Ophyd async is included on a provisional basis until the v1.0 release and may change API on minor release numbers before then

Write Tests for Devices#

Testing ophyd-async devices using tools like mocking, patching, and fixtures can become complicated very quickly. The library provides several utilities to make it easier.

Async Tests#

pytest-asyncio is required for async tests. It is should be included as a dev dependency of your project. Tests can either be decorated with @pytest.mark.asyncio or the project can be automatically configured to detect async tests.

# pyproject.toml

asyncio_mode = "auto"

Sim Backend#

Ophyd devices initialized with a sim backend behave in a similar way to mocks, without requiring you to mock out all the dependencies and internals. The DeviceCollector can initialize any number of devices, and their signals and sub-devices (recursively), with a sim backend.

async def sim_sensor() -> demo.Sensor:
    async with DeviceCollector(sim=True):
        sim_sensor = demo.Sensor("SIM:SENSOR:")
        # Signals connected here

    assert == "sim_sensor"
    return sim_sensor

Sim Utility Functions#

Sim signals behave as simply as possible, holding a sensible default value when initialized and retaining any value (in memory) to which they are set. This model breaks down in the case of read-only signals, which cannot be set because there is an expectation of some external device setting them in the real world. There is a utility function, set_sim_value, to mock-set values for sim signals, including read-only ones.

async def test_sensor_reading_shows_value(sim_sensor: demo.Sensor):
    # Check default value
    assert (await sim_sensor.value.get_value()) == pytest.approx(0.0)
    assert (await == {
        "sim_sensor-value": {
            "alarm_severity": 0,
            "timestamp": ANY,
            "value": 0.0,

    # Check different value
    set_sim_value(sim_sensor.value, 5.0)
    assert (await == {
        "sim_sensor-value": {
            "alarm_severity": 0,
            "timestamp": ANY,
            "value": 5.0,

There is another utility function, set_sim_callback, for hooking in logic when a sim value changes (e.g. because someone puts to it).

async def test_mover_stopped(sim_mover: demo.Mover):
    callbacks = []
    set_sim_callback(sim_mover.stop_, lambda r, v: callbacks.append(v))

    assert callbacks == [None]
    await sim_mover.stop()
    assert callbacks == [None, None]