Clock

E.now() reads the current time and E.sleep(duration) pauses, both through the implicit E.Clock service, so neither enters R. now() returns a timezone-aware UTC datetime and is what the logger stamps LogData.date with. Sleeping depends on the runner, so the Clock protocol has no default(); instead there are two live clocks and each runner injects the matching one: run_sync installs E.Clock.SyncLive, whose sleep blocks the thread, and the run_async family (including run_main) installs E.Clock.AsyncLive, whose sleep awaits asyncio.sleep, keeps the loop turning and is interrupted by a cancellation like any coroutine effect.

E.run_sync(E.now())  # datetime.now(UTC), no setup needed
E.run_sync(E.sleep(timedelta(seconds=1)))  # blocks for a second
E.run_async(E.sleep(timedelta(seconds=1)))  # awaits asyncio.sleep(1)

In tests, use E.Clock.Test and move it by hand with adjust(delta) or set_time(time). Both return effects and are async only, like the clock's sleep, which parks until a move reaches its wake time (a sleep of zero returns at once). effecton ships a pytest plugin, loaded automatically wherever the package is installed, that runs a test returning an effect (typically a @E.gen function) under the async runner and reports a failure as its cause. A test that requests the test_clock fixture gets that clock provided to its effect, so E.now(), E.sleep() and the movers all see it:

@E.gen
def test_reads_move_with_the_clock(test_clock: E.Clock.Test) -> E.EffectGen[None]:
    yield from test_clock.set_time(datetime(2024, 1, 1, tzinfo=UTC))
 
    first
 = yield from E.now()
    yield from test_clock.adjust(timedelta(minutes=5))
    second = yield from E.now()
 
    assert second == first + timedelta(minutes=5)

A sleeping program has to run concurrently with the moves: E.fork it and settle on the returned fiber. Each move yields to the loop before and after, so a program forked just before reaches its sleep, and a woken program progresses before the test continues.

Provide clocks with .provide(E.Clock.Protocol)(...): provide_implicit is keyed by type(value), so it would register a Test clock under Test rather than Protocol. This repo's ruff config bans direct time reads and sleeps (datetime.now, time.time, time.monotonic, time.sleep, asyncio.sleep, ...) outside the clock module, so all code goes through E.now() and E.sleep().

More examples: test_clock.py.