Question: I am using argparse's action to add various data to a class. I would like to use that action on the default value if that arg is not provided at the command line. Is this possible?
Thanks!

Answer: 
`argparse` does not use the `action` when applying the `default`. It just uses `setattr`. It may use the `type` if the default is a string. But you can invoke the `action` directly.

Here I use a custom action class borrowed from the documentation. In the first `parse_args` nothing happens. Then I create a new `namespace`, and invoke the action on the default. Then I pass that namespace to `parse_args`. To understand this, you many need to import it into an interactive shell, and examine the attributes of the namespace and action.

```
# sample custom action from docs
class FooAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        print('Setting: %r %r %r' % (namespace, values, option_string))
        setattr(namespace, self.dest, 'action:'+values)
p = argparse.ArgumentParser()
a1 = p.add_argument('--foo', action=FooAction, default='default')
print 'action:',a1
print p.parse_args([])

ns = argparse.Namespace()
a1(p, ns, a1.default, 'no string') # call action
print p.parse_args([],ns)
print p.parse_args(['--foo','1'],ns)

```

which produces:

```
action: FooAction(option_strings=['--foo'], dest='foo', nargs=None, const=None, default='default', type=None, choices=None, help=None, metavar=None)
Namespace(foo='default')
Setting: Namespace() 'default' 'no string'
Namespace(foo='action:default')
Setting: Namespace(foo='action:default') '1' '--foo'
Namespace(foo='action:1')

```

I tailored the output to highlight when the action is being used.

---

Here's a way of performing a special action on an argument that isn't given on the command line (or given with a value == to the default). It's a simplification of the class given in <https://stackoverflow.com/a/24638908/901925>.

```
class Parser1:
    def __init__(self, desc):
        self.parser = argparse.ArgumentParser(description=desc)
        self.actions = []

    def milestone(self, help_='milestone for latest release.', default=None):
        action = self.parser.add_argument('-m', '--milestone', help=help_, default=default)
        self.actions.append(action)
        return self

    def parse(self):
        args = self.parser.parse_args()
        for a in self.actions:
            if getattr(args, a.dest) == a.default:
                print 'Please specify', a.dest
                values = raw_input('>')
                setattr(args, a.dest, values)
        return args

print Parser1('desc').milestone(default='PROMPT').parse()

```

The prompting is done after `parse_args`. I don't see any reason to call `parse_args` again.
I needed to prompt the user if an option was not specified - that's how I did it:

```
class _PromptUserAction(argparse.Action):

    def __call__(self, parser, namespace, values, option_string=None):
        if values == self.default:
            print 'Please specify', self.dest
            values = raw_input('>')
        setattr(namespace, self.dest, values)

class Parser:
    def __init__(self, desc, add_h=True):
        self.parser = argparse.ArgumentParser(description=desc, add_help=add_h,
                        formatter_class=argparse.ArgumentDefaultsHelpFormatter)
        #actions to be run on options if not specified (using default to check)
        self.actions = []

    @staticmethod
    def new(description, add_help=True):
        return Parser(description, add_help)

    # ...

    def milestone(self, help_='Specify the milestone for latest release.'):
        action = self.parser.add_argument('-m', '--milestone',
                                          dest='milestone',
                                          action=_PromptUserAction,
                                          default='PROMPT', # needed I think
                                          type=str,
                                          help=help_)
        self.actions.append(action)
        return self

    def parse(self):
        """
        Return an object which can be used to get the arguments as in:
            parser_instance.parse().milestone

        :return: ArgumentParser
        """
        args = self.parser.parse_args()
        # see: http://stackoverflow.com/a/21588198/281545
        dic = vars(args)
        ns = argparse.Namespace()
        for a in self.actions:
            if dic[a.dest] == a.default:
                a(self.parser, ns, a.default) # call action
        # duh - can I avoid it ?
        import sys
        return self.parser.parse_args(sys.argv[1:],ns)

```

I am interested if this can somehow be done without having to reparse the args (the `import sys` part). Maybe some constructor options for `argparse.Action` ?