From 4e8d68a2efca4e98019453ab5c5491a36f3ca222 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andreas=20K=C3=B6lsch?=
 <20746434+andreaskoelsch@users.noreply.github.com>
Date: Fri, 2 May 2025 00:07:52 +0200
Subject: [PATCH] Fix brightness calculation when using brightness_step_pct
 (#143786)

---
 homeassistant/components/light/__init__.py |  5 +-
 tests/components/light/test_init.py        | 59 ++++++++++++++++------
 2 files changed, 47 insertions(+), 17 deletions(-)

diff --git a/homeassistant/components/light/__init__.py b/homeassistant/components/light/__init__.py
index 7b548533058475..d2869670ba446a 100644
--- a/homeassistant/components/light/__init__.py
+++ b/homeassistant/components/light/__init__.py
@@ -442,7 +442,10 @@ async def async_handle_light_on_service(  # noqa: C901
                 brightness += params.pop(ATTR_BRIGHTNESS_STEP)
 
             else:
-                brightness += round(params.pop(ATTR_BRIGHTNESS_STEP_PCT) / 100 * 255)
+                brightness_pct = round(brightness / 255 * 100)
+                brightness = round(
+                    (brightness_pct + params.pop(ATTR_BRIGHTNESS_STEP_PCT)) / 100 * 255
+                )
 
             params[ATTR_BRIGHTNESS] = max(0, min(255, brightness))
 
diff --git a/tests/components/light/test_init.py b/tests/components/light/test_init.py
index 29604ce7595abe..014e3ec8c35e18 100644
--- a/tests/components/light/test_init.py
+++ b/tests/components/light/test_init.py
@@ -958,21 +958,6 @@ async def test_light_brightness_step(hass: HomeAssistant) -> None:
     _, data = entity1.last_call("turn_on")
     assert data["brightness"] == 40  # 50 - 10
 
-    await hass.services.async_call(
-        "light",
-        "turn_on",
-        {
-            "entity_id": [entity0.entity_id, entity1.entity_id],
-            "brightness_step_pct": 10,
-        },
-        blocking=True,
-    )
-
-    _, data = entity0.last_call("turn_on")
-    assert data["brightness"] == 116  # 90 + (255 * 0.10)
-    _, data = entity1.last_call("turn_on")
-    assert data["brightness"] == 66  # 40 + (255 * 0.10)
-
     await hass.services.async_call(
         "light",
         "turn_on",
@@ -983,7 +968,49 @@ async def test_light_brightness_step(hass: HomeAssistant) -> None:
         blocking=True,
     )
 
-    assert entity0.state == "off"  # 126 - 126; brightness is 0, light should turn off
+    assert entity0.state == "off"  # 40 - 126; brightness is 0, light should turn off
+
+
+async def test_light_brightness_step_pct(hass: HomeAssistant) -> None:
+    """Test that percentage based brightness steps work as expected."""
+    entity = MockLight("Test_0", STATE_ON)
+
+    setup_test_component_platform(hass, light.DOMAIN, [entity])
+
+    entity.supported_features = light.SUPPORT_BRIGHTNESS
+    # Set color modes to none to trigger backwards compatibility in LightEntity
+    entity.supported_color_modes = None
+    entity.color_mode = None
+    entity.brightness = 255
+    assert await async_setup_component(hass, "light", {"light": {"platform": "test"}})
+    await hass.async_block_till_done()
+
+    state = hass.states.get(entity.entity_id)
+    assert state is not None
+    assert state.attributes["brightness"] == 255  # 100%
+
+    def reduce_brightness_by_ten_percent():
+        return hass.services.async_call(
+            "light",
+            "turn_on",
+            {
+                "entity_id": [entity.entity_id],
+                "brightness_step_pct": -10,
+            },
+            blocking=True,
+        )
+
+    await reduce_brightness_by_ten_percent()
+    _, data = entity.last_call("turn_on")
+    assert round(data["brightness"] / 2.55) == 90  # 100% - 10% = 90%
+
+    await reduce_brightness_by_ten_percent()
+    _, data = entity.last_call("turn_on")
+    assert round(data["brightness"] / 2.55) == 80  # 90% - 10% = 80%
+
+    await reduce_brightness_by_ten_percent()
+    _, data = entity.last_call("turn_on")
+    assert round(data["brightness"] / 2.55) == 70  # 80% - 10% = 70%
 
 
 @pytest.mark.usefixtures("enable_custom_integrations")