Layer 0 Session 03 Python Basics

Conditionals

A program that always does the same thing is not very useful. Conditionals let a program branch — doing different things depending on the data.

A condition splits execution down one of two paths.

Estimated time: 30–35 minutes · Pre-quiz → Concept → Lab → Commit → Post-quiz

1. Learning Objective

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

Write an if statement and explain that its body only runs when the condition is True
Add elif and else to handle multiple, mutually exclusive branches
Combine conditions with and, or, and not
Explain the difference between = (assignment) and == (comparison), a very common typo
Trace, by hand, exactly which branch of an if/elif/else will run for a given value

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

Given population = 54 (population in millions) and the following code, what happens?

if population > 50:
    print("Large")

Nothing prints, because population is a variable, not a literal True "Large" prints, because population > 50 evaluates to True It raises a SyntaxError "Large" prints regardless of the comparison

The condition population > 50 evaluates to True (54 is indeed greater than 50), so the indented block underneath the if runs.

Question 2 of 5

Given the following, with population = 54, what prints?

if population > 100:
    print("Huge")
elif population > 50:
    print("Large")
else:
    print("Small")

"Huge" "Large" "Small" All three lines print

Python checks branches top to bottom and stops at the FIRST True condition. 54 is not > 100, so it checks the next branch: 54 > 50 is True, so "Large" prints, and the else is skipped entirely — only one branch ever runs.

Question 3 of 5

Given region = "Africa" and population = 54, which correctly checks "is this an African country with over 50 million people"?

if region == "Africa" or population > 50: if region == "Africa" and population > 50: if region = "Africa" and population > 50: if region == "Africa", population > 50:

and requires BOTH conditions to be True. or (option a) would match if EITHER were true, which is not what "African country AND over 50 million" means. Option c uses a single = (assignment), which is invalid inside a condition.

Question 4 of 5

What is wrong with if population = 54:?

Nothing — this correctly checks if population equals 54 It uses = (assignment) instead of == (comparison) — this is a SyntaxError in Python, unlike some other languages population should be in quotes if statements cannot use numbers, only variables

A single = assigns a value; it does not compare. Python actually catches this specific mistake as a SyntaxError rather than silently doing the wrong thing — but it is still a very common typo worth recognising immediately.

Question 5 of 5

Given is_independent = True, which correctly checks "is NOT independent"?

if is_independent = False: if not is_independent: if is_independent != False something: if -is_independent:

not flips a boolean: not True is False, and not False is True. if not is_independent: is the idiomatic way to check "this is False" rather than writing if is_independent == False:.

3. The Concept — Branching with if / elif / else

The indented block only executes when the condition is True — otherwise Python skips straight past it.

What are these words called? if, elif, and else are keywords — reserved words built into Python itself; you can never use them as a variable name. and, or, and not are technically keywords too, but they have a second, more specific name: they are logical operators (also called boolean operators) — words that combine or invert True/False values, playing the same functional role that + or > play for numbers. So: every one of these words is a keyword, but only and/or/not also count as operators.

if — running code only when a condition is True

An if statement's indented block only runs when its condition evaluates to True. If the condition is False, Python skips straight past it. (Population figures below are in millions, to keep the numbers easy to read.)

population = 54

if population > 50:
    print("Large country")

print("This line always runs, regardless of the condition")

elif and else — multiple, mutually exclusive branches

Python checks each condition top to bottom and runs only the FIRST branch whose condition is True — every remaining branch, including else, is skipped once one has matched.

population = 54

if population > 100:
    print("Huge country")
elif population > 50:
    print("Large country")
elif population > 10:
    print("Medium country")
else:
    print("Small country")

# Prints "Large country" — the first True condition wins, nothing else runs

Combining conditions with and, or, not

Real conditions often depend on more than one thing. and requires every part to be True; or requires at least one; not flips a boolean.

region = "Africa"
population = 54

if region == "Africa" and population > 50:
    print("Large African country")

if region == "Africa" or region == "Asia":
    print("Africa or Asia")

is_independent = True
if not is_independent:
    print("Not independent")
else:
    print("Independent")

The = vs == typo

A single = assigns; a double == compares. Python treats = inside a condition as a SyntaxError rather than silently reassigning the variable — a deliberate safety net, since this exact typo is extremely common when writing conditionals quickly.

