Strings are immutable

It is tempting to use the [] operator on the left side of an assignment, with the intention of changing a character in a string.

                       

The ‘object’ in this case is the string and the ‘item’ is the character tried to assign. For now, an object is the same thing as a value, but we will refine that definition later. An item is one of the values in a sequence. The reason for the error is that strings are immutable, which means you can’t change an existing string. The best you can do is create a new string that is a variation on the original:

 

This example concatenates a new first letter onto a slice of greeting. It has no effect on the original string.

 

from Thinking in Python