Set and dictionary comprehensions

List comprehensions are handy ways in which to concisely build sequences of values in Python. Other forms of comprehension are also available, which you can use to build other collection types. A set is an unordered collection: you can see what elements are in a set, but you cannot index into a set nor insert an object at a particular location in the set because the elements are not ordered. An element can only be present in a set once, whereas it could appear in a list multiple times.

Sets are frequently useful in situations where you want to quickly test whether an object is in a collection but do not need to track the order of the objects in the collection. For example, a web service might keep track of all of the active session tokens in a set so that when it receives a request, it can test whether the session token corresponds to an active session.

A dictionary is a collection of pairs of objects, where one object in the pair is called the...