new_string=old_string.replace('OLD', 'NEW')
Note that the string itself is not modified in-place, i.e. the following would not modify the content of old_string:
old_string.replace('OLD', 'NEW')
With lists, however, it is different. Extending a list with another list works inplace i.e. old_list will be modified if you write:
old_list.extend(another_list)
In fact, if you will create a bug if you try:
new_list = old_list.extend(another_list)
In this case the content of new_list, somewhat surprisingly for many, becomes None.
There are good reasons for all of this: Object that are mutable can be changed in-place, while immutable objects cannot be changed in-place. Strings are immutable, lists are mutable. But for beginners who does not know this distinction, the difference may be very confusing and lead to bugs.
Not being an expert, one might ting that methods either always or never work in-place on the object by default. Instead you have to remember this on a case-by-case basis. Once you know more of the logic of "when and why" it becomes easier.
