Python argparse - mandatory argument - either positional or optional

The mutually exclusive group mechanism can take a required parameter. It also works with one ? positional along with the optionals (flagged arguments). (more than one '?' positional doesn't make sense).

As for the help display there are 2 default groups, positonal and optional. So even if an optional (flagged) is set to required it is, by default, displayed in the optional group. The usage line is a better guide as to whether an argument is required or not. If you don't like the group labels in the help section, define your own argument groups.

In [146]: import argparse
In [147]: parser = argparse.ArgumentParser()
In [148]: gp = parser.add_mutually_exclusive_group(required=True)
In [149]: gp.add_argument('pos', nargs='?', default='foo');
In [150]: gp.add_argument('-f','--foo', default='bar');

In [151]: parser.parse_args('arg'.split())
Out[151]: Namespace(foo='bar', pos='arg')

In [152]: parser.parse_args('-f arg'.split())
Out[152]: Namespace(foo='arg', pos='foo')

In [153]: parser.parse_args('arg -f arg'.split())
usage: ipython3 [-h] [-f FOO] [pos]
ipython3: error: argument -f/--foo: not allowed with argument pos

In [154]: parser.parse_args(''.split())
usage: ipython3 [-h] [-f FOO] [pos]
ipython3: error: one of the arguments pos -f/--foo is required


In [155]: parser.parse_args('-h'.split())
usage: ipython3 [-h] [-f FOO] [pos]

positional arguments:
  pos

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

Oops, usage isn't showing the -f and pos in a mutually exlusive group. Sometimes that usage formatting is brittle.

Switching the order in which the arguments are defined gives a better usage

In [156]: parser = argparse.ArgumentParser()
In [157]: gp = parser.add_mutually_exclusive_group(required=True)
In [158]: gp.add_argument('-f','--foo', default='bar');
In [159]: gp.add_argument('pos', nargs='?', default='foo');
In [160]: 
In [160]: parser.parse_args('-h'.split())
usage: ipython3 [-h] (-f FOO | pos)

positional arguments:
  pos

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

With a user defined argument group:

In [165]: parser = argparse.ArgumentParser()
In [166]: gp = parser.add_argument_group('Mutually exclusive')
In [167]: gpm = gp.add_mutually_exclusive_group(required=True)
In [168]: gpm.add_argument('-f','--foo', default='bar');
In [169]: gpm.add_argument('pos', nargs='?', default='foo');
In [170]: 
In [170]: parser.parse_args('-h'.split())
usage: ipython3 [-h] (-f FOO | pos)

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

Mutually exclusive:
  -f FOO, --foo FOO
  pos

This is the one exception to the general rule argument_groups and mutually_exclusive_groups aren't designed for nesting.

The m-x-group was not required, usage would use []

usage: ipython3 [-h] [-f FOO | pos]