How to mock argparse nested subparsers in Python

import argparse
from unittest.mock import patch
from io import StringIO

def build_parser():
    parser = argparse.ArgumentParser(prog="app")
    subparsers = parser.add_subparsers(dest="command", required=True)

    # Outer subparser
    outer = subparsers.add_parser("outer")
    outer_sub = outer.add_subparsers(dest="subcommand", required=True)

    # Inner subparser
    inner = outer_sub.add_parser("inner")
    inner.add_argument("--value", type=int, default=0)
    return parser

def run(args):
    parser = build_parser()
    parsed = parser.parse_args(args)
    if parsed.command == "outer" and parsed.subcommand == "inner":
        return f"inner value: {parsed.value}"
    return "unknown"

if __name__ == "__main__":
    # Test with actual argparse
    print(run(["outer", "inner", "--value", 42]))

    # Demonstrate mocking for testing
    with patch("sys.argv", ["app", "outer", "inner", "--value", 7]):
        with patch("sys.stdout", new_callable=StringIO) as mock_stdout:
            print(run([]))