Argparse: Required arguments listed under "optional arguments"?

Question

I use the following simple code to parse some arguments; note that one of them is required. Unfortunately, when the user runs the script without providing the argument, the displayed usage/help text does not indicate that there is a non-optional argument, which I find very confusing. How can I get python to indicate that an argument is not optional?

Here is the code:

import argparse
if __name__ == '__main__':
    parser = argparse.ArgumentParser(
        description='Foo')
    parser.add_argument('-i','--input', help='Input file name', required=True)
    parser.add_argument('-o','--output', help='Output file name', default="stdout")
    args = parser.parse_args()
    print ("Input file: %s" % args.input )
    print ("Output file: %s" % args.output )

When running above code without providing the required argument, I get the following output:

usage: foo.py [-h] -i INPUT [-o OUTPUT]

Foo

optional arguments:
    -h, --help            show this help message and exit
    -i INPUT, --input INPUT
                          Input file name
    -o OUTPUT, --output OUTPUT
                          Output file name

Answer 1

Parameters starting with - or -- are usually considered optional. All other parameters are positional parameters and as such required by design (like positional function arguments). It is possible to require optional arguments, but this is a bit against their design. Since they are still part of the non-positional arguments, they will still be listed under the confusing header “optional arguments” even if they are required. The missing square brackets in the usage part however show that they are indeed required.

See also the documentation:

In general, the argparse module assumes that flags like -f and --bar indicate optional arguments, which can always be omitted at the command line.

Note: Required options are generally considered bad form because users expect options to be optional, and thus they should be avoided when possible.

That being said, the headers “positional arguments” and “optional arguments” in the help are generated by two argument groups in which the arguments are automatically separated into. Now, you could “hack into it” and change the name of the optional ones, but a far more elegant solution would be to create another group for “required named arguments” (or whatever you want to call them):

parser = argparse.ArgumentParser(description='Foo')
parser.add_argument('-o', '--output', help='Output file name', default='stdout')
requiredNamed = parser.add_argument_group('required named arguments')
requiredNamed.add_argument('-i', '--input', help='Input file name', required=True)
parser.parse_args(['-h'])

usage: [-h] [-o OUTPUT] -i INPUT

Foo

optional arguments:
  -h, --help            show this help message and exit
  -o OUTPUT, --output OUTPUT
                        Output file name

required named arguments:
  -i INPUT, --input INPUT
                        Input file name

Answer 2

Since I prefer to list required arguments before optional, I hack around it via:

parser = argparse.ArgumentParser()
parser._action_groups.pop()
required = parser.add_argument_group('required arguments')
optional = parser.add_argument_group('optional arguments')
required.add_argument('--required_arg', required=True)
optional.add_argument('--optional_arg')
return parser.parse_args()

and this outputs:

usage: main.py [-h] --required_arg REQUIRED_ARG [--optional_arg OPTIONAL_ARG]

required arguments:
  --required_arg REQUIRED_ARG

optional arguments:
  --optional_arg OPTIONAL_ARG

I can live without -h, --help showing up in the optional arguments group.

Answer 3

Building off of @Karl Rosaen

parser = argparse.ArgumentParser()
optional = parser._action_groups.pop() # Edited this line
required = parser.add_argument_group('required arguments')
# remove this line: optional = parser...
required.add_argument('--required_arg', required=True)
optional.add_argument('--optional_arg')
parser._action_groups.append(optional) # added this line
return parser.parse_args()

and this outputs:

usage: main.py [-h] [--required_arg REQUIRED_ARG]
           [--optional_arg OPTIONAL_ARG]

required arguments:
  --required_arg REQUIRED_ARG

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

Answer 4

One more time, building off of @RalphyZ

This one doesn't break the exposed API.

from argparse import ArgumentParser, SUPPRESS
# Disable default help
parser = ArgumentParser(add_help=False)
required = parser.add_argument_group('required arguments')
optional = parser.add_argument_group('optional arguments')

# Add back help 
optional.add_argument(
    '-h',
    '--help',
    action='help',
    default=SUPPRESS,
    help='show this help message and exit'
)
required.add_argument('--required_arg', required=True)
optional.add_argument('--optional_arg')

Which will show the same as above and should survive future versions:

usage: main.py [-h] [--required_arg REQUIRED_ARG]
           [--optional_arg OPTIONAL_ARG]

required arguments:
  --required_arg REQUIRED_ARG

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

Answer 5

You don't need to override the optional group.

Just do:

parser = argparse.ArgumentParser()
required = parser.add_argument_group('required arguments')
required.add_argument('--required_arg', required=True)
# All arguments set via parser directly will automatically go to the optional group
parser.add_argument('--optional_arg')
parser.print_help()

will print out

usage: [-h] --required_arg REQUIRED_ARG [--optional_arg OPTIONAL_ARG]

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

required arguments:
  --required_arg REQUIRED_ARG

If you wish to have required arguments before optional, you can do the following:

parser = argparse.ArgumentParser()
optional = parser._action_groups.pop()
required = parser.add_argument_group('required arguments')
parser._action_groups.append(optional)
required.add_argument('--required_arg', required=True)
optional.add_argument('--optional_arg')
parser.print_help()

that will print groups in the correct order:

usage: [-h] --required_arg REQUIRED_ARG [--optional_arg OPTIONAL_ARG]

required arguments:
  --required_arg REQUIRED_ARG

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

Answer 6

NB: per Christophe Vu-Brugier, the code below does not work starting from Python version 3.10.

by default there're 2 argument groups in parser._action_groups: positional arguments and named arguments (titled 'optional arguments'). you can add your named optional arguments to the existing 'optional arguments' group, and required named arguments to a new 'required arguments' group. After that you can re-order groups:

import argparse

parser = argparse.ArgumentParser(description='Foo')

required = parser.add_argument_group('required arguments')

required.add_argument('-i','--input', help='Input file name', required=True)
parser.add_argument('-o','--output', help='Output file name', default="stdout")

groups_order = {
    'positional arguments': 0,
    'required arguments': 1,
    'optional arguments': 2
}
parser._action_groups.sort(key=lambda g: groups_order[g.title])

parser.parse_args(['-h'])

output:

usage: argparse_argument_groups.py [-h] -i INPUT [-o OUTPUT]

Foo

required arguments:
  -i INPUT, --input INPUT
                        Input file name

optional arguments:
  -h, --help            show this help message and exit
  -o OUTPUT, --output OUTPUT
                        Output file name

Answer 7

The required arguments are usually "positional" arguments when using argparse.

    parser = argparse.ArgumentParser(description="Foo")
    parser.add_argument("username")
    parser.add_argument("password")

This will add two positional arguments which are required.