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Code reading under pressureSafe extensionRule addition without breaking existing logic
⏰ 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
Order could be paid only oncePayment amount had to match total exactlyOrder can receive multiple paymentsOrder becomes PAID only when full amount is collectedOverpayment is not allowed
CREATEDPARTIALLY_PAIDPAIDCANCELLED
Same as beforeCan be added only in CREATED state
Payment amount must be > 0Total paid amount must never exceed order totalMultiple payments allowed until fully paid
Order can be cancelled only if NOT PAIDCancel clears any partial paymentCancelled order cannot be paid
Add private variables (e.g. self.paid_amount)Update internal logicAdd helper methods (private only)
Change method signaturesRemove existing validationsModify OrderItem or Payment classes
Order status is CREATED or PARTIALLY_PAIDPayment amount > 0Total paid + payment.amount ≤ total amountPayment is successfulAdd payment amount to paid totalUpdate state:If paid < total → PARTIALLY_PAIDIf paid == total → PAID
Track paid_amountCompare against totalUpdate state correctlyPreserve all Day 8 behavior
All original Day 8 tests passAll extension tests passNo signature changesNo breaking of existing behaviorCorrect state transitionsClean handling of partial payments
What new state did you introduce and why?How did you ensure backward compatibility?Where did you store paid amount and why?What was the riskiest change you made?How would you test this further?
Candidate can handle changing requirementsCan extend real backend workflowsIs ready for strong SDE-1 / borderline SDE-2 interviewsDemonstrates real engineering maturityAdd extension round to Day 9Combine Days 7.5–9 into one mega mockCreate interviewer scoring rubricTurn this into a paid assessment
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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:
🟢 Day 8 — Extension Round (Mid-Interview Change)
This simulates the interviewer saying: “Okay, now add one more requirement.”
⏱ Extension Rules (Tell Candidate)
🧩 Extension Requirement — Partial Payment Support
New Business Requirement
The system should now support partial payments.
🔄 What Changes Conceptually?
Before:
Now:
🧱 Updated Order States
Order can now be:
🚫 Updated Rules (Read Carefully)
Items
Payments
New Rules
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)
🧱 Allowed Code Changes (VERY IMPORTANT)
You MAY:
You may NOT:
🔁 Existing Method — Updated Behavior
make_payment(payment)
Now should:
Return True if:
Then:
Else return False.
🧠 What Candidate Must Figure Out
Without being told explicitly, candidate must:
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")
order.add_item(OrderItem("I1", 100, 2)) # total = 200
assert order.get_total_amount() == 200
assert order.get_status() == "CREATED"
assert order.make_payment(Payment(50)) is True
assert order.get_status() == "PARTIALLY_PAID"
assert order.make_payment(Payment(100)) is True
assert order.get_status() == "PARTIALLY_PAID"
assert order.make_payment(Payment(50)) is True
assert order.get_status() == "PAID"
print("✅ Partial payment flow tests passed")
def test_overpayment_not_allowed():
print("Running overpayment tests...")
order = Order("O_EXT_2")
order.add_item(OrderItem("I2", 100, 1)) # total = 100
assert order.make_payment(Payment(70)) is True
assert order.make_payment(Payment(40)) is False # exceeds total
print("✅ Overpayment tests passed")
def test_cancel_after_partial_payment():
print("Running cancel after partial payment tests...")
order = Order("O_EXT_3")
order.add_item(OrderItem("I3", 100, 1))
assert order.make_payment(Payment(30)) is True
assert order.get_status() == "PARTIALLY_PAID"
assert order.cancel() is True
assert order.get_status() == "CANCELLED"
assert order.make_payment(Payment(70)) is False
print("✅ Cancel after partial payment tests passed")
def test_no_payment_after_paid():
print("Running no payment after PAID tests...")
order = Order("O_EXT_4")
order.add_item(OrderItem("I4", 50, 2)) # total = 100
assert order.make_payment(Payment(100)) is True
assert order.get_status() == "PAID"
assert order.make_payment(Payment(10)) is False
print("✅ No payment after PAID tests passed")
if __name__ == "__main__":
test_partial_payment_flow()
test_overpayment_not_allowed()
test_cancel_after_partial_payment()
test_no_payment_after_paid()
print("\n🎉 ALL DAY 8 EXTENSION TESTS PASSED SUCCESSFULLY")
✅ Pass Criteria for Extension Round
Candidate passes Day 8 Extension if:
🧠 Post-Extension Reflection (Must Answer)
Ask candidate:
⭐ Extension Golden Rule
Anyone can write new code. Engineers extend code without breaking it.
🎯 What This Achieves
With Day 8 + Extension:
If you want next, I can:
Just tell me what’s next 🚀
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