Lists

A list is a data structure which holds any number of values, separated by commas and surrounded by square brackets. For example, [], [50], and [100, 200, 300, 400] are all lists of numbers (although [] is an empty list).

Iteration

Iteration is the process of going through a collection of things one at a time. There are plenty of everyday situations where you iterate through collections of things. For example, you might iterate through a pile of dollar bills to count how much money you have, or you might iterate through all today's mail to find out whether you got a birthday card from your friend.

Here are some reasons you might iterate through a collection in Python:

You're looking for something that's in the collection.
You want to do something to each item in the collection.
There's some action that you want to repeat multiple times.

In Python, something is iterable if it can contain multiple elements. For now, we will be working with two iterables: lists and ranges. A list is any number of items inside of a pair of brackets ([]), separated by commas. For example, [1, 3, 5, 7, 9]. The range function creates a list of numbers from 0 up to the number provided. For example:

>>> for number in range(4):
...     print(number)
0
1
2
3