Python argparse value range help message appearance

Question

I have an argument for a program that is an integer from 1-100 and I just don\'t like the way that it shows up in the -h help message when using argparse (it literally lists 0,

Answer 1

You can customize action, e.g:

#!/usr/bin/env python
import argparse


class Range(argparse.Action):
    def __init__(self, minimum=None, maximum=None, *args, **kwargs):
        self.min = minimum
        self.max = maximum
        kwargs["metavar"] = "[%d-%d]" % (self.min, self.max)
        super(Range, self).__init__(*args, **kwargs)

    def __call__(self, parser, namespace, value, option_string=None):
        if not (self.min <= value <= self.max):
            msg = 'invalid choice: %r (choose from [%d-%d])' % \
                (value, self.min, self.max)
            raise argparse.ArgumentError(self, msg)
        setattr(namespace, self.dest, value)


norse = argparse.ArgumentParser('Norse')
norse.add_argument('--threshold', required=False, type=int, min=0, max=100,
                   action=Range,
                   help='Threshold [%(min)d-%(max)d] denoting at what threat \
                         level to provide additional data on an IP address. \
                         Default is %(default)s.', default=49)
args = norse.parse_args()
print args

Test it:

~: user$ ./test.py --threshold 10
Namespace(threshold=10)
~: user$ ./test.py --threshold -1
usage: Norse [-h] [--threshold [0-100]]
Norse: error: argument --threshold: invalid choice: -1 (choose from [0-100])
~: user$ ./test.py -h
usage: Norse [-h] [--threshold [0-100]]

optional arguments:
  -h, --help           show this help message and exit
  --threshold [0-100]  Threshold [0-100] denoting at what threat level to
                       provide additional data on an IP address. Default is
                       49.

Answer 2

With a custom type, it is easier to control the error message (via the ArgumentTypeError). I still need the metavar to control the usage display.

import argparse

def range_type(astr, min=0, max=101):
    value = int(astr)
    if min<= value <= max:
        return value
    else:
        raise argparse.ArgumentTypeError('value not in range %s-%s'%(min,max))

parser = argparse.ArgumentParser()
norse = parser.add_argument_group('Norse')
...
norse.add_argument('--range', type=range_type, 
    help='Value in range: Default is %(default)s.',
    default=49, metavar='[0-101]')
parser.print_help()
print parser.parse_args()

producing:

2244:~/mypy$ python2.7 stack25295487.py --ran 102
usage: stack25295487.py [-h] [-n] [--threshold [0:101]] [--range [0-101]]

optional arguments:
  -h, --help           show this help message and exit

Norse:
  ...
  --range [0-101]      Value in range: Default is 49.
usage: stack25295487.py [-h] [-n] [--threshold [0:101]] [--range [0-101]]
stack25295487.py: error: argument --range: value not in range 0-101

I could use functools.partial to customize the range values:

type=partial(range_type, min=10, max=90)

Answer 3

Use the metavar parameter of add_argument().

For example:

norse = parser.add_argument_group('Norse')
norse.add_argument('-n', '--norse', required=False, help='Run the Norse IPViking scan.', action='store_true')
norse.add_argument('--threshold', required=False, type=int, choices=range(0,101),
                   metavar="[0-100]", 
                   help='Threshold (0-100) denoting at what threat level to provide additional data on an IP \
                        address. Default is 49.', default=49)

Test:

from argparse import ArgumentParser

norse = ArgumentParser()

norse.add_argument('-n', '--norse', required=False, help='Run the Norse IPViking scan.', action='store_true')
norse.add_argument('--threshold', required=False, type=int, choices=range(0,101), metavar="[0-100]", help='Threshold (0-100) denoting at what threat level to provide additional data on an IP address. Default is 49.', default=49)


norse.print_help()

Results

usage: -c [-h] [-n] [--threshold [0-100]]

optional arguments:
  -h, --help           show this help message and exit
  -n, --norse          Run the Norse IPViking scan.
  --threshold [0-100]  Threshold (0-100) denoting at what threat level to
                       provide additional data on an IP address. Default is
                       49.

Answer 4

Here are a couple ways you can do it instead

def parseCommandArgs():
    parser = argparse.ArgumentParser()
    parser.add_argument('-i', dest='myDest', choices=range(1,101), type=int, required=True, metavar='INT[1,100]', help='my help message')
    return parser.parse_args()

You can also use action instead, which I highly recommend since it allows more customization

def verify():
    class Validity(argparse.Action):
        def __call__(self, parser, namespace, values, option_string=None):
            if values < 1 or values > 100:
                # do something
                pass
    return Validity

def parseCommandArgs():
    parser = argparse.ArgumentParser()
    parser.add_argument('-i', dest='myDest', required=True, metavar='INT[1,100]', help='my help message', action=verify())
    return parser.parse_args()