Dictionaries and Sets

Store things by name, and keep a list of unique things

Big Idea: A dictionary stores pairs of key -> value, so you can look something up by name instead of by position. A set is a bag of items where every item is unique — duplicates disappear.

By the end you will be able to:

create a dictionary and read a value by its key,
add, change, and remove dictionary items safely,
loop over keys, values, and pairs,
build a set to remove duplicates and check membership fast.

1. What is a dictionary?

Think of a real paper dictionary: you look up a word and find its meaning. A Python dictionary works the same way — you look up a key and get its value.

A phone's contacts app is a perfect example: you type a name (the key) and it gives you a phone number (the value). You do not care that the name is the 5th contact — you find it by name.

Each key on the left points to exactly one value on the right.

We write a dictionary with curly braces { }, and each pair is key: value:

# A dictionary maps keys to values
ages = {"Sam": 12, "Mia": 11, "Leo": 13}

print(ages["Mia"])

Output:

The keys here are the names "Sam", "Mia", "Leo". Writing ages["Mia"] asks the dictionary: "what value is stored under Mia?" The answer is 11.

2. Adding, changing, and removing

A dictionary can grow and change. Use dict[key] = value to add a new pair or to change an existing one, and del to remove a pair:

ages = {"Sam": 12, "Mia": 11}

ages["Leo"] = 13     # add a new key
ages["Sam"] = 15     # change an existing value
del ages["Mia"]      # remove a key

print(ages)

Output:

{'Sam': 15, 'Leo': 13}

Because "Sam" already existed, writing to it changed the value from 12 to 15. Because "Leo" was new, it was added. And del ages["Mia"] removed Mia completely.

Dictionaries and Sets