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Day 8 — Extension Round (Mid-Interview Change)

Perfect choice 👍 This is exactly how real machine-coding rounds work.
Below is Day 8 – Extension Round, designed to be applied after the candidate completes Day 8 base implementation.
This teaches:
Code reading under pressure
Safe extension
Rule addition without breaking existing logic
⁠

🟢 Day 8 — Extension Round (Mid-Interview Change)

This simulates the interviewer saying: ​“Okay, now add one more requirement.”
⁠

⏱ Extension Rules (Tell Candidate)

⏰ Time for extension: 25–30 minutes
❌ Do NOT rewrite existing logic
❌ Do NOT break existing tests
❌ Do NOT change existing method signatures
✅ You may add new methods only if explicitly allowed
✅ All old behavior must still work
⁠

🧩 Extension Requirement — Partial Payment Support

New Business Requirement

The system should now support partial payments.
⁠

🔄 What Changes Conceptually?

Before:
Order could be paid only once
Payment amount had to match total exactly
Now:
Order can receive multiple payments
Order becomes PAID only when full amount is collected
Overpayment is not allowed
⁠

🧱 Updated Order States

Order can now be:
CREATED
PARTIALLY_PAID
PAID
CANCELLED
⁠

🚫 Updated Rules (Read Carefully)

Items

Same as before
Can be added only in CREATED state
⁠

Payments

New Rules
Payment amount must be > 0
Total paid amount must never exceed order total
Multiple payments allowed until fully paid
⁠

State Transitions

Current State
Action
Result State
CREATED
partial payment
PARTIALLY_PAID
PARTIALLY_PAID
partial payment
PARTIALLY_PAID
PARTIALLY_PAID
final payment
PAID
CREATED
full payment
PAID
PAID
any payment
❌
CANCELLED
any payment
❌
There are no rows in this table
⁠
⁠

Cancel Rules (Updated)

Order can be cancelled only if NOT PAID
Cancel clears any partial payment
Cancelled order cannot be paid
⁠

🧱 Allowed Code Changes (VERY IMPORTANT)

You MAY:

Add private variables (e.g. self.paid_amount)
Update internal logic
Add helper methods (private only)

You may NOT:

Change method signatures
Remove existing validations
Modify OrderItem or Payment classes
⁠

🔁 Existing Method — Updated Behavior

make_payment(payment)

Now should:
Return True if:
Order status is CREATED or PARTIALLY_PAID
Payment amount > 0
Total paid + payment.amount ≤ total amount
Payment is successful
Then:
Add payment amount to paid total
Update state:
If paid < total → PARTIALLY_PAID
If paid == total → PAID
Else return False.
⁠

🧠 What Candidate Must Figure Out

Without being told explicitly, candidate must:
Track paid_amount
Compare against total
Update state correctly
Preserve all Day 8 behavior
This is where strong candidates stand out.
⁠

🧪 Extension Test File (Run After Base Tests)

📄 test_day8_extension.py

from solution import Order, OrderItem, Payment


def test_partial_payment_flow():
print("Running partial payment tests...")

order = Order("O_EXT_1")

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