Explanation for argparse python modul behaviour: Where do the capital placeholders come from?

Question

I am trying to write a command line interface (for the first time) and after reading up about argparse, optparse and getopt I chose argparse because of several recommendations here on SO and elswhere in the net. Adapting a little of the advice of Mr. van Rossum I hooked up my first command line interface like this:

def main(argv=None):
    if argv is None:
        argv = sys.argv
    desc = u'some description'
    parser = argparse.ArgumentParser(description=desc)

    parser.add_argument('-s', '--search', help='Search for someone.')
    parser.add_argument('-c', '--do_something_else', help='Do something else.')

    args = parser.parse_args()
    print args

if __name__ == '__main__':
    sys.exit(main())

Doing python myscript.py -h results in:

usage: dblp.py [-h] [-s SEARCH] [-c DO_SOMETHING_ELSE]

some description

optional arguments:
  -h, --help            show this help message and exit
  -s SEARCH, --search SEARCH
                        Search for someone.
  -c DO_SOMETHING_ELSE, --do_something_else DO_SOMETHING_ELSE
                        Do something else.

So my first question is: Why are SEARCH and DO_SOMETHING_ELSE written in CAPITAL LETTERS? The second question would be: Do I break any standards? Is there a better way (ore a nice real world example I can learn from) how to build clean and useful command line interfaces with python? And are there pitfalls one should avoid, when writing cmd interfaces?

Answer 1

The capital letter items are just value placeholders; they're taken from the destination of the option. You can specify alternative placeholders via the metavar= param of add_argument:

http://docs.python.org/dev/library/argparse.html#metavar