input = 1212

output = 2121

for i in range(len(listofnumbers)):
  if listofnumbers[i] == 1:
    listofnumbers[i] = 2
  else:
    listofnumbers[i] = 1

Python 2.7.15 (default, May 27 2018, 08:15:19)
[GCC 5.5.0 20171010] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import string
>>> "12121212".translate(string.maketrans("12", "21"))
'21212121'