[Tutor] checking input parameters

[Peter Otten]

Bala subramanian wrote:


> I use the following way to check for the input parameters. I would
> like to know if there is a any better way to show or describe the
> script usage. So when the user just runs it without any input params.,
> the program shd not execute but just shows the documentation.
> 
> from sys import argv
> if not argv[1:]:
>       print 'Program usage: python script file-list1 file-list2'
>       print 'file list should contain the names of the atom list'
>       --------
>       --------

I may look like a lot of overhead at first, but in the long run it will pay 
off if you use argparse:

$ cat argparse_demo.py

def main():
    import argparse
    parser = argparse.ArgumentParser(
        description="what the script is supposed to do")
    parser.add_argument(
        "file_list1", metavar="file-list1",
        help="your help text for file-list1")
    parser.add_argument(
        "file_list2", metavar="file-list2",
        help="your help text for file-list2")
    args = parser.parse_args()

    print "Do something with {!r} and {!r}".format(
        args.file_list1, args.file_list2)

if __name__ == "__main__":
    main()
$ python argparse_demo.py 
usage: argparse_demo.py [-h] file-list1 file-list2
argparse_demo.py: error: too few arguments
$ python argparse_demo.py -h
usage: argparse_demo.py [-h] file-list1 file-list2

what the script is supposed to do

positional arguments:
  file-list1  your help text for file-list1
  file-list2  your help text for file-list2

optional arguments:
  -h, --help  show this help message and exit
$ python argparse_demo.py alpha.txt beta.txt
Do something with 'alpha.txt' and 'beta.txt'


For more see the documentation at 
http://docs.python.org/howto/argparse.html#id1