What are Data Structures?

Data structures are ways of organizing and storing data so that a program can access, update and process it efficiently.

Python provides several built-in data structures that are very easy to use.

• List
• Tuple
• Set
• Dictionary

Choosing the correct data structure makes your Python programs easier to write, understand and maintain.

Python Data Structures at a Glance

1. List

A List is an ordered and mutable collection. It can store multiple values in a single variable.

Creating a List

courses = ["Python", "Java", ".NET", "Data Science"]

print(courses)

Output:

['Python', 'Java', '.NET', 'Data Science']

Accessing List Elements

courses = ["Python", "Java", ".NET"]

print(courses[0])
print(courses[1])
print(courses[2])

Output:

Python
Java
.NET

Adding Elements

courses = ["Python", "Java"]

courses.append(".NET")

print(courses)

Output:

['Python', 'Java', '.NET']

Updating Elements

courses = ["Python", "Java", ".NET"]

courses[1] = "MERN"

print(courses)

Output:

['Python', 'MERN', '.NET']

Removing Elements

courses = ["Python", "Java", ".NET"]

courses.remove("Java")

print(courses)

Output:

['Python', '.NET']

Common List Methods

2. Tuple

A Tuple is an ordered collection that cannot be changed after creation.

Tuples are useful when data should remain unchanged.

student = ("Rahul", 22, "Python")

print(student)

Output:

('Rahul', 22, 'Python')

Accessing Tuple Elements

student = ("Rahul", 22, "Python")

print(student[0])
print(student[2])

Output:

Rahul
Python

Tuple Cannot Be Modified

student = ("Rahul", 22, "Python")

student[1] = 23

This produces a TypeError because tuples are immutable.

3. Set

A Set is a collection used when unique values are required.

Duplicate values are automatically removed.

courses = {"Python", "Java", "Python", ".NET"}

print(courses)

Example Output:

{'Python', 'Java', '.NET'}

Adding an Item

courses = {"Python", "Java"}

courses.add(".NET")

print(courses)

Removing an Item

courses = {"Python", "Java", ".NET"}

courses.remove("Java")

print(courses)

Set Operations

python_students = {"Rahul", "Amit", "Sneha"}

java_students = {"Amit", "Sneha", "Priya"}

print(python_students | java_students)

print(python_students & java_students)

print(python_students - java_students)

Operations:

• | → Union
• & → Intersection
• - → Difference

4. Dictionary

A Dictionary stores data in key-value pairs.

student = {
    "name": "Rahul",
    "age": 22,
    "course": "Python"
}

print(student)

Output:

{'name': 'Rahul', 'age': 22, 'course': 'Python'}

Accessing Values

student = {
    "name": "Rahul",
    "age": 22,
    "course": "Python"
}

print(student["name"])
print(student["course"])

Output:

Rahul
Python

Adding a New Key

student["city"] = "Pune"

print(student)

Updating a Value

student["age"] = 23

print(student["age"])

Output:

23

Removing a Key

student.pop("city")

print(student)

Common Dictionary Methods

student = {
    "name": "Rahul",
    "course": "Python",
    "city": "Pune"
}

print(student.keys())
print(student.values())
print(student.items())

Nested Data Structures

Python data structures can contain other data structures.

students = [
    {
        "name": "Rahul",
        "course": "Python"
    },
    {
        "name": "Sneha",
        "course": ".NET"
    }
]

print(students[0]["name"])
print(students[1]["course"])

Output:

Rahul
.NET

This type of structure is very common when working with JSON data and REST APIs.

CIIT Example 📩🤓

Suppose CIIT needs to store student details. A dictionary can represent one student and a list can store multiple students.

students = [
    {
        "name": "Rahul",
        "course": "Python",
        "fees": 35000
    },
    {
        "name": "Sneha",
        "course": ".NET",
        "fees": 35000
    },
    {
        "name": "Amit",
        "course": "Java",
        "fees": 30000
    }
]

for student in students:

    print(student["name"])
    print(student["course"])
    print(student["fees"])
    print("----------------")

Output:

Rahul
Python
35000
----------------
Sneha
.NET
35000
----------------
Amit
Java
30000
----------------

Which Data Structure Should You Use?

Common Mistakes

1. Trying to modify a Tuple.
2. Assuming Set elements always have a fixed display order.
3. Accessing a Dictionary key that does not exist without using get().
4. Confusing List indexes with Dictionary keys.
5. Choosing a data structure without considering whether the data needs to change.

Interview Points

• What is the difference between List and Tuple?
• Why are Sets used?
• What is a Dictionary?
• Can a List contain different data types?
• How do you remove duplicate values from a collection?
• What is the difference between List, Set and Dictionary?

CIIT Practice Tasks

1. Create a list of 10 course names and display them using a loop.
2. Create a Tuple containing student name, age and course.
3. Create a Set containing duplicate course names and observe the result.
4. Create a Dictionary for a student with name, email, course and fees.
5. Create a list of dictionaries containing five CIIT students.
6. Find students enrolled in the Python course from the list of dictionaries.