Race-Free Order Processing State

HardPython

Problem

How would you structure the data models and state transitions for an order processing service to prevent race conditions during concurrent updates?

Implement a pure function using optimistic concurrency control. An order has id, status, and version; an update succeeds only when expected_version matches the current version and the action is valid for the current state. Return the updated order or a structured failure without mutating the input.

Signature: apply_order_update(order, action, expected_version)

Constraints

Each order contains string fields id and status, plus a nonnegative integer version.
Valid statuses are pending, paid, shipped, delivered, and cancelled.
Valid actions are pay, ship, deliver, and cancel.
The input order must not be mutated.
A successful update increments the version by exactly one.

Function Signature

def apply_order_update(order, action, expected_version):

Code

Run your code to see test output here.