They are also immutable in Python.

What are you talking about?

http://docs.python.org/2/library/string.html

  >>> a = "string"
  >>> a[1] = 'g'
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: 'str' object does not support item assignment
  >>> b = a.replace('t', 'g')
  >>> b
  'sgring'
  >>> a
  'string'
  >>> id(b)
  3065252256L
  >>> id(a)
  3075481632L

Also from here[1]:

  Strings and tuples are immutable sequence types: such
  objects cannot be modified once created.

[1] http://docs.python.org/2/library/stdtypes.html#mutable-seque...

Given that they're immutable, you cannot change them. Instead all the operations returns a new, modified, string.