In Python argparse, is it possible to have paired --no-something/--something arguments?

Well, none of the answers so far are quite satisfactory for a variety of reasons. So here is my own answer:

class ActionNoYes(argparse.Action):
    def __init__(self, opt_name, dest, default=True, required=False, help=None):
        super(ActionNoYes, self).__init__(['--' + opt_name, '--no-' + opt_name], dest, nargs=0, const=None, default=default, required=required, help=help)
    def __call__(self, parser, namespace, values, option_string=None):
        if option_string.starts_with('--no-'):
            setattr(namespace, self.dest, False)
        else:
            setattr(namespace, self.dest, True)

And an example of use:

>>> p = argparse.ArgumentParser()
>>> p._add_action(ActionNoYes('foo', 'foo', help="Do (or do not) foo. (default do)"))
ActionNoYes(option_strings=['--foo', '--no-foo'], dest='foo', nargs=0, const=None, default=True, type=None, choices=None, help='Do (or do not) foo. (default do)', metavar=None)
>>> p.parse_args(['--no-foo', '--foo', '--no-foo'])
Namespace(foo=False)
>>> p.print_help()
usage: -c [-h] [--foo]

optional arguments:
  -h, --help       show this help message and exit
  --foo, --no-foo  Do (or do not) foo. (default do)

Unfortunately, the _add_action member function isn't documented, so this isn't 'official' in terms of being supported by the API. Also, Action is mainly a holder class. It has very little behavior on its own. It would be nice if it were possible to use it to customize the help message a bit more. For example saying --[no-]foo at the beginning. But that part is auto-generated by stuff outside the Action class.

I modified the solution of @Omnifarious to make it more like the standard actions:

import argparse

class ActionNoYes(argparse.Action):
    def __init__(self, option_strings, dest, default=None, required=False, help=None):

        if default is None:
            raise ValueError('You must provide a default with Yes/No action')
        if len(option_strings)!=1:
            raise ValueError('Only single argument is allowed with YesNo action')
        opt = option_strings[0]
        if not opt.startswith('--'):
            raise ValueError('Yes/No arguments must be prefixed with --')

        opt = opt[2:]
        opts = ['--' + opt, '--no-' + opt]
        super(ActionNoYes, self).__init__(opts, dest, nargs=0, const=None, 
                                          default=default, required=required, help=help)
    def __call__(self, parser, namespace, values, option_strings=None):
        if option_strings.startswith('--no-'):
            setattr(namespace, self.dest, False)
        else:
            setattr(namespace, self.dest, True)

You can add the Yes/No argument as you would add any standard option. You just need to pass ActionNoYes class in the action argument:

parser = argparse.ArgumentParser()
parser.add_argument('--foo', action=ActionNoYes, default=False)

Now when you call it:

>> args = parser.parse_args(['--foo'])
Namespace(foo=True)
>> args = parser.parse_args(['--no-foo'])
Namespace(foo=False)
>> args = parser.parse_args([])
Namespace(foo=False)  

Does the add_mutually_exclusive_group() of argparse help?

parser = argparse.ArgumentParser()
exclusive_grp = parser.add_mutually_exclusive_group()
exclusive_grp.add_argument('--foo', action='store_true', help='do foo')
exclusive_grp.add_argument('--no-foo', action='store_true', help='do not do foo')
args = parser.parse_args()

print 'Starting program', 'with' if args.foo else 'without', 'foo'
print 'Starting program', 'with' if args.no_foo else 'without', 'no_foo'

Here's how it looks when run:

./so.py --help
usage: so.py [-h] [--foo | --no-foo]

optional arguments:
  -h, --help  show this help message and exit
  --foo       do foo
  --no-foo    do not do foo

./so.py
Starting program without foo
Starting program without no_foo

./so.py --no-foo --foo
usage: so.py [-h] [--foo | --no-foo]
so.py: error: argument --foo: not allowed with argument --no-foo

This is different from the following in the mutually exclusive group allows neither option in your program (and I'm assuming that you want options because of the -- syntax). This implies one or the other:

parser.add_argument('--foo=', choices=('y', 'n'), default='y',
                    help="Do foo? (default y)")

If these are required (non-optional), maybe using add_subparsers() is what you're looking for.

Update 1

Logically different, but maybe cleaner:

...
exclusive_grp.add_argument('--foo', action='store_true', dest='foo', help='do foo')
exclusive_grp.add_argument('--no-foo', action='store_false', dest='foo', help='do not do foo')
args = parser.parse_args()

print 'Starting program', 'with' if args.foo else 'without', 'foo'

And running it:

./so.py --foo
Starting program with foo
./so.py --no-foo
Starting program without foo
./so.py
Starting program without foo