incorrect number of arguments")
    e = args[0]
    b = args[1]
    no = args[2]
    # ...

if __name__ == "__main__":
    main()

For using optparse here s what I normally do:

1. separate command line parsing from your main code base for better modularization. (Put the processing right after "if __name__ == "__main__"")

2. Let optparse handle your help. (Use parser.print_help())

3. Use the optparse constructs, if you don t like them just parse sys.argv yourself. (use add_option to define your necessary options)

Here s how I would write your sample using the optparse library:

def main():
    print  Welcome to the main event! 


if __name__ ==  __main__ :
    import optparse
    parser = optparse.OptionParser()
    parser.add_option("-e", "--eksponentti", dest="eksponentti",
                      help="This is help for <eksponentti>.",
                      default=None)
    parser.add_option("-s", "--siirre", dest="siirre",
                      help="This is help for <siirre>.",
                      default=None)
    parser.add_option("-f", "--figu", dest="figu",
                      help="This is help for <figu>.",
                      default=None)

    (options, args) = parser.parse_args()
    if options.eksponentti and options.siirre and options.figu:
        main()
    else:
        print "ERROR -- Expected Arguments not given!"
        parser.print_help()

Running this without the necessary options will give the following output: (It s a little prettier than I can format it here...)

ERROR -- Expected Arguments not given! Usage: parm.py [options]

Options:   
    -h, --help            show this help message and exit   
    -e EKSPONENTTI, --eksponentti=EKSPONENTTI This is help for <eksponentti>.   
    -s SIIRRE, --siirre=SIIRRE This is help for <siirre>.   
    -f FIGU, --figu=FIGU  This is help for <figu>.