import subprocess
from unittest.mock import patch

def update_conf():
    output = subprocess.check_output(['ls'], stderr=subprocess.STDOUT)

@patch('subprocess.check_output')
def test_update_conf(self, subprocess_function, is_correct_func):
    def check_update_conf(args, stderr):
        pass
    def check_proxy_update(args, stderr):
        raise AssertionError()
    subprocess_function.side_effect = check_update_conf
    update_conf()
    subprocess_function.side_effect = check_proxy_update
    update_conf()