🐍 Unit 2 – Python Programming · Lesson 09
Dictionaries in Python
What is a dictionary?
A dictionary stores data as key : value pairs in curly brackets. Each key must be unique and immutable; values can be anything. Where a list answers "what is at position 2?", a dictionary answers "what is the price of a pen?".
| key | value |
|---|---|
| 'title' | 'Wings of Fire' |
| 'author' | 'A. P. J. Abdul Kalam' |
| 'price' | 299 |
Creating and accessing
Python
book = {'title': 'Wings of Fire', 'author': 'A. P. J. Abdul Kalam', 'price': 299}
print(book['title'])
print(book.get('price'), book.get('pages'), book.get('pages', 'unknown'))
print(len(book), 'author' in book)Wings of Fire
299 None unknown
3 True
Using d[key] for a missing key raises KeyError; get() is the safe way.
Adding, modifying, deleting
Python
stock = {'pen': 120, 'pencil': 80}
stock['eraser'] = 45 # add
stock['pen'] = 100 # modify
print(stock)
del stock['pencil'] # delete
print(stock)
print(stock.pop('eraser'), stock){'pen': 100, 'pencil': 80, 'eraser': 45}
{'pen': 100, 'eraser': 45}
45 {'pen': 100}
Traversing
Python
population = {'Delhi': 33, 'Mumbai': 22, 'Kolkata': 15, 'Chennai': 12} # lakh, approx.
for city in population:
print(city, end=', ')
print()
for city, p in population.items():
print(f'{city:<8}{"●" * (p // 3)} {p}')Delhi, Mumbai, Kolkata, Chennai,
Delhi ●●●●●●●●●●● 33
Mumbai ●●●●●●● 22
Kolkata ●●●●● 15
Chennai ●●●● 12
Dictionary methods
Python
marks = {'Maths': 88, 'Physics': 76, 'IP': 95}
print(list(marks.keys()))
print(list(marks.values()))
print(list(marks.items()))
marks.update({'English': 81, 'Maths': 90})
print(marks)
print(sorted(marks), max(marks.values()))
print(marks.popitem(), marks)['Maths', 'Physics', 'IP']
[88, 76, 95]
[('Maths', 88), ('Physics', 76), ('IP', 95)]
{'Maths': 90, 'Physics': 76, 'IP': 95, 'English': 81}
['English', 'IP', 'Maths', 'Physics'] 95
('English', 81) {'Maths': 90, 'Physics': 76, 'IP': 95}
| Method | Purpose |
|---|---|
| keys(), values(), items() | All keys / values / pairs |
| get(k, default) | Safe access |
| update(other) | Merge another dictionary (overwrites same keys) |
| pop(k), popitem() | Remove a key / the last pair |
| copy(), clear() | Copy / empty |
| len(), sorted(), max(), min(), sum(d.values()) | Built-in helpers |
Programs
1. Counting votes
Python
votes = ['A', 'B', 'A', 'C', 'A', 'B', 'A']
result = {}
for v in votes:
result[v] = result.get(v, 0) + 1
print(result)
winner = max(result, key=result.get)
print('Winner:', winner, 'with', result[winner], 'votes'){'A': 4, 'B': 2, 'C': 1}
Winner: A with 4 votes
2. Employee salary dictionary
Python
n = int(input('Number of employees: '))
salary = {}
for i in range(n):
name = input('Name: ')
salary[name] = int(input('Salary: '))
print(salary)
print('Total salary bill:', sum(salary.values()))
for name, s in salary.items():
if s > 40000:
print(name, 'earns more than 40000')Number of employees: 3
Name: Ravi
Salary: 38000
Name: Meena
Salary: 52000
Name: Joseph
Salary: 41000
{'Ravi': 38000, 'Meena': 52000, 'Joseph': 41000}
Total salary bill: 131000
Meena earns more than 40000
Joseph earns more than 40000
3. Dictionary comprehension and zip()
Python
fruits = ['apple', 'mango', 'kiwi']
lengths = {f: len(f) for f in fruits} # dictionary comprehension
print(lengths)
print(dict(zip(['a', 'b', 'c'], [1, 2, 3]))){'apple': 5, 'mango': 5, 'kiwi': 4}
{'a': 1, 'b': 2, 'c': 3}
Nested dictionaries
A value can itself be a dictionary or list — this is how real records (JSON data from websites and apps) look.
Python
students = {
101: {'name': 'Asha', 'marks': [78, 85, 90]},
102: {'name': 'Dev', 'marks': [66, 72, 58]},
}
for roll, info in students.items():
avg = sum(info['marks']) / len(info['marks'])
print(roll, info['name'], round(avg, 2))101 Asha 84.33
102 Dev 65.33
Summary & practice
- Dictionary = unique keys mapped to values in { }; mutable.
- Access with d[k] or d.get(k); add/modify with d[k] = v; delete with del, pop.
- Traverse with items(); methods keys, values, update, popitem.
- Counting pattern: d[x] = d.get(x, 0) + 1.
✅ Practice
1) Store the monthly electricity units of a house for 6 months and print the month with the highest use. 2) Count how many times each word appears in a sentence. 3) Create a dictionary of 5 products and their prices, and print products costing more than ₹100.
Frequently Asked Questions
Why is a dictionary useful in data handling?
Real data often comes as labelled values — a student’s name, roll and marks, or a product’s code and price. A dictionary keeps each value with its label (key), so it is easy to read and fast to look up.
Are dictionary keys case-sensitive?
Yes. "Maths" and "maths" are different keys.
What does items() return?
A view of (key, value) pairs, ideal for a for loop like: for k, v in d.items().
