from os import system

from random import randint

i = 0

spisok = [randint(-100, 100) for i in range(6)]

print('Список до:  ', spisok)

for i in range(len(spisok)):

   if spisok[i] < 0:

       spisok[i] = 'x'

   else:

       pass

print('Список после:  ', spisok)

system('pause')