'print_usage(file : file = stdout)
Print the usage message for the current program (self.usage) to
\'file\' (default stdout).  Any occurrence of the string "%prog" in
self.usage is replaced with the name of the current program
(basename of sys.argv[0]).  Does nothing if self.usage is empty
or not defined.'

def print_usage(self, file=None):

    if self.usage:
        print >>file, self.get_usage()
