When should I use a dictionary instead of a list?

Use a dictionary when you look items up by a meaningful key (like a user ID or name). Use a list when order and position matter and you mostly loop through everything.

Can any value be a dictionary key?

Keys must be hashable, meaning immutable types such as strings, numbers, and tuples. Lists and other dictionaries can't be keys.

Python Dictionaries and Sets: When to Use Each

Introduction

Dictionaries and sets are two of Python's most useful built-in data structures. A dictionary maps keys to values for fast lookups, while a set stores unique items and supports mathematical set operations. This guide shows how each one works and how to choose between them.

Prerequisites

Python 3.8 or newer installed
Familiarity with lists and for-loops

Step-by-Step Guide

Step 1: Create a dictionary and read values

A dictionary stores key-value pairs inside curly braces. Use square brackets to read a value, or get() to supply a fallback when a key might be missing.

user = {"name": "Ava", "age": 30}

print(user["name"])                    # Ava
print(user.get("email", "not set"))    # not set

Step 2: Add, update, and remove entries

Assign to a key to add or change it, and use del or pop() to remove it.

user["age"] = 31                  # update
user["email"] = "ava@example.com" # add
del user["age"]                   # remove
print(user)

Step 3: Loop over a dictionary

Use items() to get each key and value together.

for key, value in user.items():
    print(key, "->", value)

Step 4: Create a set of unique values

A set automatically discards duplicates. Sets are unordered, so don't rely on the print order.

tags = {"python", "code", "python"}
print(tags)              # {'python', 'code'} (order may vary)
print("code" in tags)     # True

Step 5: Combine sets with set operations

Sets support union, intersection, and difference, which are handy for comparing collections.

a = {1, 2, 3}
b = {3, 4}

print(a | b)   # union:        {1, 2, 3, 4}
print(a & b)   # intersection: {3}
print(a - b)   # difference:   {1, 2}

FAQs

When should I use a dictionary instead of a list?: Use a dictionary when you look items up by a meaningful key (like a user ID or name). Use a list when order and position matter and you mostly loop through everything.
Can any value be a dictionary key?: Keys must be hashable, meaning immutable types such as strings, numbers, and tuples. Lists and other dictionaries can't be keys.

Summary

Reach for a dictionary when you need fast lookups by key, and a set when you need unique values or want to compare groups of items. Both offer near-constant-time membership checks on average.