Argparse with required subparser

Solution 1:

You need to give subparsers a dest.

parser = argparse.ArgumentParser()
subparsers = parser.add_subparsers(dest='cmd')
subparsers.required = True

Now:

1909:~/mypy$ argdev/python3 stack23349349.py
usage: stack23349349.py [-h] {foo} ...
stack23349349.py: error: the following arguments are required: cmd

In order to issue this 'missing arguments' error message, the code needs to give that argument a name. For a positional argument (like subparses), that name is (by default) the 'dest'. There's a (minor) note about this in the SO answer you linked.

One of the few 'patches' to argparse in the last Python release changed how it tests for 'required' arguments. Unfortunately it introduced this bug regarding subparsers. This needs to be fixed in the next release (if not sooner).

update

If you want this optional subparsers behavior in Py2, it looks like the best option is to use a two stage parser as described in

How to Set a Default Subparser using Argparse Module with Python 2.7

There has been some recent activity in the related bug/issue

https://bugs.python.org/issue9253

update

A fix to this is in the works: https://github.com/python/cpython/pull/3027