Layer 1 Session 06 Python Foundations

Lists and Iteration

A single country dictionary is not a country explorer. We need to hold many of them — that means lists, indexing, and loops.

An ordered, indexed sequence of values.

Estimated time: 35–40 minutes · Pre-quiz → Concept → Lab → Commit → Post-quiz

1. Learning Objective

By the end of this session you will be able to:

Create a list using literal syntax and understand that it is ordered and mutable
Access items by index, including negative indexing
Use len() and iterate with a for loop
Explain the difference between mutating methods (.append, .sort) and non-mutating operations
Store a list of dictionaries — the shape our whole project will use

2. Pre-Coding Quiz

Answer these before reading the concept explanation or writing any code. It is fine to get these wrong — that is the point. You need 4/5 to proceed to the lab.

Question 1 of 5

You have countries = ["Kenya", "Ghana", "Peru"]. What does countries[1] return?

"Kenya" "Ghana" "Peru" An IndexError

Python lists are zero-indexed. Index 0 is "Kenya", index 1 is "Ghana", index 2 is "Peru".

Question 2 of 5

What does countries[-1] return for the same list?

An IndexError — negative numbers are invalid "Kenya" — the first item "Peru" — the last item None

Negative indices count from the end. -1 is always the last item, -2 the second-to-last, and so on. This avoids writing countries[len(countries) - 1].

Question 3 of 5

What is printed by:
nums = [3, 1, 2] nums.append(9) print(nums)

[3, 1, 2] — append() returns a new list and the original is unchanged [3, 1, 2, 9] — append() mutates the list in place [9, 3, 1, 2] A TypeError

.append() is a mutating method — it adds to the existing list object and returns None. It does not create a new list.

Question 4 of 5

You loop with for country in countries:. On each iteration, what does country refer to?

The index of the current item, starting at 0 The entire list, unchanged each time The value of the current item in the list A copy of the whole list

A for ... in loop over a list binds the loop variable to each element's value in turn — not the index. Use enumerate() when you need both the index and the value.

Question 5 of 5

You want each item in the loop to be a dictionary you can read fields from, e.g. country["name"]. What must countries be?

A list of strings A dictionary of dictionaries A list of dictionaries It does not matter, Python infers it automatically

For country["name"] to work inside the loop, each element yielded by the loop must itself be a dict. That means countries is a list whose elements are dicts — exactly the shape we build in this session's lab.

3. The Concept — Python Lists and Iteration

Index counts up from 0 on the left, and down from -1 on the right — both point into the same list.

What is a list?

A list is an ordered, mutable sequence of values. Unlike a dictionary, items are accessed by position (index), not by name.

countries = ["Kenya", "Ghana", "Peru"]

print(countries[0])   # "Kenya"  — first item, index 0
print(countries[1])   # "Ghana"
print(countries[-1])  # "Peru"   — last item, negative index
print(len(countries)) # 3

Iterating with a for loop

The most common way to process every item in a list is a for ... in loop. No manual index counting required.

countries = ["Kenya", "Ghana", "Peru"]

for country in countries:
    print(country)
# Kenya
# Ghana
# Peru

# When you need the index too, use enumerate()
for i, country in enumerate(countries):
    print(i, country)
# 0 Kenya
# 1 Ghana
# 2 Peru

Mutating vs non-mutating operations

Some list methods change the list in place and return None. Others return a new value and leave the original untouched. Confusing the two is a very common bug.

nums = [3, 1, 2]

nums.append(9)     # mutates in place -> [3, 1, 2, 9]
nums.sort()         # mutates in place -> [1, 2, 3, 9]

# sorted() is non-mutating — it returns a NEW list
original = [3, 1, 2]
result = sorted(original)
print(original)  # [3, 1, 2] — unchanged
print(result)    # [1, 2, 3] — new list

.sort() mutates the original object; sorted() leaves it alone and hands back a new list.

A list of dictionaries — our project's core data shape

Combining what we learned in Session 05 with lists gives us exactly what a real application needs: a collection of structured records.

countries = [
    {"name": "Kenya", "region": "Africa", "population": 54000000},
    {"name": "Ghana", "region": "Africa", "population": 31000000},
    {"name": "Peru",  "region": "Americas", "population": 33000000},
]

for country in countries:
    print(country["name"], "-", country["region"])
# Kenya - Africa
# Ghana - Africa
# Peru - Americas

Why this matters for the project: Every screen of our Country Explorer app — search, filtering, display — starts from exactly this shape: a list of dictionaries. Everything from here forward operates on this structure.

4. Lab

Lab objective: Store three countries as a list of dictionaries, iterate over it, and prove the mutating vs non-mutating distinction.

What you will build

A file called countries.py that builds on Session 05's single dictionary.

