argparse with required subcommands

Question

With python's argparse, how do I make a subcommand a required argument? I want to do this because I want argparse to error out if a subcommand is not specified. I override the error method to print help instead. I have 3-deep nested subcommands, so it's not a matter of simply handling zero arguments at the top level.

In the following example, if this is called like so, I get:

$./simple.py
$

What I want it to do instead is for argparse to complain that the required subcommand was not specified:

import argparse

class MyArgumentParser(argparse.ArgumentParser):
    def error(self, message):
        self.print_help(sys.stderr)
        self.exit(0, '%s: error: %s\n' % (self.prog, message))

def main():
    parser = MyArgumentParser(description='Simple example')
    subs = parser.add_subparsers()
    sub_one = subs.add_parser('one', help='does something')
    sub_two = subs.add_parser('two', help='does something else')

    parser.parse_args()

if __name__ == '__main__':
    main()

Answer 1

There was a change in 3.3 in the error message for required arguments, and subcommands got lost in the dust.

http://bugs.python.org/issue9253#msg186387

There I suggest this work around, setting the required attribute after the subparsers is defined.

parser = ArgumentParser(prog='test')
subparsers = parser.add_subparsers()
subparsers.required = True
subparsers.dest = 'command'
subparser = subparsers.add_parser("foo", help="run foo")
parser.parse_args()

update

A related pull-request: https://github.com/python/cpython/pull/3027

Answer 2

In addition to hpaulj's answer: you can also use the required keyword argument with ArgumentParser.add_subparsers() since Python 3.7. You also need to pass dest as argument. Otherwise you will get an error: TypeError: sequence item 0: expected str instance, NoneType found.

Example file example.py:

import argparse

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='command', required=True)
foo_parser = subparsers.add_parser("foo", help="command foo")
args = parser.parse_args()

Output of the call without an argument:

$ python example.py
usage: example.py [-h] {foo} ...
example.py: error: the following arguments are required: command

Answer 3

How about using required=True? More info here.

Answer 4

You can use the dest argument, which is documented in the last example in the documentation for add_subparsers():

# required_subparser.py
import argparse

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='subparser_name')
one = subparsers.add_parser('one')
two = subparsers.add_parser('two')

args = parser.parse_args()

Running with Python 2.7:

$python required_subparser.py 
usage: required_subparser.py [-h] {one,two} ...
required_subparser.py: error: too few arguments
$python required_subparser.py one
$# no error