Python Lists

Example

mylist = ["apple", "banana", "cherry"]

List

A list lets one variable hold several items at once.

Python has 4 built-in types for storing collections of data. Lists are one of them; Tuple, Set and Dictionary are the others, and each has its own qualities and uses.

To create a list, put the items between square brackets:

Example

Building a simple list:

Create a Python list with the items "apple", "banana", and "cherry".
Print the list.
> Output: complete Python program

Prompt

Create a Python list with the items "apple", "banana", and "cherry". Print the list.

Output: complete Python program

List Items

Three things define list items: they keep their order, they can be changed, and the same value may appear more than once.

Each item also has an index. Counting starts at zero, so index [0] is the first item, index [1] is the next one, and so on.

Ordered

Being ordered means a list keeps its items in a set sequence, and that sequence stays put.

Anything new you add goes after the items already there.

Note: A few list methods do rearrange the items, but apart from those, the sequence stays as it is.

Changeable

You are free to modify a list once it exists: items can be changed, added and removed at any time.

Allow Duplicates

Because every item has its own index, two items can share a value without any conflict:

Example

Here "apple" and "cherry" each appear twice:

Create a Python list that contains duplicate values: "apple", "banana", "cherry", "apple", "cherry".
Print the list to show that duplicates are allowed.
> Output: complete Python program

Prompt

Create a Python list that contains duplicate values: "apple", "banana", "cherry", "apple", "cherry". Print the list to show that duplicates are allowed.

Output: complete Python program

thislist = ["apple", "banana", "cherry", "apple", "cherry"]
print(thislist)

List Length

The len() function tells you how many items a list holds:

Example

Count the items and print the result:

Create a list with three fruits and use the len() function to print how many items are in the list.
> Output: complete Python program

Prompt

Create a list with three fruits and use the len() function to print how many items are in the list.

Output: complete Python program

List Items - Data Types

A list can hold values of any data type:

Example

One list each of strings, integers and booleans:

Create three lists: one with strings, one with integers, and one with boolean values.
Do not print the lists.
> Output: complete Python program

Prompt

Create three lists: one with strings, one with integers, and one with boolean values. Do not print the lists.

Output: complete Python program

list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]

Example

A single list can also mix those types:

Create a list that contains different data types: a string, an integer, a boolean, another integer, and another string.
> Output: complete Python program

Prompt

Create a list that contains different data types: a string, an integer, a boolean, another integer, and another string.

Output: complete Python program

type()

To Python, a list is an object whose data type is 'list':

Example

Ask Python for the type of a list:

Create a list and use the type() function to print the data type of the list.
> Output: complete Python program

Prompt

Create a list and use the type() function to print the data type of the list.

Output: complete Python program

The list() Constructor

You can also build a new list by calling the list() constructor.

Example

Passing a tuple to list() gives us a list:

Use the list() constructor to create a list with "apple", "banana", and "cherry".
Print the list.
> Output: complete Python program

Prompt

Use the list() constructor to create a list with "apple", "banana", and "cherry". Print the list.

Output: complete Python program

Python Collections (Arrays)

Python gives you four collection data types, and each one behaves a little differently:

List keeps its order and can be changed. Duplicates are fine.
Tuple keeps its order but cannot be changed. Duplicates are fine.
Set has no order and no indexes, and is unchangeable*. Duplicates are not allowed.
Dictionary keeps its order** and can be changed. Duplicates are not allowed.

*You can't change the items of a set once they are in it, but you are free to add new ones or take existing ones out at any time.

**Dictionaries are ordered from Python 3.7 onward. In Python 3.6 and earlier they are unordered.

It pays to know these properties before you pick a type for your data. The right choice can keep the meaning of the data intact, and it can also make your program more efficient or more secure.