Sample code:

#!/usr/bin/env python
import argparse

print("\n\narg=foo, nargs=+")
parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('foo', nargs='+', help='foos', default=['foo1', 'foo2'])
parser.print_help()

print("\n\narg=foo, nargs=*")
parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('foo', nargs='*', help='foos', default=['foo1', 'foo2'])
parser.print_help()

print("\n\narg=--foo, nargs=+")
parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('--foo', nargs='+', help='foos', default=['foo1', 'foo2'])
parser.print_help()

print("\n\narg=--foo, nargs=*")
parser = argparse.ArgumentParser(formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('--foo', nargs='*', help='foos', default=['foo1', 'foo2'])
parser.print_help()


The first example's help text for 'foo' is missing "(default: ['foo1', 'foo2'])". I tested 2.7.2, 2.7.3, 3.2.3.

Looks like this behavior is intentional.  This subclass adds a '%(default)s' string to the help lines if:

    if '%(default)' not in action.help:
        if action.default is not SUPPRESS:
            defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
            if action.option_strings or action.nargs in defaulting_nargs:
                help += ' (default: %(default)s)'

So it adds the default if it is an `optional`, or a `positional` with '*' or '?'.  

A `default` value for a required positional (including '+') does not make sense, since the user input always overwrites the default.  

(I recommend closing this issue).

Closing per paul.j3's last comment which also is mentioned in the documentation (https://docs.python.org/3.8/library/argparse.html#default
):

> For positional arguments with nargs equal to ? or *, the default value is used when no command-line argument was present