From 8109aa96f628711482501d2bd5de1406eb764bb8 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Sun, 13 Sep 2026 23:51:42 -0500
Subject: [PATCH] [light] Skip the flash transition setter and the empty effect
 list (#19228)

---
 esphome/components/light/__init__.py          | 14 +++++++--
 esphome/components/light/light_state.h        |  2 +-
 .../light/config/transitions.yaml             | 29 +++++++++++++++++++
 .../light/test_default_setters.py             | 19 ++++++++++++
 4 files changed, 60 insertions(+), 4 deletions(-)
 create mode 100644 tests/component_tests/light/config/transitions.yaml
 create mode 100644 tests/component_tests/light/test_default_setters.py

diff --git a/esphome/components/light/__init__.py b/esphome/components/light/__init__.py
index dbcc28d64a3..ab9624c3649 100644
--- a/esphome/components/light/__init__.py
+++ b/esphome/components/light/__init__.py
@@ -340,6 +340,10 @@ RESTORE_MODES = {
     "RESTORE_AND_ON": LightRestoreMode.LIGHT_RESTORE_AND_ON,
 }
 
+# Schema default that also matches the C++ initializer in light_state.h; codegen
+# skips the setter when the config equals it.
+DEFAULT_FLASH_TRANSITION_LENGTH = "0s"
+
 LIGHT_SCHEMA = (
     cv.ENTITY_BASE_SCHEMA.extend(web_server.WEBSERVER_SORTING_SCHEMA)
     .extend(cv.MQTT_COMMAND_COMPONENT_SCHEMA)
@@ -387,7 +391,7 @@ BRIGHTNESS_ONLY_LIGHT_SCHEMA = LIGHT_SCHEMA.extend(
             CONF_DEFAULT_TRANSITION_LENGTH, default="1s"
         ): cv.positive_time_period_milliseconds,
         cv.Optional(
-            CONF_FLASH_TRANSITION_LENGTH, default="0s"
+            CONF_FLASH_TRANSITION_LENGTH, default=DEFAULT_FLASH_TRANSITION_LENGTH
         ): cv.positive_time_period_milliseconds,
         cv.Optional(CONF_EFFECTS): validate_effects(MONOCHROMATIC_EFFECTS),
     }
@@ -502,9 +506,12 @@ async def setup_light_core_(light_var, config, output_var):
         default_transition_length := config.get(CONF_DEFAULT_TRANSITION_LENGTH)
     ) is not None:
         cg.add(light_var.set_default_transition_length(default_transition_length))
+    # Skip the setter when the config matches the C++ initializer.
     if (
         flash_transition_length := config.get(CONF_FLASH_TRANSITION_LENGTH)
-    ) is not None:
+    ) is not None and flash_transition_length != cv.time_period(
+        DEFAULT_FLASH_TRANSITION_LENGTH
+    ):
         cg.add(light_var.set_flash_transition_length(flash_transition_length))
     if (gamma_correct := config.get(CONF_GAMMA_CORRECT)) is not None:
         cg.add(light_var.set_gamma_correct(gamma_correct))
@@ -514,7 +521,8 @@ async def setup_light_core_(light_var, config, output_var):
     effects = await cg.build_registry_list(
         EFFECTS_REGISTRY, config.get(CONF_EFFECTS, [])
     )
-    cg.add(light_var.add_effects(effects))
+    if effects:
+        cg.add(light_var.add_effects(effects))
 
     for conf in config.get(CONF_ON_TURN_ON, []):
         trigger = cg.new_Pvariable(conf[CONF_TRIGGER_ID], light_var)
diff --git a/esphome/components/light/light_state.h b/esphome/components/light/light_state.h
index 3a3f8fc368c..eafa161f51e 100644
--- a/esphome/components/light/light_state.h
+++ b/esphome/components/light/light_state.h
@@ -356,7 +356,7 @@ class LightState : public EntityBase, public Component {
   /// Default transition length for all transitions in ms.
   uint32_t default_transition_length_{};
   /// Transition length to use for flash transitions.
-  uint32_t flash_transition_length_{};
+  uint32_t flash_transition_length_{};  // Keep in sync with DEFAULT_FLASH_TRANSITION_LENGTH in __init__.py
   /// Gamma correction factor for the light.
   float gamma_correct_{};
 #ifdef USE_LIGHT_GAMMA_LUT
diff --git a/tests/component_tests/light/config/transitions.yaml b/tests/component_tests/light/config/transitions.yaml
new file mode 100644
index 00000000000..ecb33b0ea80
--- /dev/null
+++ b/tests/component_tests/light/config/transitions.yaml
@@ -0,0 +1,29 @@
+---
+esphome:
+  name: test
+
+esp32:
+  board: esp32dev
+
+output:
+  - platform: ledc
+    id: out_a
+    pin: GPIO4
+  - platform: ledc
+    id: out_b
+    pin: GPIO5
+
+light:
+  - platform: monochromatic
+    id: plain_light
+    output: out_a
+    flash_transition_length: 0s
+  - platform: monochromatic
+    id: fancy_light
+    output: out_b
+    flash_transition_length: 500ms
+    effects:
+      - pulse:
+  - platform: monochromatic
+    id: bare_light
+    output: out_a
diff --git a/tests/component_tests/light/test_default_setters.py b/tests/component_tests/light/test_default_setters.py
new file mode 100644
index 00000000000..a4fc24a7cbf
--- /dev/null
+++ b/tests/component_tests/light/test_default_setters.py
@@ -0,0 +1,19 @@
+"""Tests that light codegen skips setters for default values."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+
+def test_default_flash_length_and_empty_effects_are_not_emitted(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    """A 0 ms flash transition and an empty effect list match the C++ defaults."""
+    main_cpp = generate_main(component_config_path("transitions.yaml"))
+
+    assert "plain_light->set_flash_transition_length(" not in main_cpp
+    assert "plain_light->add_effects(" not in main_cpp
+    assert "bare_light->set_flash_transition_length(" not in main_cpp
+    assert "bare_light->add_effects(" not in main_cpp
+    assert "fancy_light->set_flash_transition_length(500);" in main_cpp
+    assert "fancy_light->add_effects({" in main_cpp