import os
os.system('net user username')

import subprocess

s = subprocess.Popen('cat /etc/passwd'.split(),
                     stdout=subprocess.PIPE).communicate()[0]

with open('file.txt', 'w', encoding='utf-8') as f:
    print(repr(s.splitlines()[0].split(b':')), file=f)
    print('ok')

[guest@localhost tests]$ python3 t.py
ok
[guest@localhost tests]$ cat file.txt
[b'root', b'x', b'0', b'0', b'root', b'/root', b'/bin/bash']
[guest@localhost tests]$

import subprocess

s = subprocess.Popen('net user'.split(),
                     stdout=subprocess.PIPE).communicate()[0]

with open('file.txt', 'w', encoding='utf-8') as f:
    print(repr(s), file=f)
    print('ok')

>>> s = by.decode('cp866')
>>> print(s)

Учетные записи пользователей для \\201-02

-------------------------------------------------------------------------------
HelpAssistant            SUPPORT_388945a0         Администратор            
Гость                    
Команда выполнена успешно.


>>>