Lists

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.

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The most popular kind of data collection in Python is the list. Lists have two primary 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 amino acids.

PYTHON

amino_acids = ['alanine', 'leucine', 'cysteine', 'isoleucine']
amino_acids

OUTPUT

['alanine', 'leucine', 'cysteine', 'isoleucine']

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

PYTHON

len(amino_acids)

OUTPUT

4

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

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

PYTHON

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

OUTPUT

First item: alanine
The first three items: ['alanine', 'leucine', 'cysteine']

Reassign list values with their index.

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

PYTHON

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

OUTPUT

List was: ['alanine', 'leucine', 'cysteine', 'isoleucine']
List is now: ['methionine', 'leucine', 'cysteine', 'isoleucine']

Character strings are immutable.

Unlike lists, we cannot change the characters in a string using its index value. In other words strings are immutable (cannot be changed in-place after creation), while lists are mutable: they can be modified in place. Python considers the string to be a single value with parts, not a collection of values.

PYTHON

ile = 'Osoleucine' # oops, typo!
ile[0] = 'I'

ERROR

TypeError: 'str' object does not support item assignment

Appending items to a list lengthens it.

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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. You can use the syntax of object.method() to call methods.

PYTHON

print(f'list was:{amino_acids}')
amino_acids.append('glutamine')
print(f'list is now: {amino_acids}')

OUTPUT

list was: ['methionine', 'leucine', 'cysteine', 'isoleucine']
list is now: ['methionine', 'leucine', 'cysteine', 'isoleucine', 'glutamine']

Lists may contain values of different types.

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

OUTPUT

First item in sublist: 10

Lists can contain lists.

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Lists which contain other lists can be very useful. Imagine you poll your friends on their favorite pizza toppings.

PYTHON

pizza_toppings = ['pepperoni', 'mushroom', 'pineapple']
print(pizza_toppings)

OUTPUT

['pepperoni', 'mushroom', 'pineapple']

One of your friends likes Buffalo chicken pizza. Buffalo chicken is really three toppings combined: chicken, Buffalo sauce, and blue cheese. Here is how we can represent that in the list.

PYTHON

buffalo_chicken = ['chicken', 'buffalo sauce', 'blue cheese']
pizza_toppings.append(buffalo_chicken)
print(pizza_toppings)

OUTPUT

['pepperoni', 'mushroom', 'pineapple', ['chicken', 'buffalo sauce', 'blue cheese']]

We can access the items in the sublist as shown above:

PYTHON

print(pizza_toppings[3][2])

OUTPUT

blue cheese

Use del to remove items from a list entirely.

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del list_name[index] removes an item from a list and shortens the list. Unlike .append(), del is not a method, but a “statement” in Python. In the example below, del performs an “in-place” operation on a list of prime numbers. This means that the primes variable will be reassigned when you use the del statement, without needing to use an assignment operator (e.g., primes = ...) .

PYTHON

primes = [2, 3, 5, 7, 11]
print(f'primes before: {primes}')
del primes[4]
print(f'primes after: {primes}')

OUTPUT

primes before: [2, 3, 5, 7, 11]
primes after: [2, 3, 5, 7]

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.

Show me the solution

PYTHON

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

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.

Show me the solution

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]

Bonus Challenge: More Pizza

Another person has responded to your pizza poll. Their favorite pizza is Veggie (green pepper, onion, mushroom, broccoli, and olive).

1. Create a variable that represents the toppings making up a veggie pizza.
2. Add the veggie pizza to your pizza list.
3. Print the updated pizza list.
4. Use list indexing to print the second item that goes on a veggie pizza.

Here is the current pizza list to get you started:

PYTHON

pizza_toppings = ['pepperoni', 'mushroom', 'pineapple', ['chicken', 'buffalo sauce', 'blue cheese']]
print(pizza_toppings)

OUTPUT

['pepperoni', 'mushroom', 'pineapple', ['chicken', 'buffalo sauce', 'blue cheese']]

Show me the solution

PYTHON

veggie = ['green pepper', 'onion', 'mushroom', 'broccoli', 'olive']
pizza_toppings.append(veggie)
print(pizza_toppings)
print(pizza_toppings[4][1])

OUTPUT

['pepperoni', 'mushroom', 'pineapple', ['chicken', 'buffalo sauce', 'blue cheese'], ['green pepper', 'onion', 'mushroom', 'broccoli', 'olive']]
onion

Bonus 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]

Show me the solution

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]

Bonus 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 del resources[-1] do?

Show me the solution

OUTPUT

databases

1. A negative index begins at the final element.
2. It removes 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.
• Appending items to a list lengthens it.
• Use del to remove items from a list entirely.
• 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.