Python argparse: Mutually exclusive required group with a required option

Question

I am trying to have a required mutually exclusive group with one required parameter. Below is the code which I have put

#!/usr/bin/python

import argparse
import sys

# Check for the option provided as part of arguments
def parseArgv():
    parser = argparse.ArgumentParser()
    group = parser.add_mutually_exclusive_group()
    group.add_argument("-v", "--verbose", choices=[1,2,3,4],
            help = "Increase verbosity")
    group.add_argument("-q", "--quiet", action="store_true", help = "Run quietly")
    name = parser.add_mutually_exclusive_group(required=True)
    name.add_argument("-n", "--name", help = "Name of the virtual machine")
    name.add_argument("-t", "--template", help = "Name of the template to use \
            for creating vm. If path is not provided then it will be looked \
            under template directory.")
    parser.add_argument("-s", "--save", help = "Save the machine template. If \
            path is not provided then it will be saved under template directory.");
    #parser.add_argument("-k", "--kick_start", required = True, help = "Name of the \
    #        kick start file. If path is not provided then it will be look into http \
    #        directory.")
    if len(sys.argv) == 1:
        parser.print_help()
    args = parser.parse_args()

if __name__ == '__main__':
    parseArgv()

Now the output of this program as follow

$ python test.py 
usage: test.py [-h] [-v {1,2,3,4} | -q] (-n NAME | -t TEMPLATE) [-s SAVE]

optional arguments:
  -h, --help            show this help message and exit
  -v {1,2,3,4}, --verbose {1,2,3,4}
                        Increase verbosity
  -q, --quiet           Run quietly
  -n NAME, --name NAME  Name of the virtual machine
  -t TEMPLATE, --template TEMPLATE
                        Name of the template to use for creating vm. If path
                        is not provided then it will be looked under template
                        directory.
  -s SAVE, --save SAVE  Save the machine template. If path is not provided
                        then it will be saved under template directory.
usage: test.py [-h] [-v {1,2,3,4} | -q] (-n NAME | -t TEMPLATE) [-s SAVE]
test.py: error: one of the arguments -n/--name -t/--template is required

But if I un-comment the from line 20 - 22 then the output change as below

$ python test.py 
usage: test.py [-h] [-v {1,2,3,4} | -q] (-n NAME | -t TEMPLATE) [-s SAVE] -k
               KICK_START

optional arguments:
  -h, --help            show this help message and exit
  -v {1,2,3,4}, --verbose {1,2,3,4}
                        Increase verbosity
  -q, --quiet           Run quietly
  -n NAME, --name NAME  Name of the virtual machine
  -t TEMPLATE, --template TEMPLATE
                        Name of the template to use for creating vm. If path
                        is not provided then it will be looked under template
                        directory.
  -s SAVE, --save SAVE  Save the machine template. If path is not provided
                        then it will be saved under template directory.
  -k KICK_START, --kick_start KICK_START
                        Name of the kick start file. If path is not provided
                        then it will be look into http directory.
usage: test.py [-h] [-v {1,2,3,4} | -q] (-n NAME | -t TEMPLATE) [-s SAVE] -k
               KICK_START
test.py: error: argument -k/--kick_start is required

But I want that either -n / -t along with -k become mandatory. How to achieve the same.

Solution

You have already achieved it! Argparse only prints the first error it finds, so while it may look like it's only checking -k, it actually recuires -n/-t too. You can see this by actually giving it the -k argument.

If you provide the -k argument, the error message will change from test.py: error: argument -k/--kick_start is required to test.py: error: one of the arguments -n/--name -t/--template is required.

Comments

[Abhinav]

Thanks, I was not knowing it as I am learning Python :). One more thing how to check if argument has some value. Is there some empty(C++) like function.

[Aleksi Torhamo]

You should really ask a separate question if it isn't related (after checking it hasn't been asked already, of course), but... It depends on what exactly you want to do. Python has a concept of "truthiness", so you can just use the variable directly in an if-clause or whatever, eg. if variable: or if not variable:. Aside from that, you can use eg. len(variable) == 0 on containers or variable is None to check that it is None specifically. Or just compare to the empty value for the type in question.

[hpaulj]

The default value for your argparse arguments is None. Thus print args.name is None will print True. Default for args.quiet is False.