Lesson 10: Dictionaries in Python

🎯 Lesson Objective

To understand how to create, access, and manipulate dictionaries — one of Python’s most powerful data structures for storing key-value pairs.

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🧩 1. What Is a Dictionary?

A dictionary is a collection of key-value pairs enclosed in curly braces {}.
Each key is unique, and it is used to access its corresponding value.

✅ Example:

student = {
    "name": "Sameer",
    "age": 25,
    "city": "Warangal"
}

print(student)

Output:

{'name': 'Sameer', 'age': 25, 'city': 'Warangal'}

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⚙️ 2. Accessing Dictionary Elements

You can access values using their keys.

print(student["name"])      # Using key directly
print(student.get("age"))   # Using get() method

Output:

Sameer
25

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🧱 3. Adding and Updating Items

✅ Add a New Key-Value Pair

student["course"] = "Python"
print(student)

✅ Update Existing Value

student["age"] = 26
print(student)

Output:

{'name': 'Sameer', 'age': 26, 'city': 'Warangal', 'course': 'Python'}

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❌ 4. Removing Items

Method	Description	Example
pop(key)	Removes key and returns value	student.pop("city")
del	Deletes key-value pair	del student["age"]
clear()	Removes all items	student.clear()

✅ Example:

student.pop("city")
print(student)

Output:

{'name': 'Sameer', 'age': 25}

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🔁 5. Looping Through a Dictionary

✅ Loop Through Keys:

for key in student:
    print(key)

✅ Loop Through Values:

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

✅ Loop Through Both:

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

Output:

name : Sameer
age : 25
city : Warangal

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🧮 6. Checking for Keys

You can verify if a key exists using the in operator.

if "name" in student:
    print("Name exists!")

Output:

Name exists!

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📦 7. Dictionary Methods

Method	Description	Example
keys()	Returns all keys	student.keys()
values()	Returns all values	student.values()
items()	Returns key-value pairs	student.items()
copy()	Returns a shallow copy	student.copy()
update()	Adds or updates multiple items	student.update({"country": "India"})

✅ Example:

student.update({"country": "India", "language": "English"})
print(student)

Output:

{'name': 'Sameer', 'age': 25, 'city': 'Warangal', 'country': 'India', 'language': 'English'}

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🔗 8. Nested Dictionaries

You can store dictionaries inside other dictionaries.

✅ Example:

students = {
    "s1": {"name": "Ali", "age": 22},
    "s2": {"name": "Sara", "age": 21},
    "s3": {"name": "John", "age": 23}
}

print(students["s2"]["name"])

Output:

Sara

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🔄 9. Dictionary Comprehension

Create a dictionary in one line using comprehension.

✅ Example:

squares = {x: x*x for x in range(5)}
print(squares)

Output:

{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

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🌍 10. Real-Life Example – Contact Book

contacts = {
    "Ali": "9876543210",
    "Sara": "9123456789",
    "John": "9988776655"
}

name = input("Enter name: ")
if name in contacts:
    print(f"{name}'s number is {contacts[name]}")
else:
    print("Contact not found.")

Output:

Enter name: Sara
Sara's number is 9123456789

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