What is a Dictionary?

A Dictionary is a built-in Python data structure used to store data in the form of key-value pairs.

Instead of accessing data using numeric indexes like a List, we access Dictionary values using their keys.

Dictionaries are widely used in real-world applications for storing structured data such as student details, employee records, product information and API responses.

• Stores key-value pairs
• Mutable
• Keys must be unique
• Values can be duplicated
• Keys are used to access values

Dictionary Structure

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

Here:

• "name" is a key and "Rahul" is its value.
• "age" is a key and 22 is its value.
• "course" is a key and "Python" is its value.

Creating a Dictionary

Dictionaries are commonly created using curly braces {}.

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

print(student)

Output:

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

Accessing Dictionary Values

We can access a Dictionary value by using its key inside square brackets.

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

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

Output:

Rahul
Python

Using get()

The get() method is useful for safely accessing a Dictionary value.

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

print(student.get("name"))
print(student.get("course"))

Output:

Rahul
Python

Missing Key

print(student.get("email"))

Output:

None

Unlike direct bracket access, using get() returns None when the key is not found unless a default value is provided.

Adding a New Key-Value Pair

A new key-value pair can be added by assigning a value to a new key.

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

student["city"] = "Pune"

print(student)

Output:

{'name': 'Rahul', 'course': 'Python', 'city': 'Pune'}

Updating a Dictionary Value

Since Dictionaries are mutable, existing values can be changed.

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

student["age"] = 23

print(student)

Output:

{'name': 'Rahul', 'age': 23, 'course': 'Python'}

Updating using update()

The update() method can update existing values and add new key-value pairs.

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

student.update({
    "course": "Python Full Stack",
    "city": "Pune"
})

print(student)

Output:

{
    'name': 'Rahul',
    'course': 'Python Full Stack',
    'city': 'Pune'
}

Removing Dictionary Items

pop()

The pop() method removes a specified key and returns its value.

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

removed_value = student.pop("age")

print(removed_value)
print(student)

Output:

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

popitem()

The popitem() method removes and returns the last inserted key-value pair.

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

item = student.popitem()

print(item)
print(student)

del

The del statement can remove a specific key.

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

del student["age"]

print(student)

clear()

The clear() method removes all key-value pairs.

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

student.clear()

print(student)

Output:

{}

Dictionary keys()

The keys() method returns the Dictionary's keys.

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

print(student.keys())

We can also loop through the keys:

for key in student.keys():
    print(key)

Output:

name
age
course

Dictionary values()

The values() method returns the Dictionary's values.

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

for value in student.values():
    print(value)

Output:

Rahul
22
Python

Dictionary items()

The items() method returns key-value pairs.

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

for key, value in student.items():

    print(key, ":", value)

Output:

name : Rahul
age : 22
course : Python

Checking if a Key Exists

The in operator can check whether a key exists in a Dictionary.

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

print("name" in student)
print("email" in student)

Output:

True
False

Finding Dictionary Length

The len() function returns the number of key-value pairs.

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

print(len(student))

Output:

3

Different Types of Values

Dictionary values can contain different Python data types.

student = {
    "name": "Rahul",
    "age": 22,
    "marks": 85.5,
    "passed": True,
    "skills": ["Python", "SQL"]
}

print(student)

A Dictionary can therefore represent structuredz real-world information.

List inside a Dictionary

A Dictionary value can itself be a List.

student = {
    "name": "Rahul",
    "skills": [
        "Python",
        "SQL",
        "Git"
    ]
}

print(student["skills"])
print(student["skills"][0])

Output:

['Python', 'SQL', 'Git']
Python

Nested Dictionary

A Dictionary can contain another Dictionary as a value.

student = {
    "name": "Rahul",

    "address": {
        "city": "Pune",
        "state": "Maharashtra"
    }
}

print(student["address"]["city"])

Output:

Pune

List of Dictionaries

A List can contain multiple Dictionaries. This structure is very common in API and database-related applications.

students = [

    {
        "name": "Rahul",
        "course": "Python"
    },

    {
        "name": "Sneha",
        "course": ".NET"
    },

    {
        "name": "Amit",
        "course": "Java"
    }

]

for student in students:

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

Output:

Rahul - Python
Sneha - .NET
Amit - Java

CIIT Example 🤓📩

Suppose CIIT wants to maintain student information including name, course, fees and placement status.

students = [

    {
        "name": "Rahul",
        "course": "Python",
        "fees": 35000,
        "placed": True
    },

    {
        "name": "Sneha",
        "course": ".NET",
        "fees": 35000,
        "placed": False
    },

    {
        "name": "Amit",
        "course": "Java",
        "fees": 30000,
        "placed": True
    }

]

for student in students:

    print("Name:", student["name"])
    print("Course:", student["course"])
    print("Fees:", student["fees"])
    print("Placed:", student["placed"])
    print("----------------------")

This type of List + Dictionary structure is commonly used while working with structured application data.

Name: Rahul
Course: Python
Fees: 35000
Placed: True
----------------------
Name: Sneha
Course: .NET
Fees: 35000
Placed: False
----------------------
Name: Amit
Course: Java
Fees: 30000
Placed: True
----------------------

Dictionary Comprehension Preview

Python also provides a shorter way to create Dictionaries using Dictionary Comprehension.

numbers = [1, 2, 3, 4, 5]

squares = {
    number: number * number
    for number in numbers
}

print(squares)

Output:

{1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Dictionary Comprehension will be covered in detail in the Comprehensions topic.

Important Dictionary Methods

Common Mistakes

1. Trying to access a Dictionary using a numeric index like a List.
2. Using duplicate keys and expecting multiple separate values for the same key.
3. Accessing a missing key directly with square brackets.
4. Confusing Dictionary keys with values.
5. Forgetting that Dictionary values can themselves contain Lists or Dictionaries.

Interview Points

• What is a Dictionary in Python?
• What is a key-value pair?
• Can Dictionary keys be duplicated?
• What is the difference between get() and square bracket access?
• What is the difference between keys(), values() and items()?
• How do you add and update Dictionary values?
• How do you remove a key from a Dictionary?
• What is a nested Dictionary?

CIIT Practice Tasks

1. Create a Dictionary containing student name, age, course and city.
2. Add a new email key to the Dictionary.
3. Update the student's course.
4. Remove one key using pop().
5. Print all keys, values and key-value pairs.
6. Create a nested Dictionary for student address details.
7. Create a List containing five student Dictionaries and display their names.
8. From a List of student Dictionaries, display only students whose course is Python.