argparse default option based on another option

Suppose I have an argparse python script:

import argparse

parser = argparse.ArgumentParser()
    
parser.add_argument("--foo", required=True)

Now I want to add another option --bar, which would default to appending "_BAR" to whatever was specified by --foo argument.

My goal:

>>> parser.parse_args(['--foo', 'FOO'])
>>> Namespace(foo='FOO', bar="FOO_BAR")

AND

>>> parser.parse_args(['--foo', 'FOO', '--bar', 'BAR'])
>>> Namespace(foo='FOO', bar="BAR")

I need something like this:

parser.add_argument("--bar", default=get_optional_foo + "_BAR")

Solution

Here's another attempt at writing a custom Action class

import argparse
class FooAction(argparse.Action):
    # adapted from documentation
    def __call__(self, parser, namespace, values, option_string=None):
        setattr(namespace, self.dest, values)
        defaultbar = getattr(namespace, 'bar')
        try:
            defaultbar = defaultbar%values
        except TypeError:
            # BAR has already been replaced
            pass
        setattr(namespace, 'bar', defaultbar)

parser = argparse.ArgumentParser()
parser.add_argument("--foo", required=True, action=FooAction)
parser.add_argument("--bar", default="%s_BAR")

args = parser.parse_args(['--foo', 'Foo', '--bar', 'Bar'])
# Namespace(bar='Bar', foo='Foo')
args = parser.parse_args(['--foo', 'Foo'])
# Namespace(bar='Foo_BAR', foo='Foo')
args = parser.parse_args(['--bar', 'Bar', '--foo', 'Foo'])
# Namespace(bar='Bar', foo='Foo')

Note that the class has to know the dest of the --bar argument. Also I use a '%s_BAR' to readily distinguish between a default value, and a non default one. This handles the case where --bar appears before --foo.

Things that complicate this approach are:

default values are evaluated at add_argument time.
default values are placed in the Namespace at the start of parse_args.
flagged (optionals) arguments can occur in any order
the Action class is not designed to handle interacting arguments.
the bar action will not be called in the default case.
the bar default could be a function, but something would have to check after parse_args whether it needs to be evaluated or not.

While this custom Action does the trick, I still think the addbar function in my other answer is a cleaner solution.