In this chapter, we will present iteration using loops and iterators. We will show examples of how this can be used with lists and generators. Iteration is one of the fundamental operations a computer is useful for. Traditionally, iteration is achieved by a for loop. A for loop is a repetition of a block of instructions a certain number of times. Inside the loop, one has access to a loop variable, in which the iteration number is stored.

The Python idiom is slightly different. A for loop in Python is primarily designed to exhaust a list, that is, to enumerate the elements of a list. The effect is similar to the repetition effect just described if one uses a list containing the first n integers.

A for loop only needs one element of the list at a time. It is therefore desirable to use a for loop with objects that are able to create those elements on demand, one at a time. This is what iterators achieve in Python.