Python: How to make an option to be required in optparse?

You can implement a required option easily.

parser = OptionParser(usage="usage: %prog [options] arguments")
parser.add_option('-f', '--file', 
                        dest="filename",
                        help='foo help')
(options, args) = parser.parse_args()
if not options.filename:   # if filename is not given
    parser.error('Filename not given')