python optparse, default values for optional options

This is more like a code design question. what are good default values for optional options that are of type string/directory/fullname of files?

Let us say I have code like this:

import optparse
parser = optparse.OptionParser()
parser.add_option('-i', '--in_dir', action = "store", default = 'n', help = 'this is an optional arg')
(options, args) = parser.parse_args()  

Then I do:

if options.in_dir == 'n':
    print 'the user did not pass any value for the in_dir option'
else:
    print 'the user in_dir=%s' %(options.in_dir)

Basically I want to have default values that mean the user did not input such option versus the actual value. Using 'n' was arbitrary, is there a better recommendation?