examples/strings/string_change.py
from __future__ import print_function

x = "abcd"
print(x)     # abcd

x[2] = 'Y'

# Traceback (most recent call last):
#   File "examples/strings/string_change.py", line 6, in <module>
#     x[2] = 'Y'
# TypeError: 'str' object does not support item assignment

Replace part of a string