6.20.2.3 Setting default values
All of the above examples involve setting some variable (the ``destination'') when certain
command-line options are seen. What happens if those options are never seen? Since we didn't
supply any defaults, they are all set to None. Sometimes, this is just fine
(which is why it's the default), but sometimes, you want more control. To address that need, optparse lets you supply a default value for each destination, which is
assigned before the command-line is parsed.
First, consider the verbose/quiet example. If we want optparse to
set verbose to True unless -q
is seen, then we can do this:
parser.add_option("-v", action="store_true", dest="verbose", default=True)
parser.add_option("-q", action="store_false", dest="verbose")
Oddly enough, this is exactly equivalent:
parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose", default=True)
Those are equivalent because you're supplying a default value for the option's destination,
and these two options happen to have the same destination (the verbose
variable).
Consider this:
parser.add_option("-v", action="store_true", dest="verbose", default=False)
parser.add_option("-q", action="store_false", dest="verbose", default=True)
Again, the default value for verbose will be True: the
last default value supplied for any particular destination is the one that counts.
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