python subprocessで標準出力もエラーもファイル出力する

greeting.py

import argparse
import time

def arg_res():
    parser = argparse.ArgumentParser()
    parser.add_argument('--nickname',required=True)

    args = parser.parse_args()

    nickname = args.nickname

    # エラーなし
    greeting_sentence = f"僕のニックネームは{nickname}"
    print(greeting_sentence)

    # エラーあり
    greeting_sentence = f"僕のニックネームは{nicknames}"
    print(greeting_sentence)

    return greeting_sentence

def test_main():

    greeting_sentence = arg_res()
    print(greeting_sentence)

if __name__=="__main__":
    test_main()

run_greeting.py

import subprocess
from time import time

name_list = ["john","power","smith","jack"]

with open('out.txt', 'w') as fp:

    for i in name_list:
        print(i)
        # コンソール実行時の場合に空白で区切る部分を分割してリストに入れる"
        # python greeting.py --nickname XXXX の場合下記のようになる。
        command = ["python","greeting.py","--nickname", f"{i}"]
        proc = subprocess.Popen(command, stdout=fp,stderr=fp)

出力結果

out.txt

僕のニックネームはsmith
僕のニックネームはjohn
僕のニックネームはpower
Traceback (most recent call last):
  File "greeting.py", line 28, in <module>
    test_main()
  File "greeting.py", line 24, in test_main
    greeting_sentence = arg_res()
  File "greeting.py", line 17, in arg_res
    greeting_sentence = f"僕のニックネームは{nicknames}"
NameError: name 'nicknames' is not defined
Traceback (most recent call last):
  File "greeting.py", line 28, in <module>
    test_main()
  File "greeting.py", line 24, in test_main
    greeting_sentence = arg_res()
  File "greeting.py", line 17, in arg_res
    greeting_sentence = f"僕のニックネームは{nicknames}"
NameError: name 'nicknames' is not defined
Traceback (most recent call last):
  File "greeting.py", line 28, in <module>
    test_main()
  File "greeting.py", line 24, in test_main
    greeting_sentence = arg_res()
  File "greeting.py", line 17, in arg_res
    greeting_sentence = f"僕のニックネームは{nicknames}"
NameError: name 'nicknames' is not defined
僕のニックネームはjack
Traceback (most recent call last):
  File "greeting.py", line 28, in <module>
    test_main()
  File "greeting.py", line 24, in test_main
    greeting_sentence = arg_res()
  File "greeting.py", line 17, in arg_res
    greeting_sentence = f"僕のニックネームは{nicknames}"
NameError: name 'nicknames' is not defined