argparse.ArgumentParser is slow when parsing a very long option list

Bug report

The following parser exhibits O(N^2) behavior when parsing a very long option list (e.g. --item=a --item=b ...):

parser = argparse.ArgumentParser(description="test")
parser.add_argument('--item', dest='accumulate', action='append')

Here is a simple repro that measures the time parse_args() take depending on the number of options:

#!/usr/bin/env python3
import argparse
from datetime import datetime


def run_test(num_iterations):
    """Parse a list with 'append' action and measure the time it takes."""
    parser = argparse.ArgumentParser(description="test")
    parser.add_argument('--item', dest='accumulate', action='append')

    start = datetime.now()

    args = parser.parse_args(['--item=x' for _ in range(num_iterations)])

    end = datetime.now()
    diff = end - start

    print(f"{num_iterations:8} iterations: {diff} s")


def main():
    for i in range(8, 18):
        run_test(pow(2, i))


if __name__ == "__main__":
    main()

The output on my machine:

> ./repro.py
     256 iterations: 0:00:00.002699 s
     512 iterations: 0:00:00.008444 s
    1024 iterations: 0:00:00.028508 s
    2048 iterations: 0:00:00.104813 s
    4096 iterations: 0:00:00.389232 s
    8192 iterations: 0:00:01.552962 s
   16384 iterations: 0:00:06.132139 s
   32768 iterations: 0:00:25.233547 s
   65536 iterations: 0:01:51.921170 s
  131072 iterations: 0:07:42.047914 s

Your environment

• CPython versions tested on: HEAD of main (a9d58feccfd956dc99195af6872b06446738d7db)
• Operating system and architecture: Fedora release 36 (Thirty Six)

Linked PRs

• gh-124740
• gh-124745
• gh-124909

Start with argparse.ArgumentParser and parse_args(), using the issue's repeated --item=x reproduction to observe the scaling. Inspect the linked PRs for work already addressing the behavior. Done means parsing a very long option list no longer exhibits the reported O(N^2) growth.