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