Step-by-step instructions

Create the file

# countries.py

Build a list of at least 3 country dictionaries

Reuse the shape from Session 05 — each item needs at least name, region, and population.

countries = [
    {"name": "Kenya", "region": "Africa", "population": 54000000},
    {"name": "Ghana", "region": "Africa", "population": 31000000},
    {"name": "Peru",  "region": "Americas", "population": 33000000},
]

Iterate and print each country's name

Use a plain for loop, no enumerate yet.

for country in countries:
    print(country["name"])

Iterate with enumerate() to also print position

for i, country in enumerate(countries):
    print(f"{i}: {country['name']}")

Add a country with .append()

Print the list before and after to confirm it mutated in place.

print("Before:", len(countries))
countries.append({"name": "Japan", "region": "Asia", "population": 125000000})
print("After:", len(countries))

Prove sorted() does not mutate

Sort the country names alphabetically without touching the original list.

names = [c["name"] for c in countries] if False else None  # ignore, next session
name_list = ["Peru", "Ghana", "Kenya", "Japan"]
sorted_names = sorted(name_list)
print("Original order:", name_list)
print("Sorted copy:", sorted_names)

Access the last item with negative indexing

print("Last country added:", countries[-1]["name"])

5. Expected Files Changed

File	Action	Why
`countries.py`	Created	Builds a list of dictionaries and demonstrates iteration.
`docs/sessions/session-06/index.html`	Created	This session document.

If you find yourself editing any other file, stop. This session touches exactly 2 files.

6. Commit Checkpoint

Once the lab is complete and you can explain every line, make this exact commit:

git add countries.py docs/sessions/session-06/index.html
git commit -m "session-06: store countries in a list, iterate, and separate mutating from non-mutating ops"

Do not commit until you can answer out loud: "What is the difference between .sort() and sorted(), and why does it matter?"

7. Code Review Checklist

Go through your code line by line and check each item:

The list is declared with [] literal syntax containing at least 3 dictionaries

A plain for ... in loop is used and prints correctly

enumerate() is used at least once with both index and value

.append() is demonstrated and shown to mutate in place

sorted() is demonstrated and shown to NOT mutate the original

Negative indexing is used at least once

I can explain every line without looking at the concept section

8. Post-Coding Quiz

Same 5 questions. Take it again now that you have written and run the code. You need 4/5 to mark this session complete.

Question 1 of 5

You have countries = ["Kenya", "Ghana", "Peru"]. What does countries[1] return?

"Kenya" "Ghana" "Peru" An IndexError

Python lists are zero-indexed. Index 0 is "Kenya", index 1 is "Ghana", index 2 is "Peru".

Question 2 of 5

What does countries[-1] return for the same list?

An IndexError — negative numbers are invalid "Kenya" — the first item "Peru" — the last item None

Negative indices count from the end. -1 is always the last item, -2 the second-to-last, and so on. This avoids writing countries[len(countries) - 1].

Question 3 of 5

What is printed by:
nums = [3, 1, 2] nums.append(9) print(nums)

[3, 1, 2] — append() returns a new list and the original is unchanged [3, 1, 2, 9] — append() mutates the list in place [9, 3, 1, 2] A TypeError

.append() is a mutating method — it adds to the existing list object and returns None. It does not create a new list.

Question 4 of 5

You loop with for country in countries:. On each iteration, what does country refer to?

The index of the current item, starting at 0 The entire list, unchanged each time The value of the current item in the list A copy of the whole list

A for ... in loop over a list binds the loop variable to each element's value in turn — not the index. Use enumerate() when you need both the index and the value.

Question 5 of 5

You want each item in the loop to be a dictionary you can read fields from, e.g. country["name"]. What must countries be?

A list of strings A dictionary of dictionaries A list of dictionaries It does not matter, Python infers it automatically

9. Reflection Questions

Think through these after the post-quiz. No right answer — they are for discussion.

Why do you think Python chose zero-based indexing instead of starting at 1?
If .append() mutates in place and returns None, what would countries = countries.append(x) incorrectly leave countries as? Try it.
When would negative indexing actually save you from an error-prone calculation?
Why is "a list of dictionaries" a more useful shape than three separate parallel lists (names, regions, populations)?

10. What Breaks If This Knowledge Is Missing?

The append-return bug: A very common beginner mistake is writing x = some_list.append(item), expecting x to be the updated list. It is actually None, because mutating methods return nothing. This trips people up for years if never explained explicitly.
List comprehensions (Session 07): The next session builds directly on for-loop iteration. If you cannot trace what a for loop does step by step, list comprehensions will look like meaningless syntax instead of a shorthand for something you already understand.
Testing (Layer 5): Later, you will write tests that assert on list contents and order. Without understanding indexing and iteration, you cannot reason about what a test assertion is actually checking.

11. What We Learned

Python concept mastered: Lists — indexing (including negative), len(), for-loop iteration, enumerate(), mutating vs non-mutating operations.

Unlocks: You can now hold and process a real collection of records — the shape our whole Country Explorer project uses.

Next session: Session 07 — List Comprehensions. We will learn to transform and filter this list without a manual for-loop, using list comprehensions.