Python Arrays – Python Lists

Does python have an array?

• Arrays are used to store multiple values in one single variable.
• Python does not have built-in support for Arrays, but Python Lists can be used instead.

What is Python Lists/Python Arrays?

Python Lists are used to store multiple items in a single variable. For example, fruits_list has 4 items apple, banana, cherry, orange

fruits_list = ["apple", "banana", "cherry", "orange"]

Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.

Lists are created using square brackets []

Python List/Array Methods

Python has a set of built-in methods that you can use on lists/arrays.

Python Lists/Python Arrays Items

• List items are ordered, changeable, and allow duplicate values.
• List items are indexed, the first item has index [0], the second item has index [1] etc. For example, fruits_list = ["apple", "banana", "cherry", "orange"] then fruits_list[0]="apple", fruits_list[1]=banana

Ordered

• When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
• If you add new items to a list, the new items will be placed at the end of the list.
• Note: There are some list methods that will change the order, but in general: the order of the items will not change.
• Changeable
• The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.

Python List Items Allow Duplicates

• Since lists are indexed, lists can have items with the same value:

• Example

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

Python List Length

• To determine how many items a list has, use the len() function:

Example

Print the number of items in the list:

fruits_list = ["apple", "banana", "cherry", "apple", "cherry"]
len(fruits_list)

List Items – Data Types

• List items can be of any data type.

Example: String, int and boolean data types:

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

• A list can contain different data types:

Example: A list with strings, integers and boolean values:

my_list = ["abc", 34, True, 40, "male"]

type()

• From Python’s perspective, lists are defined as objects with the data type ‘list’:
• <class ‘list’>

• Example: What is the data type of a list?

mylist = ["apple", "banana", "cherry"]
print(type(mylist))

The list() Constructor

• It is also possible to use the list() constructor when creating a new list.

• Example

  Using the list() constructor to make a List:

  thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
  print(thislist)

Python List of Lists (Python List in list)

A list of lists in Python is a list object where each list element is a list by itself. Create a list of list in Python by using the square bracket notation to create a nested list [[1, 2, 3], [4, 5, 6], [7, 8, 9]].

Create a List of Lists in Python

Create a list of lists by using the square bracket notation. For example, to create a list of lists of integer values, use [[1, 2], [3, 4]]. Each list element of the outer list is a nested list itself.

Convert List of Lists to One List

Say, you want to convert a list of lists [[1, 2], [3, 4]] into a single list [1, 2, 3, 4]. How to achieve this? There are different options:

• List comprehension [x for l in lst for x in l] assuming you have a list of lists lst.
• Unpacking [*lst[0], *lst[1]] assuming you have a list of two lists lst.
• Using the extend() method of Python lists to extend all lists in the list of lists.

Find examples of all three methods in the following code snippet:

lst = [[1, 2], [3, 4]]

# Method 1: List Comprehension
flat_1 = [x for l in lst for x in l]

# Method 2: Unpacking
flat_2 = [*lst[0], *lst[1]]

# Method 3: Extend Method
flat_3 = []
for l in lst:
flat_3.extend(l)

## Check results:
print(flat_1)
# [1, 2, 3, 4]

print(flat_2)
# [1, 2, 3, 4]

print(flat_3)
# [1, 2, 3, 4]