Defining lists in python

Python List

In Python, lists are used to store multiple data at once.

Suppose we need to record the ages of 5 students. Instead of creating 5 separate variables, we can simply create a list.

Create a List

We create a list by placing elements inside [] , separated by commas. For example,

ages = [19, 26, 23] print(ages) # Output: [19, 26, 23]

Here, we have created a list named ages with 3 integer items.

# list with elements of different data types list1 = [1, "Hello", 3.4] # list with duplicate elements list1 = [1, "Hello", 3.4, "Hello", 1] # empty list list3 = []

Note: We can also create a list using the list() constructor.

Access List Elements

In Python, lists are ordered and each item in a list is associated with a number. The number is known as a list index.

The index of the first element is 0, second element is 1 and so on. For example,

languages = ["Python", "Swift", "C++"] # access item at index 0 print(languages[0]) # Python # access item at index 2 print(languages[2]) # C++

In the above example, we have created a list named languages .

Here, we can see each list item is associated with the index number. And we have used the index number to access the items.

Remember: The list index always starts with 0. Hence, the first element of a list is present at index 0, not 1.

Negative Indexing in Python

Python allows negative indexing for its sequences. The index of -1 refers to the last item, -2 to the second last item and so on.

languages = ["Python", "Swift", "C++"] # access item at index 0 print(languages[-1]) # C++ # access item at index 2 print(languages[-3]) # Python

Note: If the specified index does not exist in a list, Python throws the IndexError exception.

Slicing of a List

In Python, it is possible to access a portion of a list using the slicing operator : . For example,

# List slicing in Python my_list = ['p','r','o','g','r','a','m','i','z'] # items from index 2 to index 4 print(my_list[2:5]) # items from index 5 to end print(my_list[5:]) # items beginning to end print(my_list[:])

['o', 'g', 'r'] ['a', 'm', 'i', 'z'] ['p', 'r', 'o', 'g', 'r', 'a', 'm', 'i', 'z']

• my_list[2:5] returns a list with items from index 2 to index 4.
• my_list[5:] returns a list with items from index 5 to the end.
• my_list[:] returns all list items

Note: When we slice lists, the start index is inclusive, but the end index is exclusive.

Add Elements to a List

Lists are mutable (changeable). Meaning we can add and remove elements from a list.

Python list provides different methods to add items to a list.

1. Using append()

The append() method adds an item at the end of the list. For example,

numbers = [21, 34, 54, 12] print("Before Append:", numbers) # using append method numbers.append(32) print("After Append:", numbers)

Before Append: [21, 34, 54, 12] After Append: [21, 34, 54, 12, 32]

In the above example, we have created a list named numbers . Notice the line

Here, append() adds 32 at the end of the array.

2. Using extend()

We use the extend() method to add all the items of an iterable (list, tuple, string, dictionary, etc.) to the end of the list. For example,

numbers = [1, 3, 5] even_numbers = [4, 6, 8] # add elements of even_numbers to the numbers list numbers.extend(even_numbers) print("List after append:", numbers) 

 List after append: [1, 3, 5, 4, 6, 8]

Here, numbers.extend(even_numbers) adds all the elements of even_numbers to the numbers list.

3. Using insert()

We use the insert() method to add an element at the specified index.

numbers = [10, 30, 40] # insert an element at index 1 (second position) numbers.insert(1, 20) print(numbers) # [10, 20, 30, 40]

Change List Items

Python lists are mutable. Meaning lists are changeable. And we can change items of a list by assigning new values using the = operator. For example,

languages = ['Python', 'Swift', 'C++'] # changing the third item to 'C' languages[2] = 'C' print(languages) # ['Python', 'Swift', 'C']

Here, initially the value at index 3 is ‘C++’ . We then changed the value to ‘C’ using

Remove an Item From a List

1. Using del Statement

In Python we can use the del statement to remove one or more items from a list. For example,

languages = ['Python', 'Swift', 'C++', 'C', 'Java', 'Rust', 'R'] # deleting the second item del languages[1] print(languages) # ['Python', 'C++', 'C', 'Java', 'Rust', 'R'] # deleting the last item del languages[-1] print(languages) # ['Python', 'C++', 'C', 'Java', 'Rust'] # delete the first two items del languages[0 : 2] # ['C', 'Java', 'Rust'] print(languages)

2. Using remove()

We can also use the remove() method to delete a list item. For example,

languages = ['Python', 'Swift', 'C++', 'C', 'Java', 'Rust', 'R'] # remove 'Python' from the list languages.remove('Python') print(languages) # ['Swift', 'C++', 'C', 'Java', 'Rust', 'R']

Here, languages.remove(‘Python’) removes ‘Python’ from the languages list.

