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6.21.2.5 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`. 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.