3.8 Lists

Lists allow us to store multiple objects together.

A list is a sequential group of variables, denoted in Python by square brackets [], with individual entries separated by commas. A few of the neat properties of lists are:

Ordered: The list [1, 5, 3, 7] will always store those numbers in the same order.
Mixed data types: ["mercury", 13, 5.3, False] is a valid list which contains every data type we’ve seen so far.
Can contain other lists: [[2, 3], "sulfur", 12, 18]
Can contain repeat values: ["tomato", "tomato", "tomato", "sulfur"]

3.8.1 Indexing

What if we want to extract a specific value from a list? We can use indexing. To index in Python, we use the following syntax:

variable_name[index]

where index is the number of the item we wish to extract.

alchemists = ["Zosimos", "Oresme", "Flamel", "pseudo-Aristotle"]
print(alchemists[1])

## Oresme

Notice that when we printed the item at position 1, we printed out the second entry in alchemists. This is because in Python, indexing begins at 0. To print out the first entry, we would use alchemists[0].

A couple interesting things we can do with indexing:

To print multiple consecutive items, we can provide two numbers separated by a colon :.

print(alchemists[0:2])

## ['Zosimos', 'Oresme']

Note that the first number is inclusive and the second number is exclusive: we include the item at position 0 ('Zosimos'), but not the item at position 2 ('Flamel').

We can index in reverse. To index from the end of a list, we use negative numbers.

print(alchemists[-1])

## pseudo-Aristotle

Nested lists

How would we extract the number 3 from the list below?

my_list = [1, 2, [3, 4], 5]

First, we extract the [3, 4] list. This is the third item of the outer list, so it is at position 2 (remember, indexing in Python starts at 0). So we can access the interior list with my_list[2]:

print(my_list[2])

## [3, 4]

Within the interior list, 3 is the first item, so it is at position 0. It can be accessed with the syntax my_list[2][0]:

print(my_list[2][0])

## 3

3.8.1.1 Indexing Strings

We can also apply indexing to extract substrings from within a string. This is done identically to how we index a list:

creature = 'stingray'
print(creature[0:5])

## sting

3.8.2 Adding to lists

Finally, we can add entries to the end of a list. We do this with the append() method, which is used with the following syntax:

list_name.append(item)

For example:

florilegium = ['marigold', 'thistle', 'wormwood']
florilegium.append('tansy')
print(florilegium)

## ['marigold', 'thistle', 'wormwood', 'tansy']