Use a set (hashset) when possible

set.py

# do:
vowels = set('aeiouAEIOU')
# rather than:
vowels = ['a', 'e', 'i', 'o', 'u', 'A', 'E', 'I', 'O', 'U']
# because searching for X in set is O(1) on average for hashset (O(n)) worst
# vs. O(n) on average for array, which is how Python list is implemented
# internally