Dictionaries

Introduction

A dictionary is one of the most useful data structures in Python. Instead of storing values in order like a list, a dictionary stores data as key-value pairs, so you can look up a value directly by its meaningful name instead of by its numeric position.

Think of a real dictionary book: you look up a word (the key) to find its definition (the value). Python dictionaries work the same way — you look up a key to instantly retrieve its associated value.

Creating Dictionaries

A dictionary is written with curly braces {}, containing key:value pairs separated by commas. Keys map to values, and every key in a dictionary must be unique.

student = {
    "name": "Alex",
    "age": 16,
    "grade": "10th"
}

print(student)
print(type(student))

empty_dict = {}
print(type(empty_dict))

{'name': 'Alex', 'age': 16, 'grade': '10th'}
<class 'dict'>
<class 'dict'>

Accessing Values

You can retrieve a value using square brackets with its key, or the safer .get() method, which lets you supply a default value instead of crashing when the key is missing.

student = {"name": "Alex", "age": 16}

print(student["name"])
print(student.get("age"))
print(student.get("school"))
print(student.get("school", "Not Enrolled"))

Alex
16
None
Not Enrolled

student = {"name": "Alex"}
print(student["school"])

KeyError: 'school'

Keys Must Be Hashable

Dictionary keys must be hashable, which in practice means they must be immutable — strings, numbers, and tuples work fine as keys, but lists, sets, and other dictionaries do not.

coordinates = {}
coordinates[(10, 20)] = "Point A"   # tuple key works fine
print(coordinates)

try:
    coordinates[[10, 20]] = "Point B"  # list key fails
except TypeError as e:
    print("Error:", e)

{(10, 20): 'Point A'}
Error: unhashable type: 'list'

Values, on the other hand, can be absolutely anything — strings, numbers, lists, or even other dictionaries.

Common Dictionary Methods

student = {"name": "Alex", "age": 16}

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

student.update({"grade": "10th", "age": 17})
print(student)

removed_age = student.pop("age")
print("Removed age:", removed_age)
print(student)

school = student.setdefault("school", "Unknown")
print(school)
print(student)

['name', 'age']
['Alex', 16]
[('name', 'Alex'), ('age', 16)]
{'name': 'Alex', 'age': 17, 'grade': '10th'}
Removed age: 17
{'name': 'Alex', 'grade': '10th'}
Unknown
{'name': 'Alex', 'grade': '10th', 'school': 'Unknown'}

Iterating Over a Dictionary

Looping directly over a dictionary gives you its keys. To get both the key and value together, loop over .items() instead.

prices = {"apple": 40, "banana": 20, "cherry": 150}

for fruit in prices:
    print(fruit)

print("---")

for fruit, price in prices.items():
    print(f"{fruit}: ₹{price}")

apple
banana
cherry
---
apple: ₹40
banana: ₹20
cherry: ₹150

Checking Key Existence

Use the in keyword to check whether a key exists before accessing it, which avoids KeyError crashes.

student = {"name": "Alex", "age": 16}

if "age" in student:
    print("Age:", student["age"])

if "school" not in student:
    print("School is not recorded yet.")

Age: 16
School is not recorded yet.

Nested Dictionaries

Dictionary values can themselves be dictionaries, which is extremely useful for representing structured, real-world data such as records with multiple related fields.

students = {
    "s101": {"name": "Alex", "age": 16},
    "s102": {"name": "Priya", "age": 17}
}

print(students["s101"]["name"])
print(students["s102"]["age"])

for student_id, info in students.items():
    print(f"{student_id}: {info['name']}, age {info['age']}")

Alex
17
s101: Alex, age 16
s102: Priya, age 17

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

Dictionaries let you store and retrieve data by meaningful keys instead of numeric positions, making them ideal for representing structured, real-world information like user profiles, settings, or records.

Next, you will learn how to control the flow of your program using conditional statements — if, elif, and else.