Python Lists

In Python, a list is a built-in dynamic sized array (automatically grows and shrinks). We can store all types of items (including another list) in a list. A list may contain mixed type of items, this is possible because a list mainly stores references at contiguous locations and actual items maybe stored at different locations.

• List can contain duplicate items.
• List in Python are Mutable. Hence, we can modify, replace or delete the items.
• List are ordered. It maintain the order of elements based on how they are added.
• Accessing items in List can be done directly using their position (index), starting from 0.

a = [10, 20, 15]

print(a[0]) # access first item
a.append(11) # add item
a.remove(20) # remove item

print(a)

10
[10, 15, 11]

Creating a List

Here are some common methods to create a list:

Using Square Brackets

# List of integers
a = [1, 2, 3, 4, 5]

# List of strings
b = ['apple', 'banana', 'cherry']

# Mixed data types
c = [1, 'hello', 3.14, True]

print(a)
print(b)
print(c)

[1, 2, 3, 4, 5]
['apple', 'banana', 'cherry']
[1, 'hello', 3.14, True]

Using list() Constructor

We can also create a list by passing an iterable (like a string, tuple, or another list) to list() function.

# From a tuple
a = list((1, 2, 3, 'apple', 4.5))  

print(a)

[1, 2, 3, 'apple', 4.5]

Creating List with Repeated Elements

We can create a list with repeated elements using the multiplication operator.

# Create a list [2, 2, 2, 2, 2]
a = [2] * 5

# Create a list [0, 0, 0, 0, 0, 0, 0]
b = [0] * 7

print(a)
print(b)

[2, 2, 2, 2, 2]
[0, 0, 0, 0, 0, 0, 0]

Accessing List Elements

Elements in a list can be accessed using indexing. Python indexes start at 0, so a[0] will access the first element, while negative indexing allows us to access elements from the end of the list. Like index -1 represents the last elements of list.

a = [10, 20, 30, 40, 50]

# Access first element
print(a[0])    

# Access last element
print(a[-1])

10
50

Adding Elements into List

We can add elements to a list using the following methods:

• append(): Adds an element at the end of the list.
• extend(): Adds multiple elements to the end of the list.
• insert(): Adds an element at a specific position.

# Initialize an empty list
a = []

# Adding 10 to end of list
a.append(10)  
print("After append(10):", a)  

# Inserting 5 at index 0
a.insert(0, 5)
print("After insert(0, 5):", a) 

# Adding multiple elements  [15, 20, 25] at the end
a.extend([15, 20, 25])  
print("After extend([15, 20, 25]):", a) 

After append(10): [10]
After insert(0, 5): [5, 10]
After extend([15, 20, 25]): [5, 10, 15, 20, 25]

Updating Elements into List

We can change the value of an element by accessing it using its index.

a = [10, 20, 30, 40, 50]

# Change the second element
a[1] = 25 

print(a)  

[10, 25, 30, 40, 50]

Removing Elements from List

We can remove elements from a list using:

• remove(): Removes the first occurrence of an element.
• pop(): Removes the element at a specific index or the last element if no index is specified.
• del statement: Deletes an element at a specified index.

a = [10, 20, 30, 40, 50]

# Removes the first occurrence of 30
a.remove(30)  
print("After remove(30):", a)

# Removes the element at index 1 (20)
popped_val = a.pop(1)  
print("Popped element:", popped_val)
print("After pop(1):", a) 

# Deletes the first element (10)
del a[0]  
print("After del a[0]:", a)  

After remove(30): [10, 20, 40, 50]
Popped element: 20
After pop(1): [10, 40, 50]
After del a[0]: [40, 50]

Iterating Over Lists

We can iterate the Lists easily by using a for loop or other iteration methods. Iterating over lists is useful when we want to do some operation on each item or access specific items based on certain conditions. Let’s take an example to iterate over the list using for loop.

Using for Loop

a = ['apple', 'banana', 'cherry']

# Iterating over the list
for item in a:
    print(item)

apple
banana
cherry

To learn various other methods, please refer to iterating over lists.

Nested Lists in Python

A nested list is a list within another list, which is useful for representing matrices or tables. We can access nested elements by chaining indexes.

matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

# Access element at row 2, column 3
print(matrix[1][2]) 

6

To learn more, please refer to Multi-dimensional lists in Python

Basic Example on Python List

• Length of a List
• Maximum in a List
• Swap two items in a list
• Check if an Element Exists
• Index of an Item
• Remove Duplicates from a List
• Reverse a List in Python
• Sum of a List
• Concatenate Two Lists
• Sort a List in Python
• Check for Sublist

Related Articles:

• List Method
• List Comprehensions
• List Slicing

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