Check if Python argparse used explicit CLI argument or default value?

Question

I derive some settings from my parser that I latter store on disk to be reloaded again, during the reload I want to overwrite some values but keep the first ones, similar to new default values.

I want to have a priority of args2 > args1 > default values, however I face the challenge on how to determine the highest priority, especially when I pass the default value explicitly in arg2, it should not fall back to args1, i.e. a simple comparison with the default is not sufficient.

args1 = ["-a", "1", "--bar", "2"]  # Run 1
args2 = ["--bar", "default-bar", "-c", "3"]  # Run 2, override bar with default
args2_b = ["-c", "3"]  # Run 2b, bar should be restored from Run 1

# Equivalent full args for Run 2
wanted = ["-a", "1", "--bar", "default-bar", "-c", "3"]
wanted_b = ["-a", "1", "--bar", "2", "-c", "3"]

from argparse import ArgumentParser

parser = ArgumentParser()
# required arguments without defaults can be present too
parser.add_argument("--foo", "-a", default="default-foo")
parser.add_argument("--bar", "-b", default="default-bar")
parser.add_argument("--baz", "-c", default="default-baz")

results1 = parser.parse_args(args1)
print(results1) # Namespace(foo='1', bar='2', baz='default-baz')
results2 = parser.parse_args(args1)
print(results2) # Namespace(foo='default-foo', bar='default-bar', baz='2')

# merge results
# ...?

I do not want to make things too complicated and hard to maintain, therefore manually looking at sys.argv does not look feasible (I have many arguments, shorthand codes, usage of dest, ...).
I use tap Typed Argument Parser - if that info helps to find a better solution, also I would like to keep most of tap's features, i.e. to define most arguments in a class and keep meaningful default values there.

TL;DR: how could I differentiate between these two cases in Run 2?

# Run 1
file.py -a 1 --bar 2
# Run 2, that references some state from Run 1
# Use passed value:
file.py -c 3 --restore-run 1  --bar "default-bar"

# Use value from Run 1
file.py -c 3 --restore-run 1

Related questions:

python get changed (non-default) cli arguments? - helpful and closest to my problem, but fails with the restored default value
Python command line arguments check if default or given - problem and solution are far off
Python argparse: default value or specified value - asks for const

I don't know what the question is. Let's start with run 2 arguments and what you want results2 to be. — Hai Vu
– Hai Vu, Commented Sep 29 at 21:06
I have many arguments and many with defaults. I want to take the arguments of run 1 as the base - without the need to remember or rewrite them to avoid mistakes. The problem is when is when run 1 has a non-default argument. For run 2, I want to explicitly change it back to the default value. For run 2b, I want to keep the restored value of run 1. My question is how do I differentiate these two cases. I cannot simply do a if args_in_run2.bar == default_value: use value from run1; as I do not know if args_in_run2.bar is implicit or explicitly the default value. — Daraan
– Daraan, Commented Oct 1 at 8:40

Daraan · Accepted Answer · 2025-07-25 10:47:47Z

First I'll provide a more general answer, that reveals which actions were actually parsed by user arguments:

import argparse
from copy import deepcopy


parser = argparse.ArgumentParser()
# required arguments without defaults can be present too
parser.add_argument("--foo", "-a", default="default-foo")
parser.add_argument("--bar", "-b", default="default-bar")
parser.add_argument("--baz", "-c", default="default-baz")


new_parser = deepcopy(parser)
for action in new_parser ._actions:
    action.default = argparse.SUPPRESS

# get args that are explicitly passed in CLI:
cli_args = new_parser.parse_args()
# Namespace()

print(new_parser.parse_args(["--bar", "default-bar", "-c", "3"]))
# Namespace(bar='default-bar', baz='3')
print(new_parser.parse_args(["-c", "2"]))
# Namespace(baz='2')

# Use parser to get the normally parsed args and use cli_args in whatever way necessary

In my case I do it similarly, but I construct an updated new parser without using argparse.SUPPRESS instead I use the restored values as new default values:

import argparse
from copy import deepcopy

parser = argparse.ArgumentParser()
# required arguments without defaults can be present too
parser.add_argument("--foo", "-a", default="default-foo")
parser.add_argument("--bar", "-b", default="default-bar")
parser.add_argument("--baz", "-c", default="default-baz")

# parse args and detect I want to restore:
from types import SimpleNamespace
restored_namespace = SimpleNamespace(foo=1, bar=2)

new_parser = deepcopy(parser)

# update defaults or new_parser with restored values
actions: list[argparse.Action] = new_parser ._actions
for action in actions:
    action.default = getattr(restored_namespace, action.dest, action.default)  # set new default values

# now CLI args2 > restored args 1> defaults (during args1)
final_args = new_parser.parse_args()

print(new_parser.parse_args(["--bar", "default-bar", "-c", "3"]))
# Namespace(foo=1, bar='default-bar', baz='3')
print(new_parser.parse_args(["-c", "2"]))
# Namespace(foo=1, bar=2, baz='2')

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