Python Lists

In Python, a list is a built-in data structure that allows us to store multiple items in a single variable. Lists are one of the most used data structures in Python because they are mutable, ordered, and can hold different types of elements. Lists provide a flexible way to handle collections of data and come with many useful built-in methods.

Let us take a look at a simple example of a list.

Example

# creating a list  

L = [20, 'Python App', 35.75, [20, 40, 60]]  

  

print(L)

Output:

[20, 'Python App', 35.75, [20, 40, 60]]

Explanation:

In the above example, we have created a simple list consisting of four elements of different data types. Each element in this list is indexed, meaning it can be accessed using its position in the list, as shown below:

• L[0] = 20 (Integer)
• L[1] = ‘Python App’ (String)
• L[2] = 35.75 (Float)
• L[3] = [20, 40, 60] (Nested List)

Characteristics of Python Lists

List in Python is a data structure, which is:

• Ordered: The data elements of list maintain their order of insertion. If an element is inserted at a specific index, it remains there unless it is explicitly changed.
• Changeable (Mutable): Lists in Python allow modification of their elements after creation.

Let us look at the following example showing that Lists are changeable.

Example

# creating a list  

my_lst = [25, 16.25, 'Python App', [20, 40, 60], 3e8]  

  

# changing the element of the list  

my_lst[1] = 'welcome'  

print(my_lst)

Output:

[25, 'welcome', 'Python App', [20, 40, 60], 300000000.0]

In this example, we have initialized a list and used the indexing to change its element.

• Heterogeneous (Different Data Types): A single list can store multiple data types like integers, strings, floats, and even another list.

Let us take a look at the following example showing that List is a heterogeneous data structure.

Example

# creating a list  

mixed_lst = [10, 'Hello', 5.23, [7, 12]]  

  

print(mixed_lst)

Output:

[10, 'Hello', 5.23, [7, 12]]

In this example, we have initialized a list consisting of multiple data types, such as integer, string, float, and a list.

• Contains Duplicates: List is a data structure that can store duplicate values.

Let us take an example showing that List allows duplicates.

Example

# creating a list  

dup_lst = [12, 51, 12, 5, 3, 5, 77, 42, 2, 12, 12, 6, 6]  

  

print(dup_lst)

Output:

[12, 51, 12, 5, 3, 5, 77, 42, 2, 12, 12, 6, 6]

In this example, we have initialized a list consisting of duplicate values.

Operations on List in Python

There are many operations that we can perform on Python list. Some of them are listed below:

1. Creating a List
2. Accessing List Elements
3. Adding Elements to List
4. Updating Elements into List
5. Removing Elements from the List
6. Iterating Over Lists

Let us now see the implementation of these list operations with examples.

Creating a List

Lists in Python can be created by enclosing elements within square brackets [], separated by commas.

Let us see an example showing the way of creating a list.

Example

# Creating lists  

empty_list = []  

numbers = [11, 4, 23, 71, 58]  

mixed_list = [1, "Hello", 6.74, True]  

  

print(empty_list)  

print(numbers)  

print(mixed_list)

Output:

[]
[11, 4, 23, 71, 58]
[1, 'Hello', 6.74, True]

Explanation:

In the above example, we have created three types of lists – empty list, list consisting of only integer values, and list consisting of elements of different data types. We have then printed them for reference.

Accessing List Elements

Elements in a list can be accessed using indexing. Python uses zero-based indexing, meaning the first element has an index of 0. We can use the index number enclosed with the square brackets ‘[]’ to access the element present at the given position.

Here is an example showing the method of accessing the element in a given list.

Example

# Creating lists  

my_list = [15, 23, 47, 32, 94]  

  

print(my_list[0])    

print(my_list[-1])

Output:

15
94

Explanation:

In the above example, we have created three types of lists – empty list, list consisting of only integer values, and list consisting of elements of different data types. We have then printed them for reference.

Adding Elements to the List

Python offers accessibility to add elements to a list. This can be achieved using the various options available for us:

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

Here is an example showing different methods to add elements to a list.

