Dictionaries and Sets

What a dictionary is

A list finds values by position: scores[0]. That's awkward when the values have names. Is the student's department student[1] or student[2]? A dictionary stores key: value pairs, so you look values up by name, like a word in a real dictionary.

Write it with curly brackets, a colon between each key and its value, and commas between pairs:

{'name': 'Musa', 'department': 'Accounting', 'level': 300, 'cgpa': 3.9}

• Keys are usually text, and each key appears only once.
• Values can be anything: text, numbers, lists, even other dictionaries.
• Dictionaries keep the order you added things in.

Reading values

Put the key in square brackets:

Musa
300

If the key doesn't exist, that's an error. get() is the safe way: it returns None, or a default you choose, instead:

3.9
None
Not assigned

Check whether a key exists with in:

True
False

Adding, changing and removing

Assigning to a key adds it if it's new and changes it if it exists:

{'name': 'Musa', 'department': 'Accounting', 'level': 400, 'cgpa': 3.9, 'hall': 'Jaja'}

{'name': 'Musa', 'department': 'Accounting', 'level': 400, 'cgpa': 4.0, 'hall': 'Jaja'}
Removed: 0803 000 0000

Looping through a dictionary

Three methods give you the parts:

Food
Transport
Data
46500
Food: ₦28,000
Transport: ₦12,500
Data: ₦6,000

.items() is the one you'll use most: each round gives you the key and its value, unpacked into two variables. (Looping over the dictionary itself, for item in spending:, gives the keys.)

Finding the largest

max on a dictionary compares keys by default. To find the key with the largest value, tell it to compare using spending.get:

Food
28000

Counting with a dictionary

A very common job: count how often each value appears. Start with an empty dictionary and add 1 for each item, using get with a default of 0 for keys you haven't seen yet:

{'Accounting': 3, 'Economics': 2, 'Law': 1}

The first time Python sees "Accounting", counts.get("Accounting", 0) gives 0, so it becomes 1. Next time it gives 1, so it becomes 2, and so on.

Nested data

Values can be lists or other dictionaries. That's how real data is often shaped, for example data from websites and apps:

ECO 201
Ibadan

Read it from left to right: student["address"] is the inner dictionary, and ["city"] looks inside that.

A list of dictionaries is a natural way to store a table, one dictionary per row:

Adaeze
Tobi

Sets: unique values

A set is a collection with no duplicates and no particular order. Write it with curly brackets (but no colons), or make one from a list with set():

3
True

Duplicates disappear, so len(set(items)) counts distinct values. Sets also compare groups. Students taking each of two courses:

['Musa', 'Tobi']
['Adaeze', 'Chidi', 'Musa', 'Tobi']
['Adaeze']

(sorted turns each set into an ordered list, so the output is always the same.)

When it goes wrong

print(student["Name"])

KeyError: 'Name'

A key that isn't there. Keys must match exactly, capitals included. Use student.get("Name", "unknown") when a key might be missing.

empty = {}
print(type(empty))

<class 'dict'>

{} is an empty dictionary, not a set. Make an empty set with set().

for item, amount in spending:
    print(item, amount)

ValueError: too many values to unpack (expected 2)

Forgetting .items(). Looping over a dictionary gives only the keys; add .items() to get pairs.

Summary

Try it