add arwn sensor platform (#3846)

This adds a sensor component that builds sensors based on the arwn
project (https://github.com/sdague/arwn). This uses a 433mhz receiver
to collect weather data and publish it over mqtt in a well defined
schema, which home-assistant can display as sensors.

.coveragerc

@@ -218,6 +218,7 @@ omit =
     homeassistant/components/openalpr.py
     homeassistant/components/scene/hunterdouglas_powerview.py
     homeassistant/components/sensor/arest.py
+    homeassistant/components/sensor/arwn.py
     homeassistant/components/sensor/bitcoin.py
     homeassistant/components/sensor/bom.py
     homeassistant/components/sensor/coinmarketcap.py

homeassistant/components/sensor/arwn.py

@@ -0,0 +1,126 @@
+"""Support for collecting data from the ARWN project.
+
+For more details about this platform, please refer to the
+documentation at https://home-assistant.io/components/sensor.arwn/
+
+"""
+import json
+import logging
+from homeassistant.helpers.entity import Entity
+import homeassistant.components.mqtt as mqtt
+from homeassistant.const import (TEMP_FAHRENHEIT, TEMP_CELSIUS)
+from homeassistant.util import slugify
+
+DEPENDENCIES = ['mqtt']
+
+DOMAIN = "arwn"
+TOPIC = 'arwn/#'
+SENSORS = {}
+
+_LOGGER = logging.getLogger(__name__)
+
+
+def discover_sensors(topic, payload):
+    """Given a topic, dynamically create the right sensor type."""
+    parts = topic.split('/')
+    unit = payload.get('units', '')
+    domain = parts[1]
+    if domain == "temperature":
+        name = parts[2]
+        if unit == "F":
+            unit = TEMP_FAHRENHEIT
+        else:
+            unit = TEMP_CELSIUS
+        return (ArwnSensor(name, 'temp', unit),)
+    if domain == "barometer":
+        return (ArwnSensor("Barometer", 'pressure', unit),)
+    if domain == "wind":
+        return (ArwnSensor("Wind Speed", 'speed', unit),
+                ArwnSensor("Wind Gust", 'gust', unit),
+                ArwnSensor("Wind Direction", 'direction', '°'))
+
+
+def _slug(name):
+    return "sensor.arwn_%s" % slugify(name)
+
+
+def setup_platform(hass, config, add_devices, discovery_info=None):
+    """Set up the ARWN platform."""
+    def sensor_event_received(topic, payload, qos):
+        """Process events as sensors.
+
+        When a new event on our topic (arwn/#) is received we map it
+        into a known kind of sensor based on topic name. If we've
+        never seen this before, we keep this sensor around in a global
+        cache. If we have seen it before, we update the values of the
+        existing sensor. Either way, we push an ha state update at the
+        end for the new event we've seen.
+
+        This lets us dynamically incorporate sensors without any
+        configuration on our side.
+        """
+        event = json.loads(payload)
+        sensors = discover_sensors(topic, event)
+        if not sensors:
+            return
+
+        if 'timestamp' in event:
+            del event['timestamp']
+
+        for sensor in sensors:
+            if sensor.name not in SENSORS:
+                sensor.hass = hass
+                sensor.set_event(event)
+                SENSORS[sensor.name] = sensor
+                _LOGGER.debug("Registering new sensor %(name)s => %(event)s",
+                              dict(name=sensor.name, event=event))
+                add_devices((sensor,))
+            else:
+                SENSORS[sensor.name].set_event(event)
+            SENSORS[sensor.name].update_ha_state()
+
+    mqtt.subscribe(hass, TOPIC, sensor_event_received, 0)
+    return True
+
+
+class ArwnSensor(Entity):
+    """Represents an ARWN sensor."""
+
+    def __init__(self, name, state_key, units):
+        """Initialize the sensor."""
+        self.hass = None
+        self.entity_id = _slug(name)
+        self._name = name
+        self._state_key = state_key
+        self.event = {}
+        self._unit_of_measurement = units
+
+    def set_event(self, event):
+        """Update the sensor with the most recent event."""
+        self.event = {}
+        self.event.update(event)
+
+    @property
+    def state(self):
+        """Return the state of the device."""
+        return self.event.get(self._state_key, None)
+
+    @property
+    def name(self):
+        """Get the name of the sensor."""
+        return self._name
+
+    @property
+    def state_attributes(self):
+        """Return all the state attributes."""
+        return self.event
+
+    @property
+    def unit_of_measurement(self):
+        """Unit this state is expressed in."""
+        return self._unit_of_measurement
+
+    @property
+    def should_poll(self):
+        """Should we poll."""
+        return False