Python - Circumvent argparse nargs error

Question

I have a program that works like this:

prog.py filename -r

Uses default default values given by me

prog.py filename -r 0 500 20

Uses 0, 500 and 20

I've managed to achieve this using:

class RdistAction(argparse.Action):
    def __call__(self,parser,namespace,values,option_string=None):
        if not values:
            setattr(namespace,self.dest,[0, 1000, 50])
        else:
            setattr(namespace,self.dest,values)

parser = argparse.ArgumentParser()
parser.add_argument("-r", "--rdist", action=RdistAction, nargs='*', type=int)
args = parser.parse_args()

But I want to be stubborn, as my original goal was to have nargs set to 3. But when I use nargs=3 in the above code, I get an error message stating that 3 arguments were expected.

I've googled around, and from the results my gut tells me that I have to add def __init__ and modify something in that function. Is it possible to get the same results I get in the above code when nargs='*', but with nargs=3 instead?

Answer 1

If I change your add_argument line to have nargs='3', I think I get the error you're taking about:

Traceback (most recent call last):
  File "python", line 12, in <module>
ValueError: length of metavar tuple does not match nargs

If I set nargs=3 (without the quotes), then it works for me:

import argparse

class RdistAction(argparse.Action):
    def __call__(self,parser,namespace,values,option_string=None):
      if not values:
        setattr(namespace,self.dest,[0, 1000, 50])
      else:
        setattr(namespace,self.dest,values)

parser = argparse.ArgumentParser()
parser.add_argument("-r", "--rdist", action=RdistAction, nargs=3, type=int)
print parser.parse_args('-r 0 500 20'.split())

gives

Namespace(rdist=[0, 500, 20])

Is that what you're looking for?

The trick here is that if nargs isn't one of the special characters ('?', '*', '+', etc), then it needs to be an integer, not a string.

Note that the documentation does point this out.