"There is also an optional argument cmp=cmpFn to sorted() that specifies a traditional two-argument comparison function that takes two values from the list and returns negative/0/positive to indicate their ordering."

I can't understand the following lines from the article. Please help me understand this .

>>>a='hello world'

>>>a=a.upper()

>>> a

>>>'HELLO WORLD'

these lines of code changed the string . Can you help me to understand this whole thing in detail ? It is written in the tutorial that python strings are immutable .