Lists and Dictionaries

Step 1: A list holds many values

So far each variable has held one value. A list holds many, in order.

fruits = ["apple", "pear", "fig"]
print(fruits)
print(fruits[0])
print(fruits[2])
print(len(fruits))

['apple', 'pear', 'fig']
apple
fig
3

Each item has an index, its position, starting at 0. fruits[0] is the first item. A negative index counts from the end: fruits[-1] is the last.

Step 2: Changing a list

fruits = ["apple", "pear", "fig"]
fruits.append("plum")
fruits.remove("pear")
fruits[0] = "mango"
print(fruits)

['mango', 'fig', 'plum']

• append() adds to the end.
• remove() takes out the first matching item.
• Assigning to an index replaces that item.

Step 3: Looping over a list

You don’t need range() to go through a list. Just loop over the list itself.

scores = [80, 95, 70]
total = 0
for s in scores:
    total += s
print("Average:", total / len(scores))

Average: 81.66666666666667

You can also ask whether something is in a list:

pets = ["cat", "dog"]
print("dog" in pets)
print("fish" in pets)

True
False

Step 4: A dictionary looks things up by name

A list finds things by position. A dictionary finds things by a key. It stores pairs: a key and its value.

prices = {"tea": 2, "milk": 3}
print(prices["milk"])
prices["juice"] = 4
print(prices)

3
{'tea': 2, 'milk': 3, 'juice': 4}

Check if a key exists before you use it, otherwise Python raises a KeyError:

if "cake" in prices:
    print(prices["cake"])
else:
    print("No cake today.")

No cake today.

Loop over a dictionary with .items():

for item, price in prices.items():
    print(f"{item}: ${price}")

tea: $2
milk: $3
juice: $4

Step 5: Which one should I use?

You can even combine them. A list of dictionaries is a tiny table:

students = [
    {"name": "Amara", "grade": 7},
    {"name": "Jabari", "grade": 8},
]
for s in students:
    print(s["name"], "is in grade", s["grade"])

Amara is in grade 7
Jabari is in grade 8