09-lists

09 · Lists

Part 2 — Data Structures · Estimated time: 30–40 min · Prerequisite: 07 · Loops

A list holds many values in a single, ordered container — a shopping list, a set of scores, a row of names. Lists are the workhorse data structure of Python: you can add to them, remove from them, sort them, and loop over them.

What you'll learn

• Creating lists and reading items by index (and with in)
• Measuring length with len()
• Changing a list: append, insert, extend, remove, pop
• Slicing, and sorting with sort() vs sorted()
• Looping over a list to process every item

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1. Creating and accessing

fruits = ["apple", "banana", "cherry"]
print(len(fruits))         # 3
print(fruits[0])           # apple    (indexing starts at 0)
print(fruits[-1])          # cherry   (negative counts from the end)
print("banana" in fruits)  # True     (membership test)

A list can mix types ([1, "two", 3.0]), though most lists hold one kind of thing.

▶️ Run it: python examples/01_creating_and_accessing.py

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2. Changing a list

Lists are mutable — you can change them after creating them.

nums = [1, 2, 3]
nums.append(4)        # add one item to the end   -> [1, 2, 3, 4]
nums.insert(0, 0)     # insert at a position       -> [0, 1, 2, 3, 4]
nums.extend([5, 6])   # add several items          -> [0, 1, 2, 3, 4, 5, 6]

nums.remove(0)        # remove the first matching value (0)
last = nums.pop()     # remove AND return the last item (6)
print(nums, "popped", last)

▶️ Run it: python examples/02_modifying.py

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3. Slicing and sorting

Slicing works just like it did for strings: list[start:stop].

letters = ["d", "a", "c", "b"]
print(letters[1:3])      # ['a', 'c']

print(sorted(letters))   # ['a', 'b', 'c', 'd']  -> returns a NEW list
print(letters)           # unchanged

letters.sort()           # sorts THIS list in place (returns None)
print(letters)           # ['a', 'b', 'c', 'd']

⚠️ sorted(x) gives you a new sorted list; x.sort() rearranges x itself and returns None. Writing x = x.sort() is a classic bug — it sets x to None.

▶️ Run it: python examples/03_slicing_and_sorting.py

💡 Try it yourself: reverse a list two ways — with .reverse() and with the slice x[::-1]. Which one changes the original?

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4. Looping over a list

scores = [10, 20, 30]
for score in scores:
    print(score)

total = 0
for score in scores:
    total += score
print("Total:", total)   # 60

(Python also has sum(scores) and max(scores) built in — handy shortcuts.)

▶️ Run it: python examples/04_looping.py

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Common mistakes

Mistake	What happens	Fix
fruits[3] on a 3-item list	IndexError	Valid indexes are 0..len-1; last is -1.
x = x.sort()	x becomes None	Use x.sort() alone, or x = sorted(x).
nums.append([5, 6]) to add two items	nests a list inside	Use extend([5, 6]) to add items individually.
remove(x) when x isn't there	ValueError	Check if x in list: first.

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Recap / cheat-sheet

items = [1, 2, 3]
len(items)            # 3
items[0]  items[-1]   # first / last
x in items            # membership -> True/False

items.append(4)       # add one to the end
items.insert(0, 9)    # add at a position
items.extend([5, 6])  # add several
items.remove(9)       # remove by value
items.pop()           # remove & return the last

items[1:3]            # slice
sorted(items)         # new sorted list
items.sort()          # sort in place (returns None)
items.reverse()       # reverse in place

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_third_item.py — read items by index.
2. exercises/02_shopping_list.py — add and remove items, then report the list.
3. exercises/03_total_and_max.py — total and largest value of a number list.

Try each yourself before opening the solution.

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Next → 10 · Tuples & Sets (coming soon)