Dictionaries and Sets

Introduction

A list finds things by position: "item number 2". But often you want to find things by name: "the phone number for Asha". A dictionary stores pairs of keys and values, and looks up a value from its key almost instantly.

Creating and reading a dictionary

Use curly brackets with key: value pairs.

Asking for a key that does not exist is an error. The get() method is safer and lets you set a default.

Adding, changing and removing

Looping over a dictionary

Counting with a dictionary

A very common pattern: count how often each thing appears.

For each word, we take the current count (or 0 if it has not been seen) and add one.

Dictionaries inside lists

Real data often looks like a list of dictionaries, one per record.

Sets

A set is an unordered collection of unique items. Duplicates are removed automatically, and checking membership is fast.

An easy way to remove duplicates from a list: set(items).

Sets compare groups using maths ideas:

Note that {} on its own creates an empty dictionary. Create an empty set with set().

Try it yourself

Count how many times each letter appears in the word "banana", using a dictionary, and print the result. Then print how many different letters there are, using a set.

Summary

• A dictionary maps keys to values: {"name": "Asha"}. Read with d[key] or the safer d.get(key, default).
• Add or change with d[key] = value, and loop with .items(), .keys() and .values().
• A set holds unique items and supports & (intersection), | (union) and - (difference).
• Lists of dictionaries are the usual shape of real-world records.