Complete Order - Validate and Finalize an Order State Machine

Question

## Round 1 - Coding

## Problem

Model the lifecycle of a DoorDash order as a state machine. Orders move through states: `CREATED -> ACCEPTED -> PICKED_UP -> DELIVERED`. Some transitions also allow `CANCELLED` from certain states. Implement the state machine with validation.

```python
from enum import Enum

class OrderState(Enum):
    CREATED   = "CREATED"
    ACCEPTED  = "ACCEPTED"
    PICKED_UP = "PICKED_UP"
    DELIVERED = "DELIVERED"
    CANCELLED = "CANCELLED"

VALID_TRANSITIONS = {
    OrderState.CREATED:   [OrderState.ACCEPTED, OrderState.CANCELLED],
    OrderState.ACCEPTED:  [OrderState.PICKED_UP, OrderState.CANCELLED],
    OrderState.PICKED_UP: [OrderState.DELIVERED],
    OrderState.DELIVERED: [],
    OrderState.CANCELLED: [],
}

class Order:
    def __init__(self, order_id: str): ...
    def transition(self, new_state: OrderState) -> None:
        # Raise ValueError if transition is invalid
        ...
    def is_terminal(self) -> bool: ...
    def history(self) -> list[OrderState]: ...
```

```
Example:
order = Order("ORD-001")
order.transition(OrderState.ACCEPTED)   # ok
order.transition(OrderState.DELIVERED)  # raises ValueError: invalid transition
order.transition(OrderState.PICKED_UP)  # ok
order.transition(OrderState.DELIVERED)  # ok
order.is_terminal() -> True
order.history() -> [CREATED, ACCEPTED, PICKED_UP, DELIVERED]
```

## Follow-ups

1. How would you persist order state so it survives process restarts?
2. How do you handle concurrent state transitions (e.g. customer cancels while dasher picks up)?
3. How would you add timestamps to each state transition for SLA tracking?
4. How would you unit test every valid and invalid transition?

Complete Order - Validate and Finalize an Order State Machine

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