import optparse

class OptionParser (optparse.OptionParser):

    def check_required (self, opt):
      option = self.get_option(opt)

      # Assume che le opzioni predefinite siano impostate a None!
      if getattr(self.values, option.dest) is None:
          self.error("%s option not supplied" % option)


parser = OptionParser()
parser.add_option("-v", action="count", dest="verbose")
parser.add_option("-f", "--file", default=None)
(options, args) = parser.parse_args()

print "verbose:", options.verbose
print "file:", options.file
parser.check_required("-f")