List Comprehension

A list comprehension looks like this: 

new_list = [f(o) for o in a_list if o>0]

This will return every element of a_list that is greater than 0, after passing it to the function f. There are three parts here: the collection you are iterating over (a_list), an optional filter (if o>0), and something to do to each element (f(o)).