Example

# creating a list  

my_lst = [7, 3, 4]  

  

# Method 1: using append()  

my_lst.append(6)  

print(my_lst)  

  

# Method 2: using insert()  

my_lst.insert(1, 13)  

print(my_lst)  

  

# Method 3: using extend()  

my_lst.extend([14, 43, 37])  

print(my_lst)

Output:

[7, 3, 4, 6]
[7, 13, 3, 4, 6]
[7, 13, 3, 4, 6, 14, 43, 37]

Explanation:

In the above example, we have created a list and used three different methods – append(), insert() and extend(), in order to insert items in the list. As a result, the append() method has added an item at the end of the list. The insert() method has added an item at the second place in the list. The extend() method has added multiple elements altogether at the end of the list.

Updating Elements into a List

Elements in a list can be updated using indexing. The following is an example showing the way of updating items in a list.

Example

# creating a list  

my_lst = [17, 23, 35]  

  

# Updating the second element  

my_lst[1] = 52  

  

print(my_lst)

Output:

[17, 52, 35]

Explanation:

In the above example, we have created a list. We then used the indexing to update the second element of the list.

Removing Elements from the List

Python provides several methods to remove elements from a list:

• remove(value): Removes the first occurrence of the value.
• pop(index): Removes the element at a specific index (or the last element by default).
• del statement: Deletes an element or the entire list.

Let us take a look at the following example showing several ways to remove an item from the list.

Example

# creating a list  

my_lst = [14, 25, 43, 2, 33]  

  

# Method 1: using remove()  

my_lst.remove(43)  

print(my_lst)  

  

# Method 2: using pop()  

my_lst.pop(-1)  

print(my_lst)  

  

# Method 3: using del keyword  

del my_lst[0]  

print(my_lst)

Output:

[14, 25, 2, 33]
[14, 25, 2]
[25, 2]

Explanation:

In the above example, we have created a list and used three different approaches – remove(), pop() and del keyword, in order to remove items from the list. As a result, remove() method has removed the specified item from the list. The pop() method has removed the element at the specified index from the list. The del keyword has deleted the element from the list at the specified index.

Iterating Over Lists

In Python, we can iterate over lists using the loops. Let us take an example showing the use of ‘for’ loop and ‘while’ loop to iterate over the elements of a given list.

Example

# creating a list  

my_lst = [12, 23, 9, 17, 41]  

  

print("Iterating List using 'for' loop:")  

# Using for loop  

for ele in my_lst:  

    print(ele)  

  

print("\nIterating List using 'while' loop:")  

# Using while loop  

i = 0  

while i < len(my_lst):  

    print(my_lst[i])  

    i += 1

Output:

Iterating List using 'for' loop:
12
23
9
17
41

Iterating List using 'while' loop:
12
23
9
17
41

Explanation:

In this example, we have created a list. We have then used the for loop to iterate over the list. We then used the while loop where we set the initial index value, i as 0; and incremented it while printing the element of the current index from the list.

Nested Lists in Python

Nested List is a list in Python that consists of multiple sub-lists separated by commas. The elements in each sub-list can be of different data types. The inner lists (or sub-list) can again consist of other lists, giving rise to the multiple levels of nesting.

In Python, we can use nested lists to create hierarchical data structures, matrices or simply, a list of lists. Python provides various tools to handle nested lists efficiently and effectively, allowing users to perform standard functions on them.

Let us see an example showing the different operations on nested list in Python.

Example

# creating a nested list  

nested_lst = [[14, 6, 8], [13, 18, 25], [72, 48, 69]]  

  

print("Nested List:", nested_lst)  

  

print("\nAccessing Elements of Nested List:")  

# Accessing elements  

print("nested_lst[0]:", nested_lst[0])  

print("nested_lst[0][1]:", nested_lst[0][1])  

  

print("\nIterating through Nested list using 'for' loop:")  

# Iterating through a nested list  

for sublist in nested_lst:  

  for item in sublist:  

    print(item)  

  

print("\nAdding element to Nested list:")  

# Adding elements  

nested_lst[0].append(10)  

print("New Nested List:", nested_lst)

