# optparse --- Parser for command line options — Default values

> All of the above examples involve setting some variable (the "destination") when certain command-line options are seen.

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## Knowledge

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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)

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 ~OptionParser.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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