I have a requirement as follows:

./xyifier --prox --lport lport --rport rport

for the argument prox , I use action=’store_true’ to check if it is present or not. I do not require any of the arguments. But, if –prox is set I require rport and lport as well. Is there an easy way of doing this with argparse without writing custom conditional coding.

More Code:

non_int.add_argument('--prox', action='store_true', help='Flag to turn on proxy')
non_int.add_argument('--lport', type=int, help='Listen Port.')
non_int.add_argument('--rport', type=int, help='Proxy port.')

No, there isn’t any option in argparse to make mutually inclusive sets of options.

The simplest way to deal with this would be:

if args.prox and (args.lport is None or args.rport is None):
    parser.error("--prox requires --lport and --rport.")

You’re talking about having conditionally required arguments. Like @borntyping said you could check for the error and do parser.error(), or you could just apply a requirement related to --prox when you add a new argument.

A simple solution for your example could be:

non_int.add_argument('--prox', action='store_true', help='Flag to turn on proxy')
non_int.add_argument('--lport', required='--prox' in sys.argv, type=int)
non_int.add_argument('--rport', required='--prox' in sys.argv, type=int)

This way required receives either True or False depending on whether the user as used --prox. This also guarantees that -lport and -rport have an independent behavior between each other.

How about using parser.parse_known_args() method and then adding the --lport and --rport args as required args if --prox is present.

# just add --prox arg now
non_int = argparse.ArgumentParser(description="stackoverflow question", 
                                  usage="%(prog)s [-h] [--prox --lport port --rport port]")
non_int.add_argument('--prox', action='store_true', 
                     help='Flag to turn on proxy, requires additional args lport and rport')
opts, rem_args = non_int.parse_known_args()
if opts.prox:
    non_int.add_argument('--lport', required=True, type=int, help='Listen Port.')
    non_int.add_argument('--rport', required=True, type=int, help='Proxy port.')
    # use options and namespace from first parsing
    non_int.parse_args(rem_args, namespace = opts)

Also keep in mind that you can supply the namespace opts generated after the first parsing while parsing the remaining arguments the second time. That way, in the the end, after all the parsing is done, you’ll have a single namespace with all the options.

Drawbacks:

• If --prox is not present the other two dependent options aren’t even present in the namespace. Although based on your use-case, if --prox is not present, what happens to the other options is irrelevant.
• Need to modify usage message as parser doesn’t know full structure
• --lport and --rport don’t show up in help message

Do you use lport when prox is not set. If not, why not make lport and rport arguments of prox? e.g.

parser.add_argument('--prox', nargs=2, type=int, help='Prox: listen and proxy ports')

That saves your users typing. It is just as easy to test if args.prox is not None: as if args.prox:.

The accepted answer worked great for me! Since all code is broken without tests here is how I tested the accepted answer. parser.error() does not raise an argparse.ArgumentError error it instead exits the process. You have to test for SystemExit.

with pytest

import pytest
from . import parse_arguments  # code that rasises parse.error()


def test_args_parsed_raises_error():
    with pytest.raises(SystemExit):
        parse_arguments(["argument that raises error"])

with unittests

from unittest import TestCase
from . import parse_arguments  # code that rasises parse.error()

class TestArgs(TestCase):

    def test_args_parsed_raises_error():
        with self.assertRaises(SystemExit) as cm:
            parse_arguments(["argument that raises error"])

inspired from: Using unittest to test argparse – exit errors

Is it possible to add an Argument to an python argparse.ArgumentParser without it showing up in the usage or help (script.py --help)?

Yes, you can set the help option to add_argument to argparse.SUPPRESS. Here’s an example from the argparse documentation:

>>> parser = argparse.ArgumentParser(prog='frobble')
>>> parser.add_argument('--foo', help=argparse.SUPPRESS)
>>> parser.print_help()
usage: frobble [-h]

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

I do it by adding an option to enable the hidden ones, and grab that by looking at sysv.args.

If you do this, you have to include the special arg you pick out of sys.argv directly in the parse list if you Assume the option is -s to enable hidden options.

parser.add_argument('-a', '-axis',
                    dest="axis", action="store_true", default=False,
                    help="Rotate the earth")
if "-s" in sys.argv or "-secret" in sys.argv:
    parser.add_argument('-s', '-secret',
                        dest="secret", action="store_true", default=False,
                        help="Enable secret options")
    parser.add_argument('-d', '-drill',
                        dest="drill", action="store_true", default=False,
                        help="drill baby, drill")

I would like to check whether an optional argparse argument has been set by the user or not.

Can I safely check using isset?

Something like this:

if(isset(args.myArg)):
    #do something
else:
    #do something else

Does this work the same for float / int / string type arguments?

I could set a default parameter and check it (e.g., set myArg = -1, or “” for a string, or “NOT_SET”). However, the value I ultimately want to use is only calculated later in the script. So I would be setting it to -1 as a default, and then updating it to something else later. This seems a little clumsy in comparison with simply checking if the value was set by the user.