List Methods

Python has many useful list methods that makes it really easy to work with lists.

Method	Description
append()	add an item to the end of the list
extend()	add all the items of an iterable to the end of the list
insert()	inserts an item at the specified index
remove()	removes item present at the given index
pop()	returns and removes item present at the given index
clear()	removes all items from the list
index()	returns the index of the first matched item
count()	returns the count of the specified item in the list
sort()	sort the list in ascending/descending order
reverse()	reverses the item of the list
copy()	returns the shallow copy of the list

Iterating through a List

We can use a for loop to iterate over the elements of a list. For example,

languages = ['Python', 'Swift', 'C++'] # iterating through the list for language in languages: print(language)

Check if an Element Exists in a List

We use the in keyword to check if an item exists in the list or not. For example,

languages = ['Python', 'Swift', 'C++'] print('C' in languages) # False print('Python' in languages) # True

• ‘C’ is not present in languages , ‘C’ in languages evaluates to False .
• ‘Python’ is present in languages , ‘Python’ in languages evaluates to True .

List Length

We use the len() function to find the size of a list. For example,

languages = ['Python', 'Swift', 'C++'] print("List: ", languages) print("Total Elements: ", len(languages)) # 3

List: ['Python', 'Swift', 'C++'] Total Elements: 3

List Comprehension

List comprehension is a concise and elegant way to create lists. For example,

# create a list with value n ** 2 where n is a number from 1 to 5 numbers = [n**2 for n in range(1, 6)] print(numbers) # Output: [1, 4, 9, 16, 25]

numbers = [n**2 for n in range(1, 6)]

numbers = [] for n in range(1, 6): numbers.append(n**2)

Table of Contents

• Introduction
• Create a Python List
• Access List Elements
• Negative Indexing in Python
• Slicing of a List
• Add Elements to a List
• Change List Items
• Remove an Item From a List
• List Methods
• Iterating through a List
• Check if an Item Exists
• Python List Length

Video: Python Lists and Tuples

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Python Lists (With Examples)

List can be seen as a collection: they can hold many variables. List resemble physical lists, they can contain a number of items.

A list can have any number of elements. They are similar to arrays in other programming languages. Lists can hold all kinds of variables: integers (whole numbers), floats, characters, texts and many more.

Example

Empty list

Lets create an empty list. To define an empty list you should use brackets.
Brackets is what tells Python that the object is a list.

Lists can hold both numbers and text. Regardless of contents, they are accessed in the same fashion.

To access a list add the id between the brackets, such as list[0], list[1] and so on.

Define list

An empty list was defined above. Lists can contain all kinds of data.
You can create numeric lists like this:

Lists can contain strings or characters:

You can interact item by item using a for loop.

Access list items

You can access a list item by using brackets and its index. Python starts counting at zero, that means the first element is zero.

Computer languages used to count from zero. At the time when programming languages were first created, it made sense to count from zero. These days it would just be strange to change that tradition.

To get the first item, simply add the brackets and a zero after the list name.

Every other element can be accessed by the incremental numbers, to print the second item you’d use (1), to print the thrid item you’d use (2).

List example

#!/usr/bin/python

list = [ «New York», «Los Angles», «Boston», «Denver» ]

print(list) # prints all elements
print(list[0]) # print first element

list2 = [1,3,4,6,4,7,8,2,3]

print(sum(list2))
print(min(list2))
print(max(list2))
print(list2[0])
print(list2[-1])

Exercise

states = [ ‘Alabama’,‘Alaska’,‘Arizona’,‘Arkansas’,‘California’,‘Colorado’,‘Connecticut’,‘Delaware’,‘Florida’,‘Georgia’,‘Hawaii’,‘Idaho’,‘Illinois’,‘Indiana’,‘Iowa’,‘Kansas’,‘Kentucky’,‘Louisiana’,‘Maine’,‘Maryland’,‘Massachusetts’,‘Michigan’,‘Minnesota’,‘Mississippi’,‘Missouri’,‘Montana’,‘Nebraska’,‘Nevada’,‘New Hampshire’,‘New Jersey’,‘New Mexico’,‘New York’,‘North Carolina’,‘North Dakota’,‘Ohio’,‘Oklahoma’,‘Oregon’,‘Pennsylvania’,‘Rhode Island’,‘South Carolina’,‘South Dakota’,‘Tennessee’,‘Texas’,‘Utah’,‘Vermont’,‘Virginia’,‘Washington’,‘West Virginia’,‘Wisconsin’,‘Wyoming’ ]

Functions in Python (With Examples)

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