Lists

A list contains a collection of values. You have already seen variables that hold a single value:

number = 5

A variable that references a list can hold multiple values, in order:

names = ['Maria', 'Mariano', 'Anna', 'Antonino']
numbers = [5, 8, 10, 13, 42]

A list can hold as many values as you want. We can even have an empty list:

twilight_books_i_like = []

Length

To get the length of a list, use len:

number_of_names = len(names)

Here, number_of_names will be equal to 4.

Appending to a list

One of the many functions we can perform on a list is appending items to it. This means adding items to the end of the list. For example:

pets = ['cat', 'horse', 'dog']
pets.append('emu')

This will result in the pets list now containing ['cat', 'horse', 'dog', 'emu'].

Note the syntax here — variable dot function:

You can use input loops to start with an empty list and then add more items to it. In the zip file you will find a small program in get_names.py that illustrates how to do this:

if __name__ == '__main__':
    names = []
    while True:
        name = input("Give me a name: ")
        if name == 'q':
            break
        names.append(name)

    print(names)

• We start with an empty list, using names = [].

• We use the event stream pattern, looping forever and then breaking out of the loop if the person enters ‘q’ instead of a name. (Note, if you were making a program to enter characters in Star Trek, you might not want to do it this way.)

• We use append() to add each name to the list.

If you run this program, you will see something like this:

Give me a name: me
Give me a name: you
Give me a name: everyone
Give me a name: q
['me', 'you', 'everyone']

Iteration

Once you have a list, you can iterate (or loop) through it using for ... in:

for number in numbers:
    print(number)

Each time through the for loop, Python sets the variable number to one of the numbers in the list, in order. Since we have set the numbers variable to [5, 8, 10, 13, 42], the first time through the loop, number = 5. The second time through the loop, number = 8, and so forth. So this code will print:

5
8
10
13
42

The name of the variable can be whatever you like. For example:

for cute_puppies in numbers:
    print(cute_puppies)

However, you want to be sure that your friends can read your code and make sense of it!

Your code is much more readable if you use variables that make sense.