I think that optional arguments (specified with --) are initialized to None if they are not supplied. So you can test with is not None. Try the example below:

import argparse as ap

def main():
    parser = ap.ArgumentParser(description="My Script")
    parser.add_argument("--myArg")
    args, leftovers = parser.parse_known_args()

    if args.myArg is not None:
        print "myArg has been set (value is %s)" % args.myArg

As @Honza notes is None is a good test. It’s the default default, and the user can’t give you a string that duplicates it.

You can specify another default='mydefaultvalue, and test for that. But what if the user specifies that string? Does that count as setting or not?

You can also specify default=argparse.SUPPRESS. Then if the user does not use the argument, it will not appear in the args namespace. But testing that might be more complicated:

args.foo # raises an AttributeError
hasattr(args, 'foo')  # returns False
getattr(args, 'foo', 'other') # returns 'other'

Internally the parser keeps a list of seen_actions, and uses it for ‘required’ and ‘mutually_exclusive’ testing. But it isn’t available to you out side of parse_args.

I think using the option default=argparse.SUPPRESS makes most sense. Then, instead of checking if the argument is not None, one checks if the argument is in the resulting namespace.

Example:

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("--foo", default=argparse.SUPPRESS)
ns = parser.parse_args()

print("Parsed arguments: {}".format(ns))
print("foo in namespace?: {}".format("foo" in ns))

Usage:

$ python argparse_test.py --foo 1
Parsed arguments: Namespace(foo='1')
foo in namespace?: True

$ python argparse_test.py
Parsed arguments: Namespace()
foo in namespace?: False

You can check an optionally passed flag with store_true and store_false argument action options:

import argparse

argparser = argparse.ArgumentParser()
argparser.add_argument('-flag', dest='flag_exists', action='store_true')

print argparser.parse_args([])
# Namespace(flag_exists=False)
print argparser.parse_args(['-flag'])
# Namespace(flag_exists=True)

This way, you don’t have to worry about checking by conditional is not None. You simply check for True or False. Read more about these options in the docs here

If your argument is positional (ie it doesn’t have a “-” or a “–” prefix, just the argument, typically a file name) then you can use the nargs parameter to do this:

parser = argparse.ArgumentParser(description='Foo is a program that does things')
parser.add_argument('filename', nargs='?')
args = parser.parse_args()

if args.filename is not None:
    print('The file name is {}'.format(args.filename))
else:
    print('Oh well ; No args, no problems')

Here is my solution to see if I am using an argparse variable

import argparse

ap = argparse.ArgumentParser()
ap.add_argument("-1", "--first", required=True)
ap.add_argument("-2", "--second", required=True)
ap.add_argument("-3", "--third", required=False) 
# Combine all arguments into a list called args
args = vars(ap.parse_args())
if args["third"] is not None:
# do something

This might give more insight to the above answer which I used and adapted to work for my program.

In order to address @kcpr’s comment on the (currently accepted) answer by @Honza Osobne

Unfortunately it doesn’t work then the argument got it’s default value defined.

one can first check if the argument was provided by comparing it with the Namespace object abd providing the default=argparse.SUPPRESS option (see @hpaulj’s and @Erasmus Cedernaes answers and this python3 doc) and if it hasn’t been provided, then set it to a default value.

import argparse

parser = argparse.ArgumentParser()
parser.add_argument('--infile', default=argparse.SUPPRESS)
args = parser.parse_args()
if 'infile' in args: 
    # the argument is in the namespace, it's been provided by the user
    # set it to what has been provided
    theinfile = args.infile
    print('argument \'--infile\' was given, set to {}'.format(theinfile))
else:
    # the argument isn't in the namespace
    # set it to a default value
    theinfile = 'your_default.txt'
    print('argument \'--infile\' was not given, set to default {}'.format(theinfile))

Usage

$ python3 testargparse_so.py
argument '--infile' was not given, set to default your_default.txt

$ python3 testargparse_so.py --infile user_file.txt
argument '--infile' was given, set to user_file.txt

Very simple, after defining args variable by ‘args = parser.parse_args()’ it contains all data of args subset variables too. To check if a variable is set or no assuming the ‘action=”store_true” is used…

if args.argument_name:
   # do something
else:
   # do something else

I have created a script using argparse.

The script needs to take a configuration file name as an option, and user can specify whether they need to proceed totally the script or only simulate it.

The args to be passed: ./script -f config_file -s or ./script -f config_file.

It’s ok for the -f config_file part, but It keeps asking me for arguments for the -s which is optionnal and should not be followed by any.

