Fastest way to check if a value exists in a list

Question

What is the fastest way to know if a value exists in a list (a list with millions of values in it) and what its index is?

I know that all values in the list are uniqu

Answer 1

You could put your items into a set. Set lookups are very efficient.

Try:

s = set(a)
if 7 in s:
  # do stuff

edit In a comment you say that you'd like to get the index of the element. Unfortunately, sets have no notion of element position. An alternative is to pre-sort your list and then use binary search every time you need to find an element.