python optparse, default values for optional options

Question

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?

Answer 1

You could use an empty string, "", which Python interprets as being False; you can simply test:

if options.in_dir:
    # argument supplied
else:
    # still empty, no arg

Alternatively, use None:

if options.in_dir is None:
    # no arg
else:
    # arg supplied 

Note that the latter is, per the documentation the default for un-supplied arguments.

Answer 2

How about just None?

Nothing mandates that the default values must be of the same type as the option itself.

import optparse
parser = optparse.OptionParser()
parser.add_option('-i', '--in_dir', default=None, help='this is an optional arg')
(options, args) = parser.parse_args()  
print vars(options)

(ps. action="store" isn't required; store is the default action.)