Output:

Nested List: [[14, 6, 8], [13, 18, 25], [72, 48, 69]]

Accessing Elements of Nested List:
nested_lst[0]: [14, 6, 8]
nested_lst[0][1]: 6

Iterating through Nested list using 'for' loop:
14
6
8
13
18
25
72
48
69

Adding element to Nested list:
New Nested List: [[14, 6, 8, 10], [13, 18, 25], [72, 48, 69]]

Explanation:

In the above example, we have created a nested list. We then used the indexing to access the elements of the list. In first case, we have accessed the element of the outer list using nested_lst[0], which returns the inner list as an output, i.e., [14, 6, 8]. In second case, we have accessed the element of the inner list using nested_lst[0][1], which returns the element of the inner list, i.e., 6. We then perform iteration on the nested list using the nested for loop. At last, we have added an element to the nested list using the append() method.

Finding the Length of a List

Python provides a built-in function called len() in order to determine the number of elements in a list.

Syntax:

len(list_name)  

Let us take an example showing the use of the len() function on lists.

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

  

# using the len() function to find the size of the list  

no_of_ele = len(lst_one)  

  

print("Given List:", lst_one)  

print("Number of Elements in the given list:", no_of_ele) 

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Number of Elements in the given list: 7

Explanation:

In the above example, we have created a list of some integers. We have then used the len() function to find the size of the list and store it in a variable. We then printed the list and the variable value as a result.

Sorting a List

Python provides a method called sort() that allows users to sort the elements of a list in place. Let us look at an example showing the working of sort() method.

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

print("Given List:", lst_one)  

  

# sorting the list in ascending order (by default)  

lst_one.sort()  

print("Sorted List:", lst_one)

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Sorted List: [5, 6, 15, 23, 61, 87, 93]

Explanation:

In the above example, we have created a list and used the sort() method to sort it into ascending order.

Python Sorted List Example

Python also offers a function called sorted() which works similar to the sort() method; however, it returns a new list.

Let us take a look at the following example showing the use of sorted() function.

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

  

# sorting the list  

new_lst = sorted(lst_one)  

  

print("Given List:", lst_one)  

print("Sorted List:", new_lst)

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Sorted List: [5, 6, 15, 23, 61, 87, 93]

Explanation:

In the example, we have created a list and used the sorted() function to sort the list.

Reversing a List

Python List provides a method called reverse() that allows users to reverse the order of the elements in the list.

Let us see the following example showing the use of reverse() method.

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

print("Given List:", lst_one)  

  

# reversing the list  

lst_one.reverse()  

print("Reversed List:", lst_one)

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Reversed List: [93, 5, 87, 6, 23, 61, 15]

Explanation:

In the above example, we have created a list and used the reverse() method to reverse the order of the elements in the given list.

Finding the Largest and Smallest Number in a List

To find the largest and smallest number in a list, we can make use of max() and min() functions as shown in the following example:

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

print("Given List:", lst_one)  

  

# finding largest and smallest number  

print("Largest Number:", max(lst_one))  

print("Smallest Number:", min(lst_one))

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Largest Number: 93
Smallest Number: 5

Explanation:

In the above example, we have created a list. We then used the max() function to determine the largest number in the given list. We also used the min() function to find the smallest number.

Calculating Sum of Elements in a List

In order to calculate the sum of elements in a list, we can use Python’s built-in sum() function as shown in the following example:

Example

# creating a list  

lst_one = [15, 61, 23, 6, 87, 5, 93]  

print("Given List:", lst_one)  

  

# finding sum of the elements in the list  

print("Sum of the Elements in the list:", sum(lst_one))

Output:

Given List: [15, 61, 23, 6, 87, 5, 93]
Sum of the Elements in the list: 290

Explanation:

In the above example, we have created a list. We then used the sum() function to calculate the sum of the elements in the given list.

Conclusion

Python lists provide a versatile way to handle multiple data items. With functionalities like appending, inserting, updating, and iterating, lists form the backbone of many Python programs. Mastering lists is essential for efficient programming in Python.