Add generic interface for accessing device features (#741)

This adds a generic interface for all device classes to introspect available device features,
that is necessary to make it easier to support a wide variety of supported devices with different set of features.
This will allow constructing generic interfaces (e.g., in homeassistant) that fetch and change these features without hard-coding the API calls.

`Device.features()` now returns a mapping of `<identifier, Feature>` where the `Feature` contains all necessary information (like the name, the icon, a way to get and change the setting) to present and change the defined feature through its interface.

kasa/tests/test_feature.py

@@ -0,0 +1,79 @@
+import pytest
+
+from kasa import Feature, FeatureType
+
+
+@pytest.fixture
+def dummy_feature() -> Feature:
+    # create_autospec for device slows tests way too much, so we use a dummy here
+    class DummyDevice:
+        pass
+
+    feat = Feature(
+        device=DummyDevice(),  # type: ignore[arg-type]
+        name="dummy_feature",
+        attribute_getter="dummygetter",
+        attribute_setter="dummysetter",
+        container=None,
+        icon="mdi:dummy",
+        type=FeatureType.BinarySensor,
+    )
+    return feat
+
+
+def test_feature_api(dummy_feature: Feature):
+    """Test all properties of a dummy feature."""
+    assert dummy_feature.device is not None
+    assert dummy_feature.name == "dummy_feature"
+    assert dummy_feature.attribute_getter == "dummygetter"
+    assert dummy_feature.attribute_setter == "dummysetter"
+    assert dummy_feature.container is None
+    assert dummy_feature.icon == "mdi:dummy"
+    assert dummy_feature.type == FeatureType.BinarySensor
+
+
+def test_feature_value(dummy_feature: Feature):
+    """Verify that property gets accessed on *value* access."""
+    dummy_feature.attribute_getter = "test_prop"
+    dummy_feature.device.test_prop = "dummy"  # type: ignore[attr-defined]
+    assert dummy_feature.value == "dummy"
+
+
+def test_feature_value_container(mocker, dummy_feature: Feature):
+    """Test that container's attribute is accessed when expected."""
+
+    class DummyContainer:
+        @property
+        def test_prop(self):
+            return "dummy"
+
+    dummy_feature.container = DummyContainer()
+    dummy_feature.attribute_getter = "test_prop"
+
+    mock_dev_prop = mocker.patch.object(
+        dummy_feature, "test_prop", new_callable=mocker.PropertyMock, create=True
+    )
+
+    assert dummy_feature.value == "dummy"
+    mock_dev_prop.assert_not_called()
+
+
+def test_feature_value_callable(dev, dummy_feature: Feature):
+    """Verify that callables work as *attribute_getter*."""
+    dummy_feature.attribute_getter = lambda x: "dummy value"
+    assert dummy_feature.value == "dummy value"
+
+
+async def test_feature_setter(dev, mocker, dummy_feature: Feature):
+    """Verify that *set_value* calls the defined method."""
+    mock_set_dummy = mocker.patch.object(dummy_feature.device, "set_dummy", create=True)
+    dummy_feature.attribute_setter = "set_dummy"
+    await dummy_feature.set_value("dummy value")
+    mock_set_dummy.assert_called_with("dummy value")
+
+
+async def test_feature_setter_read_only(dummy_feature):
+    """Verify that read-only feature raises an exception when trying to change it."""
+    dummy_feature.attribute_setter = None
+    with pytest.raises(ValueError):
+        await dummy_feature.set_value("value for read only feature")