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.

A list can

• store elements of different types (integer, float, string, etc.)
• store duplicate elements

# 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.

Let's see an example,

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[:])

Output

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

Here,

• 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)

Output

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

numbers.append(32)

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) 

Output

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

languages[2] = 'C'

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.

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)

Output

Python
Swift
C++

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

Here,

• '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

Output

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]

Here, this code

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

is equivalent to

numbers = []

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