Python Dictionaries (Part 1)

A dictionary is one of the most useful built-in data structures in Python. It stores data in the form of key-value pairs. Unlike lists, dictionaries access data using unique keys instead of index positions. Dictionaries are widely used to represent real-world data such as student records, employee information, product details, and configuration settings.

---

Learning Objectives

• Understand the concept of dictionaries.
• Create dictionaries using different methods.
• Access values using keys.
• Add, update, and delete dictionary elements.
• Traverse dictionaries using loops.

---

What is a Dictionary?

A dictionary is a collection of key-value pairs enclosed within curly braces { }. Every key is unique and is used to access its corresponding value.

Syntax

dictionary={
    key1:value1,
    key2:value2,
    key3:value3
}

---

Examples

Student Dictionary

student={
    "roll":101,
    "name":"Rahul",
    "class":"XI",
    "marks":92
}

Product Dictionary

product={
    "id":501,
    "name":"Laptop",
    "price":55000
}

---

Characteristics of Dictionaries

• Store data as key-value pairs.
• Keys must be unique.
• Values can be duplicated.
• Dictionaries are mutable.
• Elements are accessed using keys.
• A dictionary can contain different data types.

---

Creating a Dictionary

Using Curly Braces

employee={
    "id":101,
    "name":"Aman",
    "salary":45000
}

Empty Dictionary

data={}

---

Accessing Dictionary Values

Dictionary values are accessed using their keys.

student={
    "name":"Rahul",
    "marks":95
}

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

Output

Rahul
95

---

Adding New Elements

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

student={
    "name":"Rahul",
    "marks":95
}

student["city"]="Jaipur"

print(student)

Output

{'name':'Rahul','marks':95,'city':'Jaipur'}

---

Updating Existing Values

student={
    "name":"Rahul",
    "marks":95
}

student["marks"]=98

print(student)

Output

{'name':'Rahul','marks':98}

---

Deleting Elements

student={
    "name":"Rahul",
    "marks":95,
    "city":"Jaipur"
}

del student["city"]

print(student)

Output

{'name':'Rahul','marks':95}

---

Membership Operators

The in and not in operators check whether a key exists in a dictionary.

student={
    "name":"Rahul",
    "marks":95
}

print("name" in student)
print("city" in student)

Output

True
False

---

Traversing a Dictionary

Using for Loop

student={
    "name":"Rahul",
    "marks":95,
    "city":"Jaipur"
}

for key in student:
    print(key,student[key])

Output

name Rahul
marks 95
city Jaipur

---

Real-Life Applications

• Student information systems.
• Employee records.
• Library management systems.
• Online shopping applications.
• Hospital patient records.
• Configuration settings.

---

Common Programming Mistakes

1. Accessing a Non-Existing Key

student={
    "name":"Rahul"
}

print(student["city"])

Result: KeyError

---

2. Using Duplicate Keys

student={
    "name":"Rahul",
    "name":"Aman"
}

Explanation: Only the last value is retained because dictionary keys must be unique.

---

3. Assuming Dictionaries Use Index Numbers

Dictionary elements are accessed using keys, not numeric indexes.

---

CBSE Important Programs

1. Create a dictionary of student details.
2. Display all key-value pairs.
3. Add a new key-value pair.
4. Update the value of an existing key.
5. Delete a key-value pair.
6. Check whether a key exists.
7. Display all student details using a loop.
8. Create a product dictionary.
9. Create an employee dictionary.
10. Display values using keys.

---

Viva Questions

1. What is a dictionary?
2. How is a dictionary different from a list?
3. Why must dictionary keys be unique?
4. Can dictionary values be duplicated?
5. How are dictionary values accessed?
6. Are dictionaries mutable?
7. How do you add a new key-value pair?
8. How do you update an existing value?
9. How do you delete a dictionary element?
10. Give two real-life applications of dictionaries.

---

Exam Tips

• Remember that dictionaries store data as key-value pairs.
• Dictionary keys must be unique.
• Use keys instead of indexes to access values.
• Dictionaries are mutable.
• Practice traversing dictionaries using a for loop.

---