# if population = 54:   # SyntaxError — = cannot be used as a condition
if population == 54:      # correct — == compares
    print("Exactly 54 million")

4. Lab

Lab objective: Write a series of conditionals classifying a country by population and region, combining and/or/not correctly.

What you will build

A file called conditionals.py.

Step-by-step instructions

Create the file and classify population size with if/elif/else

# conditionals.py
population = 54

if population > 100:
    size = "Huge"
elif population > 50:
    size = "Large"
elif population > 10:
    size = "Medium"
else:
    size = "Small"

print(f"Population size category: {size}")

Combine two conditions with and

region = "Africa"

if region == "Africa" and population > 50:
    print("This is a large African country")
else:
    print("Does not match: large African country")

Combine two conditions with or

if region == "Africa" or region == "Americas":
    print("This country is in Africa or the Americas")

Use not to check a boolean is False

is_independent = True

if not is_independent:
    print("Not an independent nation")
else:
    print("An independent nation")

Trigger the = vs == typo deliberately, read the error, then fix it

Comment the broken line out after reading the SyntaxError.

# if population = 54:   # uncomment to see the SyntaxError
if population == 54:
    print("Exactly 54 million")
else:
    print("Not exactly 54 million")

5. Expected Files Changed

File	Action	Why
`conditionals.py`	Created	Demonstrates if/elif/else, and/or/not, and the = vs == distinction.
`docs/sessions/session-03/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 conditionals.py docs/sessions/session-03/index.html
git commit -m "session-03: branch program flow with if/elif/else and combined conditions"

Do not commit until you can answer out loud: "Why does only ONE branch of an if/elif/else chain ever run, even if multiple conditions would technically be True?"

7. Code Review Checklist

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

An if/elif/else chain with at least 3 branches correctly classifies population size

and is used to require two conditions to both be true

or is used to require at least one of two conditions

not is used to check that a boolean is False

The = vs == SyntaxError was triggered once, read, and understood

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

Given population = 54 (population in millions) and the following code, what happens?

if population > 50:
    print("Large")

Nothing prints, because population is a variable, not a literal True "Large" prints, because population > 50 evaluates to True It raises a SyntaxError "Large" prints regardless of the comparison

The condition population > 50 evaluates to True (54 is indeed greater than 50), so the indented block underneath the if runs.

Question 2 of 5

Given the following, with population = 54, what prints?

if population > 100:
    print("Huge")
elif population > 50:
    print("Large")
else:
    print("Small")

"Huge" "Large" "Small" All three lines print

Question 3 of 5

Given region = "Africa" and population = 54, which correctly checks "is this an African country with over 50 million people"?

if region == "Africa" or population > 50: if region == "Africa" and population > 50: if region = "Africa" and population > 50: if region == "Africa", population > 50:

Question 4 of 5

What is wrong with if population = 54:?

Question 5 of 5

Given is_independent = True, which correctly checks "is NOT independent"?

if is_independent = False: if not is_independent: if is_independent != False something: if -is_independent:

not flips a boolean: not True is False, and not False is True. if not is_independent: is the idiomatic way to check "this is False" rather than writing if is_independent == False:.

9. Reflection Questions

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

In the population-classification chain, what would happen if you reordered the branches so the "Medium" check came before "Large"? Would the result change? Why or why not?
Why does Python stop checking further elif branches once it finds one that is True, instead of checking all of them?
Can you think of a real condition that would need and, one that would need or, and one that would need not, from your own experience (not this lab)?
Why do you think Python makes = inside a condition a hard error instead of just a warning?

10. What Breaks If This Knowledge Is Missing?

Every data-filtering session ahead: From the very next Layer forward, every "find only the countries that match X" operation is built on the exact if-condition logic from this session, just applied inside a loop or comprehension instead of standalone.
The assignment/comparison typo: Writing = instead of == is one of the most common typos in any C-like or Python-like language. Python is one of the few languages that turns this into an immediate, loud SyntaxError instead of a silent, hard-to-find bug — recognising that error message on sight will save you real debugging time.

11. What We Learned

Python concept mastered: Branching program flow with if/elif/else, combining conditions with and/or/not, and the assignment-vs-comparison distinction.

Unlocks: Your programs can now make decisions based on data instead of always doing the same thing — the foundation for filtering and validating data in every layer ahead.

Next session: Session 04 — Loops. We repeat an action multiple times instead of writing it out by hand — for and while loops.