light.piglow update (#7408)

* light.piglow: brightness control logic update

Do not reset brightness when brightness is not explicitly supplied.
Based on conversation at:
https://github.com/home-assistant/home-assistant/pull/7377#discussion_r114102005

* light.piglow: add assumed state

homeassistant/components/light/piglow.py

@@ -72,6 +72,16 @@ class PiglowLight(Light):
         """Flag supported features."""
         return SUPPORT_PIGLOW
 
+    @property
+    def should_poll(self):
+        """Return if we should poll this device."""
+        return False
+
+    @property
+    def assumed_state(self) -> bool:
+        """Return True if unable to access real state of the entity."""
+        return True
+
     @property
     def is_on(self):
         """Return true if light is on."""
@@ -80,7 +90,9 @@ class PiglowLight(Light):
     def turn_on(self, **kwargs):
         """Instruct the light to turn on."""
         self._piglow.clear()
-        self._brightness = kwargs.get(ATTR_BRIGHTNESS, 255)
+
+        if ATTR_BRIGHTNESS in kwargs:
+            self._brightness = kwargs[ATTR_BRIGHTNESS]
         percent_bright = (self._brightness / 255)
 
         if ATTR_RGB_COLOR in kwargs:
@@ -92,9 +104,11 @@ class PiglowLight(Light):
             self._piglow.all(self._brightness)
         self._piglow.show()
         self._is_on = True
+        self.schedule_update_ha_state()
 
     def turn_off(self, **kwargs):
         """Instruct the light to turn off."""
         self._piglow.clear()
         self._piglow.show()
         self._is_on = False
+        self.schedule_update_ha_state()