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Workshop 8 — Dictionaries

Prerequisites

  • Workshop 3: if/elif/else, in
  • Workshop 4: for, the accumulator, counting with a condition
  • Workshop 5–6: def, return, returning several values
  • Workshop 7: lists, indexing, append, looping over a list

1. Why dictionaries

A list finds things by position. To store a phone book with a list, you would have to remember that Nina is at index 0:

names = ["Nina", "Luka"]
phones = ["555-1234", "555-9876"]
print(phones[names.index("Luka")])
# 555-9876

A dictionary finds things by key. Each key is paired with a value:

phones = {"Nina": "555-1234", "Luka": "555-9876"}
print(phones["Luka"])
print(len(phones))
# 555-9876
# 2

Curly braces, key: value pairs, commas between the pairs. {} is an empty dictionary. Keys are usually strings or numbers; values can be anything — even lists.

2. Getting a value

Square brackets with the key instead of an index:

ages = {"Nina": 19, "Luka": 22}
print(ages["Nina"])
# 19

Ask for a key that is not there and Python stops with an error:

print(ages["Ana"])
# KeyError: 'Ana'

.get() returns None instead, or a default you choose:

print(ages.get("Ana"))
print(ages.get("Ana", 0))
# None
# 0

And in asks whether a key is in the dictionary:

print("Nina" in ages)
print(19 in ages)
# True
# False

in checks keys, not values — that is why 19 in ages is False.

3. Adding, changing and removing

Assign to a key. If the key is new it is added; if it exists, its value is replaced. A key can appear only once.

ages = {"Nina": 19}
ages["Luka"] = 22      # add
ages["Nina"] = 20      # change
print(ages)
# {'Nina': 20, 'Luka': 22}

pop(key) removes a pair and returns its value, like pop on a list:

removed = ages.pop("Luka")
print(removed, ages)
# 22 {'Nina': 20}

4. Looping over a dictionary

A for loop over a dictionary gives you the keys:

prices = {"bread": 2.5, "milk": 3.0, "cheese": 9.0}
for item in prices:
    print(item, prices[item])
# bread 2.5
# milk 3.0
# cheese 9.0

.items() gives you the key and the value together — unpack them into two variables, like returning several values in Workshop 6:

total = 0
for item, price in prices.items():
    print(f"{item}: {price}")
    total += price
print(f"Total: {total}")
# bread: 2.5
# milk: 3.0
# cheese: 9.0
# Total: 14.5

.keys() and .values() give just one side. sum(prices.values()) is 14.5.

5. Counting with a dictionary

The most common dictionary pattern: count how many times each thing appears. Start empty; for each item, add 1 to its count — or start it at 1 the first time you see it:

words = ["cat", "dog", "cat", "bird", "cat"]
counts = {}
for word in words:
    if word in counts:
        counts[word] += 1
    else:
        counts[word] = 1
print(counts)
# {'cat': 3, 'dog': 1, 'bird': 1}

With .get() the if collapses into one line:

counts[word] = counts.get(word, 0) + 1

6. Tuples

A tuple is like a list, but written with round brackets and it cannot be changed:

point = (3, 5)
print(point[0])
x, y = point
print(x + y)
# 3
# 8
point[0] = 10
# TypeError: 'tuple' object does not support item assignment

You have already used tuples without knowing it: return a, b returns a tuple, and .items() gives you (key, value) tuples.

7. Putting it together

A list of dictionaries is a tiny database:

students = [
    {"name": "Nina", "score": 82},
    {"name": "Luka", "score": 95},
    {"name": "Ana", "score": 67},
]

def best(students):
    """Return the name of the student with the highest score."""
    top = students[0]
    for student in students:
        if student["score"] > top["score"]:
            top = student
    return top["name"]

print(best(students))
# Luka

Quick reference

Idea Example
Create ages = {"Nina": 19} / empty = {}
Get ages["Nina"], ages.get("Ana", 0)
Has key? "Nina" in ages
Add / change ages["Luka"] = 22
Remove ages.pop("Luka")
Loop keys for name in ages:
Loop pairs for name, age in ages.items():
One side ages.keys(), ages.values()
Count counts[x] = counts.get(x, 0) + 1
Tuple point = (3, 5), x, y = point

What's next

See workshop_8/ for runnable examples, and your homework in python-127-homework-8.