Replace arguments in ArgumentParser

I create a argparse.ArgumentParser instance from a config file, adding arguments from the name and value of entries.

Later in my code I want to specifically set some arguments, lets say --hidden_size is one of them.

When I run

parser.add_argument('--hidden_size', type=int, metavar='N', help='Hidden size')

I get argparse.ArgumentError: argument --hidden_size: conflicting option string: --hidden_size.

What would be the easiest way to replace an entry? Ideally, it should look something like this:

arg = parser['--hidden_size']
parser.add_argument('--hidden_size', type=int, metavar='N', default=arg.default, help='Hidden size')

However, neither can I access the argument by subscripting the parser, nor can I replace an argument with the method add_argument and the same name. How would this be done?

Solution

Here is a hack because it relies on hidden attribute of an ArgumentParser:

import argparse

def lookup(par, arg):
    for action in par._actions:
        if arg in action.option_strings:
            return action
    raise ValueError(f"{arg} not found")

parser = argparse.ArgumentParser()
parser.add_argument("--hidden_size")


try:
    # Lookup --hidden_size and update
    action = lookup(parser, "--hidden_size")
    action.type = int
    action.metavar = "N"
    action.help = "Hidden size"
except ValueError:
    # Lookup failed, add new
    parser.add_argument('--hidden_size', type=int, metavar='N', help='Hidden size')