argparse doesn't check for positional arguments

Question

I'm creating a script that takes both positional and optional arguments with argparse. I have gone through Doug's tutorial and the python Docs but can't find an answer.

parser = argparse.ArgumentParser(description='script to run')
parser.add_argument('inputFile', nargs='?', type=argparse.FileType('rt'),
parser.add_argument('inputString', action='store', nargs='?') 
parser.add_argument('-option1', metavar='percent', type=float, action='store')
parser.add_argument('-option2', metavar='outFile1', type=argparse.FileType('w'),
parser.add_argument('-option3', action='store', default='<10',
args = parser.parse_args()
# rest of script.... blah blah

As you can see, I want 2 positional and 3 optional arguments. However, when I try to run it in the terminal, it doesn't check for the positionals! If I try: python script.py inputfile it will run normally and output error halfway through the script when it cannot find a value for inputString. If I try: python script.py xxx ; the output is:

usage script.py [-h] [-option1] [-option2] [-option3]

Can anyone explain why it doesn't check for the positional arguments?

Answer 1

Your problem is that you're specifying nargs='?'. From the documentation:

'?'. One argument will be consumed from the command line if possible, and produced as a single item. If no command-line argument is present, the value from default will be produced.

If you leave out the nargs='?' then the argument will be required, and argparse will display an error if it is not provided. A single argument is consumed if action='store' (the default).

You can also specify nargs=1; the difference is that this produces a list containing one item, as opposed to the item itself. See the documentation for more ways you can use nargs.

Answer 2

Works for me.

Code:

#!/usr/bin/python

import argparse

parser=argparse.ArgumentParser(description='script to run')

parser.add_argument('inputFile', nargs='?', type=argparse.FileType('rt'))
parser.add_argument('inputString', action='store', nargs='?') 
parser.add_argument('-option1', metavar='percent', type=float, action='store')
parser.add_argument('-option2', metavar='outFile1', type=argparse.FileType('w'))
parser.add_argument('-option3', action='store', default='<10')
args = parser.parse_args()

Execution:

# ./blah.py -h
usage: blah.py [-h] [-option1 percent] [-option2 outFile1] [-option3 OPTION3]
               [inputFile] [inputString]

script to run

positional arguments:
  inputFile
  inputString

optional arguments:
  -h, --help         show this help message and exit
  -option1 percent
  -option2 outFile1
  -option3 OPTION3

Did you overlook the second line in the argument list?

Answer 3

It works as expected. There is no inputString if you run it as script.py inputfile (only one argument is given, but inputString is the second argument).

narg='?' means that the argument is optional (they are surrounded by [] in the help message).