clock

• before_now
  Before returning current time from now()
  outputs: Patch
• after_advance
  After FixedClock.advance() modifies time (testing only)
  outputs: SideEffectRequest
• after_set
  After FixedClock.set() changes time (testing only)
  outputs: SideEffectRequest

• TimeAdvanced v1
  After FixedClock.advance() is called (testing only)
  payload: {clock_id, before, after, delta}
• TimeSet v1
  After FixedClock.set() is called (testing only)
  payload: {clock_id, before, after}

class CreateTaskUseCase:
    def __init__(self, repo: TaskRepository, clock: Clock) -> None:
        self._repo = repo
        self._clock = clock

    async def execute(self, input: CreateTaskInput) -> CreateTaskOutput:
        now = self._clock.now()
        task = Task(id=uuid4(), created_at=now, ...)
        await self._repo.add(task)
        return CreateTaskOutput(task_id=task.id, created_at=now)

Use cases receive Clock as a dependency.

from datetime import datetime, UTC
from psp.platform.clock import FixedClock

def test_task_created_at():
    fixed_time = datetime(2024, 1, 15, 10, 30, tzinfo=UTC)
    clock = FixedClock(fixed_time)

    use_case = CreateTaskUseCase(repo=InMemoryRepo(), clock=clock)
    result = use_case.execute(CreateTaskInput(title="Test"))

    assert result.created_at == fixed_time

Control time in tests for deterministic results.

clock = FixedClock(datetime(2024, 1, 1, tzinfo=UTC))

# Create a reminder due in 1 hour
reminder = scheduler.schedule(due_at=clock.now() + timedelta(hours=1))

# Advance time by 2 hours
clock.advance(hours=2)

# Now the reminder is overdue
assert reminder.due_at < clock.now()

Simulate time passing for expiration tests.