Phase 1 · Day 4 · Learn AI

Lists: ordered and mutable

A list holds an ordered sequence of values in square brackets. Items can be different types, and you can change a list after you create it.

Index from 0. Negative indexes count from the end: -1 is the last item.

Creating and indexing lists

skills = ["python", "git", "sql"]
print(skills[0])   # python
print(skills[-1])  # sql

skills.append("docker")
skills[1] = "github"
print(skills)

print(len(skills))
print("sql" in skills)

Slicing and looping

nums = [10, 20, 30, 40, 50]
print(nums[1:4])   # [20, 30, 40]
print(nums[:2])    # [10, 20]
print(nums[::2])   # [10, 30, 50]

for n in nums:
    print(n * 2)

Common methods: append, insert, remove, pop, sort, reverse. Prefer list comprehensions later for clean transforms.

Dicts: keys map to values

A dictionary stores pairs of keys and values in curly braces. Look up a value by its key instead of by position.

Keys must be immutable (strings, numbers, tuples). Values can be anything—including lists or other dicts.

Creating and reading dicts

user = {
    "name": "Ada",
    "role": "engineer",
    "years": 3,
}

print(user["name"])
print(user.get("team", "unassigned"))

user["years"] = 4
user["team"] = "AI"
print(user)

Looping over dicts

scores = {"math": 90, "python": 95, "english": 88}

for subject, score in scores.items():
    print(f"{subject}: {score}")

print(list(scores.keys()))
print(list(scores.values()))

Use .get(key, default) when a key might be missing—avoids KeyError.

Tuples: ordered and immutable

A tuple is like a list you cannot change. Use parentheses (or just commas). They are great for fixed records and multiple return values.

Because tuples are immutable, they can be used as dict keys.

Tuples in practice

point = (3, 7)
print(point[0], point[1])

# unpacking
x, y = point
print(x, y)

def min_max(values):
    return min(values), max(values)

low, high = min_max([4, 1, 9, 2])
print(low, high)

A one-item tuple needs a trailing comma: (42,) — otherwise Python treats (42) as a plain number.

Which collection when?

list — ordered items you will add, remove, or update
dict — labeled data you look up by name/id
tuple — fixed groups, coordinates, or function returns you should not mutate

Mixing collections

students = [
    {"name": "Ada", "scores": (90, 88, 95)},
    {"name": "Grace", "scores": (92, 91, 89)},
]

for student in students:
    name = student["name"]
    avg = sum(student["scores"]) / len(student["scores"])
    print(f"{name}: {avg:.1f}")

Practice

Mini challenge: contact card

Build a dict for one contact with keys name, email, and tags (a list of strings). Print the name and email, then loop over tags. Finally pack name and email into a tuple and print it.

challenge starter

contact = {
    # TODO: name, email, tags
}

# print name and email
# loop tags
# pack (name, email) into a tuple and print

Interactive

Try It in the browser

Start with nums = [3, 1, 4, 1, 5]. Append 9, remove one 1, then print the length and the sorted copy (without changing the original permanently—use sorted()).

Output

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Errors

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Lists, Dicts & Tuples