002. Comments

Aryan writes a RAM alert function. It fires a warning if memory usage exceeds a threshold.

def check_ram(used_mb, total_mb):
    usage = used_mb / total_mb
    if usage > 0.85:
        return "WARN"
    return "OK"

Three weeks later, his teammate asks: “Why 0.85? Is that from a spec? A guess? A Stack Overflow answer?”

Without a comment, no one knows. And next month, someone will “fix” 0.85 to 0.9 because it seems more round — and the production alert will start missing real memory pressure.

Comments exist to anchor the WHY behind a decision so the number can never be changed silently. Fix the late-fee function below using the same principle: add comments explaining the rate and the cap, and correct the wrong cap value.

💡 Fun fact: The # comment style traces back to shell scripting in the 1970s. Python adopted it from Unix shells — Guido wanted the language to feel natural to system programmers who already used # for inline notes.

⚠️ Watch out: The most common beginner mistake is writing what the code does rather than why. A comment like # multiply by 0.25 is useless — the code already shows that. The valuable comment is # 25 cents per day per library policy.

🤔 Think about it: If a comment just restates the code in plain English, does it actually help the next reader — or does it create two things that can drift out of sync?

Learning objectives

Write single-line comments with
Explain business logic and non-obvious decisions in comments
Distinguish between useful and redundant comments

Key concepts

comments
documentation
readability

Try it

Concept detail

Comments exist for humans, not for Python. The interpreter skips them entirely. The rule professionals follow: comment the WHY, not the WHAT.

‘fee = days_overdue * 0.25 # multiply by 0.25’ is a useless comment — the code already shows the multiplication. But ‘# 25 cents per day per library policy’ tells the reader WHERE the number comes from, so no one changes it in ignorance.

The broken code also has a real bug: the cap is 5.0 instead of 10.0. Both problems — the missing comment and the wrong value — are symptoms of the same root cause: undocumented business rules become invisible and get corrupted over time.

Well-placed comments prevent future readers (including yourself in 6 months) from quietly breaking rules they never knew existed.

Solution

def calculate_late_fee(days_overdue):
    # 25 cents per day overdue
    fee = days_overdue * 0.25
    # Maximum fee capped at $10.00 to keep it fair
    if fee > 10.0:
        fee = 10.0
    return fee

Tests

def test_normal_fee():
    assert calculate_late_fee(4) == 1.0

def test_fee_capped_at_ten():
    assert calculate_late_fee(60) == 10.0

def test_exactly_at_cap():
    assert calculate_late_fee(40) == 10.0

def test_just_under_cap():
    result = calculate_late_fee(39)
    assert abs(result - 9.75) < 0.001