无法在Python中解析命令行参数

Question

I am trying to create a script that takes command line argument and based on the input, it calls out to relevant function. Here is how my main function looks like:

from lib.updatefeed import gather
#stdlib imports
import argparse



def main():
    print "test"
    parser = argparse.ArgumentParser(formatter_class=argparse.RawTextHelpFormatter)
    parser.add_argument('-ip', type=str, nargs='+', help="Search for a single IP")

    parser.add_argument('-list', type=str, nargs='?', help="Search for a list of IP")
    parser.add_argument('-update', type=str, nargs='?', help='Update the local storage')
    args = parser.parse_args()
    if args.ip:
        if len(args.ip) > 4:
            print "Too many"
            sys.exit(1)
        parse_ip(args.ip)
    if args.list:
        parse_ipList(list)
    if args.update:
        print "updating"
        gather()

if __name__ == '__main__':
    main()

All other Arguments are working fine and the respective functions are being called. The only problem is with the "update" argument. For some reason, the gather() function is not getting called when the -update arg is passed. I have also added a print statement before the function call but that is also not getting printed either.

Can anyone help me in identifying the root cause.

Here is a part of my gather function as well:

def gather(self):
    if not os.path.exists('New'):
        os.mkdir('New')
    print "Starting feed update process"

Answer 1

parser.add_argument('-update', type=str, nargs='?', help='Update the local storage')

declares the option -update as taking a single optional argument ( nargs='?' ); the value of the option will be either the argument, if supplied, or the value of the default key. However, you don't provide a default key, and the default default is None .

So if you just provide the command-line option -update with no argument, then the values of args.update will be None , and the test:

if args.update:
        print "updating"
        gather()

will fail, so nothing will be done.

Apparently, you only care whether -update is present in the command-line, so it should not take any arguments. To handle this case, define the option as having the action store_true , and leave out the type and nargs parameters:

parser.add_argument('-update', action='store_true', help='Update the local storage')