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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. This is usually fine, but sometimes you
want more control. 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")
Since default values apply to the destination rather than to any particular option, and these two options happen to have the same destination, this is exactly equivalent:
parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose", default=True)
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.
A clearer way to specify default values is the set_defaults() method of OptionParser, which you can call at any time before calling parse_args():
parser.set_defaults(verbose=True) parser.add_option(...) (options, args) = parser.parse_args()
As before, the last value specified for a given option destination is the one that counts. For clarity, try to use one method or the other of setting default values, not both.
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