Python Dictionaries

What Are Dictionaries?

A dictionary is a Python data structure used to store information as key-value pairs.

Unlike a list, where we normally access data using a numerical index, dictionaries allow us to access data using meaningful keys.

For example, instead of remembering that an employee's name is stored at index 0, we can simply access it using the key "name".

student = {
    "name": "Ali",
    "age": 21,
    "course": "Python"
}

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

Ali
Python

Why Dictionaries Matter

Dictionaries are extremely important because real-world information is often naturally represented using labels and values.

Consider a customer:

customer = {
    "name": "Ahmed",
    "email": "ahmed@example.com",
    "city": "Multan",
    "age": 28
}

Each piece of information has a meaningful name. This makes the data easier to understand and work with.

Creating Dictionaries

Dictionaries are created using curly braces { }.

dictionary_name = {
    key: value,
    key: value
}

Example:

person = {
    "name": "Sara",
    "age": 25,
    "city": "Lahore"
}

print(person)

{'name': 'Sara', 'age': 25, 'city': 'Lahore'}

Key-Value Pairs

Every dictionary entry consists of two parts:

• Key — identifies the data.
• Value — stores the actual data.

employee = {
    "name": "Usman",
    "salary": 75000,
    "department": "Sales"
}

Here:

• "name" is a key.
• "Usman" is its value.
• "salary" is a key.
• 75000 is its value.

Accessing Dictionary Values

You can access a value by placing its key inside square brackets.

student = {
    "name": "Ali",
    "age": 21,
    "grade": "A"
}

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

Ali
21

Using get()

The get() method is a safer way to access dictionary values.

student = {
    "name": "Ali",
    "age": 21
}

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

Ali
None

You can also provide a default value.

print(student.get("course", "Not Available"))

Not Available

Changing Dictionary Values

Dictionaries are mutable, meaning their contents can be changed after creation.

student = {
    "name": "Ali",
    "age": 21
}

student["age"] = 22

print(student)

{'name': 'Ali', 'age': 22}

Adding New Items

To add a new key-value pair, simply assign a value to a new key.

student = {
    "name": "Ali",
    "age": 21
}

student["city"] = "Multan"

print(student)

{'name': 'Ali', 'age': 21, 'city': 'Multan'}

Removing Items

Python provides several ways to remove dictionary items.

Using pop()

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan"
}

removed = student.pop("age")

print(removed)
print(student)

21
{'name': 'Ali', 'city': 'Multan'}

Using del

del student["city"]

Using clear()

student.clear()

print(student)

{}

Finding Dictionary Length

Use the len() function to count the number of key-value pairs.

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan"
}

print(len(student))

3

Membership Testing

You can use in and not in to check whether a key exists.

student = {
    "name": "Ali",
    "age": 21
}

print("name" in student)
print("email" in student)
print("city" not in student)

True
False
True

keys() & values()

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

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan"
}

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

These methods provide dynamic view objects that reflect changes to the dictionary.

print(list(student.keys()))
print(list(student.values()))

['name', 'age', 'city']
['Ali', 21, 'Multan']

items()

The items() method provides the dictionary's key-value pairs.

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan"
}

print(student.items())

It becomes especially useful when looping through a dictionary.

for key, value in student.items():
    print(key, ":", value)

name : Ali
age : 21
city : Multan

Looping Through Dictionaries

Dictionaries can be processed using for loops.

Loop Through Keys

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan"
}

for key in student:
    print(key)

Loop Through Values

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

Loop Through Key-Value Pairs

for key, value in student.items():
    print(f"{key}: {value}")

Nested Dictionaries

A dictionary can contain another dictionary as a value. This is called a nested dictionary.

students = {
    "student1": {
        "name": "Ali",
        "age": 21
    },

    "student2": {
        "name": "Sara",
        "age": 22
    }
}

You can access nested data using multiple keys.

print(students["student1"]["name"])
print(students["student2"]["age"])

Ali
22

Nested dictionaries are useful for representing structured data such as customers, products, employees, and API responses.

Copying Dictionaries

Assigning a dictionary to another variable does not create an independent copy.

original = {
    "name": "Ali",
    "age": 21
}

copy = original

copy["age"] = 30

print(original)

{'name': 'Ali', 'age': 30}

Both variables refer to the same dictionary.

To create a separate shallow copy, use copy().

original = {
    "name": "Ali",
    "age": 21
}

copy = original.copy()

copy["age"] = 30

print(original)
print(copy)

{'name': 'Ali', 'age': 21}
{'name': 'Ali', 'age': 30}

Using update()

The update() method adds new key-value pairs or changes existing values.

student = {
    "name": "Ali",
    "age": 21
}

student.update({
    "age": 22,
    "city": "Multan"
})

print(student)

{'name': 'Ali', 'age': 22, 'city': 'Multan'}

Existing keys are updated, while missing keys are added.

Real-World Dictionary Examples

Product Information

product = {
    "name": "Laptop",
    "price": 125000,
    "brand": "Dell",
    "stock": 12
}

print(f"Product: {product['name']}")
print(f"Price: Rs. {product['price']}")
print(f"Stock: {product['stock']}")

Employee Record

employee = {
    "id": 101,
    "name": "Ahmed",
    "department": "Marketing",
    "salary": 85000
}

if employee["salary"] > 80000:
    print("High-value employee")

Student Marks

marks = {
    "Math": 85,
    "English": 78,
    "Computer": 92,
    "Physics": 88
}

average = sum(marks.values()) / len(marks)

print(f"Average: {average:.2f}")

Average: 85.75

Common Dictionary Mistakes

Mistake 1: Using a Missing Key

student = {"name": "Ali"}

print(student["age"])

This causes a KeyError.

Safer approach:

print(student.get("age", "Not available"))

Mistake 2: Forgetting the Colon

# Incorrect
student = {
    "name" "Ali"
}

Key-value pairs require a colon:

# Correct
student = {
    "name": "Ali"
}

Mistake 3: Confusing Keys and Values

student = {
    "name": "Ali"
}

print("Ali" in student)

False

The in operator checks keys by default.

Mistake 4: Expecting Duplicate Keys

student = {
    "name": "Ali",
    "name": "Ahmed"
}

print(student)

{'name': 'Ahmed'}

The second value replaces the first one.

Practice Exercises

Mini Challenge — Student Record Manager

Build a small program that stores student information using a dictionary.

Your program should store:

• Student name
• Age
• City
• Course
• Marks

Then display the student's information and calculate whether the student has passed.

student = {
    "name": "Ali",
    "age": 21,
    "city": "Multan",
    "course": "Python",
    "marks": 82
}

print("Student Information")
print("-------------------")

for key, value in student.items():
    print(f"{key.title()}: {value}")

if student["marks"] >= 50:
    print("Result: Passed")
else:
    print("Result: Failed")

Student Information
-------------------
Name: Ali
Age: 21
City: Multan
Course: Python
Marks: 82
Result: Passed

Quick Knowledge Check

Test your understanding before moving to the next lesson.

Lesson Summary

Python dictionaries are one of the most useful data structures for storing structured information.