list.sort()

sort the list in place , does not return a list, so print list.sort() return None

sorted(list) return a new sorted list

sorted() works on any iterable, not just lists. Strings, tuples, dictionaries (you'll get the keys), generators, etc., returning a list containing all elements, sorted.

set() remove the duplicate , return a set , list(set()) change set to list

char.isdigit()

char.isalpha()

judge whether the char is digit or alpha

char.isupper() char.islower()