I have tried this:

parser = argparse.ArgumentParser()
parser.add_argument('-f', '--file')
#parser.add_argument('-s', '--simulate', nargs = '0')
args = parser.parse_args()
if args.file:
    config_file = args.file
if args.set_in_prod:
        simulate = True
else:
    pass

With the following errors:

File "/usr/local/lib/python2.6/dist-packages/argparse.py", line 2169, in _get_nargs_pattern
nargs_pattern = '(-*%s-*)' % '-*'.join('A' * nargs)
TypeError: can't multiply sequence by non-int of type 'str'

And same errror with '' instead of 0.

As @Felix Kling suggested use action='store_true':

>>> from argparse import ArgumentParser
>>> p = ArgumentParser()
>>> _ = p.add_argument('-f', '--foo', action='store_true')
>>> args = p.parse_args()
>>> args.foo
False
>>> args = p.parse_args(['-f'])
>>> args.foo
True

To create an option that needs no value, set the action ^[docs] of it to 'store_const', 'store_true' or 'store_false'.

Example:

parser.add_argument('-s', '--simulate', action='store_true')

The title pretty much summarizes what I’d like to have happen.

Here is what I have, and while the program doesn’t blow up on a nonpositive integer, I want the user to be informed that a nonpositive integer is basically nonsense.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("-g", "--games", type=int, default=162,
                    help="The number of games to simulate")
args = parser.parse_args()

And the output:

python simulate_many.py -g 20
Setting up...
Playing games...
....................

Output with a negative:

python simulate_many.py -g -2
Setting up...
Playing games...

Now, obviously I could just add an if to determine if args.games is negative, but I was curious if there was a way to trap it at the argparse level, so as to take advantage of the automatic usage printing.

Ideally, it would print something similar to this:

python simulate_many.py -g a
usage: simulate_many.py [-h] [-g GAMES] [-d] [-l LEAGUE]
simulate_many.py: error: argument -g/--games: invalid int value: 'a'

Like so:

python simulate_many.py -g -2
usage: simulate_many.py [-h] [-g GAMES] [-d] [-l LEAGUE]
simulate_many.py: error: argument -g/--games: invalid positive int value: '-2'

For now I’m doing this, and I guess I’m happy:

if args.games <= 0:
    parser.print_help()
    print "-g/--games: must be positive."
    sys.exit(1)

This should be possible utilizing type. You’ll still need to define an actual method that decides this for you:

def check_positive(value):
    ivalue = int(value)
    if ivalue <= 0:
        raise argparse.ArgumentTypeError("%s is an invalid positive int value" % value)
    return ivalue

parser = argparse.ArgumentParser(...)
parser.add_argument('foo', type=check_positive)

This is basically just an adapted example from the perfect_square function in the docs on argparse.

type would be the recommended option to handle conditions/checks, as in Yuushi’s answer.

In your specific case, you can also use the choices parameter if your upper limit is also known:

parser.add_argument('foo', type=int, choices=xrange(5, 10))

Note: Use range instead of xrange for python 3.x

The quick and dirty way, if you have a predictable max as well as min for your arg, is use choices with a range

parser.add_argument('foo', type=int, choices=xrange(0, 1000))

A simpler alternative, especially if subclassing argparse.ArgumentParser, is to initiate the validation from inside the parse_args method.

Inside such a subclass:

def parse_args(self, args=None, namespace=None):
    """Parse and validate args."""
    namespace = super().parse_args(args, namespace)
    if namespace.games <= 0:
         raise self.error('The number of games must be a positive integer.')
    return namespace

This technique may not be as cool as a custom callable, but it does the job.

About ArgumentParser.error(message):

This method prints a usage message including the message to the standard error and terminates the program with a status code of 2.

Credit: answer by jonatan

In case someone (like me) comes across this question in a Google search, here is an example of how to use a modular approach to neatly solve the more general problem of allowing argparse integers only in a specified range:

# Custom argparse type representing a bounded int
class IntRange:

    def __init__(self, imin=None, imax=None):
        self.imin = imin
        self.imax = imax

    def __call__(self, arg):
        try:
            value = int(arg)
        except ValueError:
            raise self.exception()
        if (self.imin is not None and value < self.imin) or (self.imax is not None and value > self.imax):
            raise self.exception()
        return value

    def exception(self):
        if self.imin is not None and self.imax is not None:
            return argparse.ArgumentTypeError(f"Must be an integer in the range [{self.imin}, {self.imax}]")
        elif self.imin is not None:
            return argparse.ArgumentTypeError(f"Must be an integer >= {self.imin}")
        elif self.imax is not None:
            return argparse.ArgumentTypeError(f"Must be an integer <= {self.imax}")
        else:
            return argparse.ArgumentTypeError("Must be an integer")

This allows you to do something like:

parser = argparse.ArgumentParser(...)
parser.add_argument('foo', type=IntRange(1))     # Must have foo >= 1
parser.add_argument('bar', type=IntRange(1, 7))  # Must have 1 <= bar <= 7

The variable foo now allows only positive integers, like the OP asked.

Note that in addition to the above forms, just a maximum is also possible with IntRange:

parser.add_argument('other', type=IntRange(imax=10))  # Must have other <= 10