Lists

Last updated on 2026-08-21 | Edit this page

Estimated time: 35 minutes

Overview

Questions

  • How can I store multiple items in a Python variable?

Objectives

  • Create collections to work with in Python using lists.
  • Write Python code to index, slice, and modify lists through assignment and method calls.

A list stores many values in a single structure.

A list is one of the most commonly used data structures in Python. Lists have two important characteristics:

  1. They are mutable, i.e, they can be changed after they are created.
  2. They are heterogeneous, i.e, they can store values of many different types.

To create a new list, you can just put some values in square brackets with commas in between. Let’s create a short list of some library metadata standards.

PYTHON 

metadata = ['marc', 'frbr', 'mets', 'mods']
metadata

OUTPUT 

['marc', 'frbr', 'mets', 'mods']

We can use len() to find out how many values are in a list.

PYTHON 

len(metadata)

OUTPUT 

4

Use an item’s index to fetch it from a list.

In the same way we used index numbers for strings, we can reference elements and slices in a list.

PYTHON 

print(f'First item: {metadata[0]}')
print(f'The first three items: {metadata[0:3]}')

OUTPUT 

First item: marc
The first three items: ['marc', 'frbr', 'mets']

Reassign list values with their index.

Use an index value along with your list variable to replace a value from the list.

PYTHON 

print(f'List was: {metadata}')
metadata[0] = 'bibframe'
print(f'List is now: {metadata}')

OUTPUT 

List was: ['marc', 'frbr', 'mets', 'mods']
List is now: ['bibframe', 'frbr', 'mets', 'mods']

Character strings are immutable.

Unlike lists, we cannot change the characters in a string using its index value. Strings and lists can both be indexed and sliced. However, strings are immutable (cannot change in place), while lists are mutable.

PYTHON 

librarian = 'Langanathan' # misspelled SR Ranganathan's name
librarian[0] = 'R'

ERROR 

TypeError: 'str' object does not support item assignment

Lists may contain values of different types.

A single list may contain numbers, strings, and anything else (including other lists!). If you’re dealing with a list within a list you can continue to use the square bracket notation to reference specific items.

PYTHON 

mixed_list = ['string', 3.2, [10, 20, 30]]
f'First item in sublist: {mixed_list[2][0]}'

OUTPUT 

First item in sublist: 10

Using list methods to make adding/removing easier.

Lists are objects, which means they provide methods that allow us to perform common operations. A method is called using the syntax object.method().

Lists provide methods for modifying their contents. We call these methods using dot notation. There are many methods associated with a list, but we will cover only a few of them.

Appending items to a list.

Use list_name.append() to add items to the end of a list. In Python, we would call .append() a method of the list object.

PYTHON 

print(f'list was:{metadata}')
metadata.append('oai-pmh')
print(f'list is now: {metadata}')

OUTPUT 

list was: ['bibframe', 'frbr', 'mets', 'mods']
list is now: ['bibframe', 'frbr', 'mets', 'mods', 'oai-pmh']

We can also call list_name.insert(index, value) to add an item at a given position in the list. The first argument specifies where to place the new item, and the second argument is the item to add.

PYTHON 

numbers = [1, 2, 3, 4, 6]
print(f'numbers before: {numbers}')
numbers.insert(4, 5)
print(f'numbers after: {numbers}')

OUTPUT 

numbers before: [1, 2, 3, 4, 6]
numbers after: [1, 2, 3, 4, 5, 6]

Removing items from a list.

Use list_name.pop() to remove and return the last item in a list. If we provide an index, .pop() removes and returns the item at that position. It raises an IndexError if the list is empty or the index is out of range.

PYTHON 

drinks = ['water', 'tea', 'coffee', 'milk']
print(f'drinks before: {drinks}')
drinks.pop()
print(f'drinks after first pop: {drinks}')
val = drinks.pop(0)
print(f'drinks after second pop: {drinks}')
print(val)

OUTPUT 

drinks before: ['water', 'tea', 'coffee', 'milk']
drinks after first pop: ['water', 'tea', 'coffee']
drinks after second pop: ['tea', 'coffee']
water

We can also call list_name.remove(value), where value is the item you want to remove. This will remove the first occurrence of the item. If there are multiple, then .remove() will need to be called several times.

PYTHON 

animals = ['dog', 'bird', 'shark', 'dog']
print(f'animals before: {animals}')
animals.remove('dog')
print(f'animals after: {animals}')

OUTPUT 

animals before: ['dog', 'bird', 'shark', 'dog']
animals after: ['bird', 'shark', 'dog']
Challenge

Lists: Length and Indexing

  1. Create a list named colors containing the strings ‘red’, ‘blue’, and ‘green’.
  2. Print the length of the list.
  3. Print the first color using indexing.

PYTHON 

colors = ['red', 'blue', 'green']
print(len(colors))
print(colors[0])
Challenge

List slicing

  1. Create a list of numbers defined as [1, 2, 3, 4, 5, 6].
  2. Print the first three items in the list using slicing.
  3. Print the last three items using slicing.

PYTHON 

numbers = [1, 2, 3, 4, 5, 6]
print(numbers[0:3])
print(numbers[3:6])

OUTPUT 

[1, 2, 3]
[4, 5, 6]

You can also leave the first and last elements in a slice blank to refer to the first and last elements in a list:

PYTHON 

print(numbers[:3])
print(numbers[3:])

OUTPUT 

[1, 2, 3]
[4, 5, 6]
Challenge

Fill in the Blanks

Fill in the blanks so that the program below produces the output shown. In the first line we create a blank list by assigning values = [].

PYTHON 

values = []
values.____(1)
values.____(3)
values.____(5)
print(f'first time: {values}')
values = values[____]
print(f'second time: {values}')

OUTPUT 

first time: [1, 3, 5]
second time: [3, 5]

PYTHON 

values = []
values.append(1)
values.append(3)
values.append(5)
print(f'first time: {values}')
values = values[1:3]
print(f'second time: {values}')

OUTPUT 

first time: [1, 3, 5]
second time: [3, 5]
Challenge

Working With the End

Run the following code on your own and answer the following questions.

PYTHON 

resources = ['books', 'DVDs', 'maps', 'databases']
print(resources[-1])
  1. How does Python interpret a negative index value?
  2. If resources is a list, what does resources.pop(-1) do?
  3. What value does resources.pop(-1) return?

OUTPUT 

databases
  1. A negative index begins at the final element.
  2. It removes the final element of the list.
  3. It will return the final element of the list.
Key Points
  • A list stores many values in a single structure.
  • Use an item’s index to fetch it from a list.
  • Lists’ values can be replaced by assigning to them.
  • Use list methods to help add and remove items
  • Lists may contain values of different types.
  • Character strings can be indexed like lists.
  • Character strings are immutable.
  • Indexing beyond the end of the collection is an error.