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?

• ecatmur: If you want inputFile and` inputString to be required arguments, why are you setting nargs='?'?
• jonoave: Ok, I tried removing nargs and it worked! I was using another example and the python docs specify nargs as consuming 1 argument. I used it so that args.inputFile won't consume both the names given for inputFile and inputString. So is it correct now to say that, you only use nargs for optional arguments? Thanks a lot btw!
• ecatmur: You can also use nargs='*' or nargs='+' for multiple arguments; see my answer below and the documentation.

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?

• jonoave: oh right, I missed the last two positional arguments stated in the usage [-h]. Sorry, my bad. Thanks for pointing it out. But I still have problems with the script running without a 2nd positional argument.
• jonoave: By the way is there a way to rearrange help display so that it will show usage blah.py [inputFile] [inputString] [option1].....? Thanks!
• jonoave: Sorry, I saw your reply before the comment on my question. The reply by @ecatmur on removing nargs solved my problem. Basically it was that the script could run with python blah.py inputFile without inputString given.

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).