Is it possible to use argparse to capture an arbitrary set of optional arguments?

Possible? possibly, but I wouldn't recommend it. argparse is the not best tool for parsing this kind of input, or conversely, this a poor argument specification from an argparse perspective.

Have you thought about what the usage line should look like? How would explain this to your users?

How would you parse this working from sys.argv directly? It looks like you could collect 3 pieces:

 prog = sys.argv[0]
 arg1 = sys.argv[1]
 keys = sys.argv[2::2]
 # maybe strip -- off each
 values = sys.argv[3::2]
 kvdict = {k:v for k, v in zip(keys, values)}

There are other SO questions asking about generic key:value pairs. Things like:

 --args key1:value1 key2:value2

This can be handled with nargs='*' and an action that splits each input string on : (or =) and stores things by key.

Your requirement is least amenable to argparse use because it requires bypassing the whole idea of matching argument flags with strings in argv. It requires, some how, turning off all the normal argparse parsing.

Looks like I suggested the same thing a couple of years ago

Parse non-pre-defined argument

or earlier

Using argparse to parse arguments of form "arg= val"

This is kind of a hackish way, but it works well:

Check, which arguments are not added and add them

import argparse
parser = argparse.ArgumentParser()
parser.add_argument("foo")
parser.add_argument("-bar", type=int)
# parser can have any arguments, whatever you want!

parsed, unknown = parser.parse_known_args() # this is an 'internal' method
# which returns 'parsed', the same as what parse_args() would return
# and 'unknown', the remainder of that
# the difference to parse_args() is that it does not exit when it finds redundant arguments

for arg in unknown:
    if arg.startswith(("-", "--")):
        # you can pass any arguments to add_argument
        parser.add_argument(arg.split('=')[0], type=<your type>, ...)

args = parser.parse_args()

For example:

python3 arbitrary_parser.py ha -bar 12 -extra1 value1 -extra2 value2

Then the result would be

args = Namespace(bar=12, foo='ha', extra1='value1' extra2='value2')