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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