PCC·04 · Chapter record

Working with Lists

Chapter 4 — looping, generating, slicing, copying, and locking lists

Done

Five sessions on doing real work with lists: looping through every item, generating numeric lists with range(), grabbing a slice, copying a list properly, and tuples — lists that refuse to change. Each one explains the concept, lists the key terms, then gives you exercises — try them yourself before revealing the code.

4.1

for Loops

Concept

A for loop repeats the same action for every item in a list, without repetitive code and without caring how long the list is.
for magician in magicians: pulls one name at a time from the list and assigns it to magician — every indented line under it runs once per item.
You can write as many indented lines as you like inside the loop — each one runs on every single pass.
A line after the loop that isn't indented runs only once, after the loop finishes — perfect for a summary message.
Python uses indentation, not braces, to know what's inside the loop — forget to indent and you get an IndentationError.

Key Terms

for loop: repeats a block of code once for each item in a list
loop variable: the temporary name (magician, cat, item…) holding the current item
IndentationError: raised when Python expects an indented line and doesn't find one

Exercise

Activity 1

Store three foods you like in a list called pizzas, then use a for loop to print a sentence about each one.

Activity 2

Store a few animals in a list, then use a for loop to print one fact about each animal.

Activity 3

Inside a loop over a list of names, print two lines per person — a greeting, then a farewell with a blank line after — then add one summary line after the loop that runs only once.

4.2

Making Numerical Lists

Concept

range() generates a series of numbers — range(1, 5) produces 1, 2, 3, 4, stopping one short of the second value, the same off-by-one rule as slicing.
Wrap list() around a range() call to turn it into an actual list: list(range(1, 6)).
A third argument to range() sets a step size, letting you skip numbers — range(2, 11, 2) gives the even numbers 2 to 10.
min(), max(), and sum() work directly on a list of numbers.
A list comprehension collapses the "empty list + loop + append" pattern into one line: squares = [value**2 for value in range(1, 11)].

Key Terms

range(start, stop, step): generates numbers from start up to (not including) stop
min() / max() / sum(): the smallest, largest, and total of a list of numbers
list comprehension: a one-line way to build a list from a loop and an expression

Exercise

Activity 4

Use a for loop with range() to print the numbers 1 through 20, inclusive.

Activity 5

Use the third argument of range() to build a list of the odd numbers from 1 to 20, then print each one with a for loop.

Activity 6

Build a list of the first 10 cubes (value ** 3) two ways: with a loop, then again as a one-line list comprehension.

4.3

Slicing a List

Concept

A slice grabs a subset of a list with list[start:stop] — like range(), it stops one before the second index.
Omit the start index to begin at the very beginning: players[:4]. Omit the stop index to go all the way to the end: players[2:].
Negative indexes work in slices too — players[-3:] grabs the last three items, however long the list is.
You can loop through just a slice with a for loop: for player in players[:3]:.

Key Terms

slice: a subset of a list, written list[start:stop]

Exercise

Activity 7

Using a list of at least 5 items, print the first three items, three items from the middle, and the last three items — each with its own slice.

Activity 8

Loop through just the first three items of a list with a for loop and a slice, printing each one in title case.

4.4

Copying a List

Concept

To copy a list, slice the whole thing: friend_foods = my_foods[:] — this makes a genuinely separate list.
Just writing friend_foods = my_foods does not copy anything — both names now point to the exact same list, so changing one changes the other.
This matters because lists are mutable: appending to a fake "copy" silently changes the original too.

Key Terms

list[:]: a slice of the entire list — the correct way to copy one
mutable: able to be changed after creation, like a list

Exercise

Activity 9

Make a list of pizzas you like, copy it properly into friend_pizzas, then add a different pizza to each list and print both to prove they're separate.

Activity 10

Repeat the exercise, but this time set friend_pizzas = pizzas directly instead of slicing — add a pizza to one list and see it appear in both.

4.5

Tuples

Concept

A tuple looks like a list but uses parentheses () instead of square brackets — and once created, it can't be changed.
Python calls this immutable. Access tuple items the same way as list items: dimensions[0].
Try to reassign a tuple item and Python raises a TypeError — tuples don't support item assignment.
You can still loop over a tuple with a for loop, exactly like a list.

Key Terms

tuple: an ordered, immutable collection written with ( )
immutable: can't be changed after creation
TypeError: raised when you try to assign to a tuple's item

Exercise

Final challenge

A rectangle's dimensions are stored as a tuple (200, 50). Try changing dimensions[0] to 250 in the widget below and read the error — then explain in one sentence why Python refuses.