Add brightness scale support for MQTT Lights.

Converts 0..255 values that HA expects into a device 0..SCALE value
Example:
  HA considers "hall light" at 25 brightness or 10% of 255
  Device considers "hall light" at 100 brightness or 10% of 1000

This allows our existing MQTT devices to not change their data format to be used in HA

homeassistant/components/light/mqtt.py

@@ -19,6 +19,7 @@ DEFAULT_QOS = 0
 DEFAULT_PAYLOAD_ON = 'ON'
 DEFAULT_PAYLOAD_OFF = 'OFF'
 DEFAULT_OPTIMISTIC = False
+DEFAULT_BRIGHTNESS_SCALE = 255
 
 DEPENDENCIES = ['mqtt']
 
@@ -44,14 +45,17 @@ def setup_platform(hass, config, add_devices_callback, discovery_info=None):
             'on': config.get('payload_on', DEFAULT_PAYLOAD_ON),
             'off': config.get('payload_off', DEFAULT_PAYLOAD_OFF)
         },
-        config.get('optimistic', DEFAULT_OPTIMISTIC))])
+        config.get('optimistic', DEFAULT_OPTIMISTIC),
+        config.get('brightness_scale', DEFAULT_BRIGHTNESS_SCALE)
+    )])
 
 
 class MqttLight(Light):
     """Provides a MQTT light."""
 
     # pylint: disable=too-many-arguments,too-many-instance-attributes
-    def __init__(self, hass, name, topic, templates, qos, payload, optimistic):
+    def __init__(self, hass, name, topic, templates, qos, payload, optimistic,
+                 brightness_scale):
 
         self._hass = hass
         self._name = name
@@ -62,6 +66,7 @@ class MqttLight(Light):
         self._optimistic_rgb = optimistic or topic["rgb_state_topic"] is None
         self._optimistic_brightness = (optimistic or
                                        topic["brightness_state_topic"] is None)
+        self._brightness_scale = brightness_scale
         self._state = False
 
         templates = {key: ((lambda value: value) if tpl is None else
@@ -84,7 +89,9 @@ class MqttLight(Light):
 
         def brightness_received(topic, payload, qos):
             """A new MQTT message for the brightness has been received."""
-            self._brightness = int(templates['brightness'](payload))
+            device_value = float(templates['brightness'](payload))
+            percent_bright = device_value / self._brightness_scale
+            self._brightness = int(percent_bright * 255)
             self.update_ha_state()
 
         if self._topic["brightness_state_topic"] is not None:
@@ -143,6 +150,7 @@ class MqttLight(Light):
 
         if ATTR_RGB_COLOR in kwargs and \
            self._topic["rgb_command_topic"] is not None:
+
             mqtt.publish(self._hass, self._topic["rgb_command_topic"],
                          "{},{},{}".format(*kwargs[ATTR_RGB_COLOR]), self._qos)
 
@@ -152,9 +160,10 @@ class MqttLight(Light):
 
         if ATTR_BRIGHTNESS in kwargs and \
            self._topic["brightness_command_topic"] is not None:
-
+            percent_bright = float(kwargs[ATTR_BRIGHTNESS]) / 255
+            device_brightness = int(percent_bright * self._brightness_scale)
             mqtt.publish(self._hass, self._topic["brightness_command_topic"],
-                         kwargs[ATTR_BRIGHTNESS], self._qos)
+                         device_brightness, self._qos)
 
             if self._optimistic_brightness:
                 self._brightness = kwargs[ATTR_BRIGHTNESS]

tests/components/light/test_mqtt.py

@@ -42,6 +42,24 @@ light:
   qos: 0
   payload_on: "on"
   payload_off: "off"
+
+config for RGB Version with brightness and scale:
+
+light:
+  platform: mqtt
+  name: "Office Light RGB"
+  state_topic: "office/rgb1/light/status"
+  command_topic: "office/rgb1/light/switch"
+  brightness_state_topic: "office/rgb1/brightness/status"
+  brightness_command_topic: "office/rgb1/brightness/set"
+  brightness_scale: 99
+  rgb_state_topic: "office/rgb1/rgb/status"
+  rgb_command_topic: "office/rgb1/rgb/set"
+  rgb_scale: 99
+  qos: 0
+  payload_on: "on"
+  payload_off: "off"
+
 """
 import unittest
 
@@ -153,6 +171,51 @@ class TestLightMQTT(unittest.TestCase):
         self.assertEqual([125, 125, 125],
                          light_state.attributes.get('rgb_color'))
 
+    def test_controlling_scale(self):
+        self.assertTrue(light.setup(self.hass, {
+            'light': {
+                'platform': 'mqtt',
+                'name': 'test',
+                'state_topic': 'test_scale/status',
+                'command_topic': 'test_scale/set',
+                'brightness_state_topic': 'test_scale/brightness/status',
+                'brightness_command_topic': 'test_scale/brightness/set',
+                'brightness_scale': '99',
+                'qos': 0,
+                'payload_on': 'on',
+                'payload_off': 'off'
+            }
+        }))
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(STATE_OFF, state.state)
+        self.assertIsNone(state.attributes.get('brightness'))
+        self.assertIsNone(state.attributes.get(ATTR_ASSUMED_STATE))
+
+        fire_mqtt_message(self.hass, 'test_scale/status', 'on')
+        self.hass.pool.block_till_done()
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(STATE_ON, state.state)
+        self.assertEqual(255, state.attributes.get('brightness'))
+
+        fire_mqtt_message(self.hass, 'test_scale/status', 'off')
+        self.hass.pool.block_till_done()
+
+        state = self.hass.states.get('light.test')
+        self.assertEqual(STATE_OFF, state.state)
+
+        fire_mqtt_message(self.hass, 'test_scale/status', 'on')
+        self.hass.pool.block_till_done()
+
+        fire_mqtt_message(self.hass, 'test_scale/brightness/status', '99')
+        self.hass.pool.block_till_done()
+
+        light_state = self.hass.states.get('light.test')
+        self.hass.pool.block_till_done()
+        self.assertEqual(255,
+                         light_state.attributes['brightness'])
+
     def test_controlling_state_via_topic_with_templates(self):
         self.assertTrue(light.setup(self.hass, {
             'light': {