Adds light switch platform (#18562)

* Adds light switch platform

* Addresses HoundCI warnings

* Addresses Flake8 reported issues

* Removes async_update call as per review

homeassistant/components/light/switch.py

@@ -0,0 +1,113 @@
+"""
+Light support for switch entities.
+
+For more information about this platform, please refer to the documentation at
+https://home-assistant.io/components/light.switch/
+"""
+import logging
+import voluptuous as vol
+
+from homeassistant.core import State, callback
+from homeassistant.components.light import (
+    Light, PLATFORM_SCHEMA)
+from homeassistant.components import switch
+from homeassistant.const import (
+    STATE_ON,
+    ATTR_ENTITY_ID,
+    CONF_NAME,
+    CONF_ENTITY_ID,
+    STATE_UNAVAILABLE
+)
+from homeassistant.helpers.typing import HomeAssistantType, ConfigType
+from homeassistant.helpers.event import async_track_state_change
+import homeassistant.helpers.config_validation as cv
+
+_LOGGER = logging.getLogger(__name__)
+
+DEFAULT_NAME = 'Light Switch'
+
+PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
+    vol.Optional(CONF_NAME, default=DEFAULT_NAME): cv.string,
+    vol.Required(CONF_ENTITY_ID): cv.entity_domain(switch.DOMAIN)
+})
+
+
+async def async_setup_platform(hass: HomeAssistantType, config: ConfigType,
+                               async_add_entities,
+                               discovery_info=None) -> None:
+    """Initialize Light Switch platform."""
+    async_add_entities([LightSwitch(config.get(CONF_NAME),
+                                    config[CONF_ENTITY_ID])], True)
+
+
+class LightSwitch(Light):
+    """Represents a Switch as a Light."""
+
+    def __init__(self, name: str, switch_entity_id: str) -> None:
+        """Initialize Light Switch."""
+        self._name = name  # type: str
+        self._switch_entity_id = switch_entity_id  # type: str
+        self._is_on = False  # type: bool
+        self._available = False  # type: bool
+        self._async_unsub_state_changed = None
+
+    @property
+    def name(self) -> str:
+        """Return the name of the entity."""
+        return self._name
+
+    @property
+    def is_on(self) -> bool:
+        """Return true if light switch is on."""
+        return self._is_on
+
+    @property
+    def available(self) -> bool:
+        """Return true if light switch is on."""
+        return self._available
+
+    @property
+    def should_poll(self) -> bool:
+        """No polling needed for a light switch."""
+        return False
+
+    async def async_turn_on(self, **kwargs):
+        """Forward the turn_on command to the switch in this light switch."""
+        data = {ATTR_ENTITY_ID: self._switch_entity_id}
+        await self.hass.services.async_call(
+            switch.DOMAIN, switch.SERVICE_TURN_ON, data, blocking=True)
+
+    async def async_turn_off(self, **kwargs):
+        """Forward the turn_off command to the switch in this light switch."""
+        data = {ATTR_ENTITY_ID: self._switch_entity_id}
+        await self.hass.services.async_call(
+            switch.DOMAIN, switch.SERVICE_TURN_OFF, data, blocking=True)
+
+    async def async_update(self):
+        """Query the switch in this light switch and determine the state."""
+        switch_state = self.hass.states.get(self._switch_entity_id)
+
+        if switch_state is None:
+            self._available = False
+            return
+
+        self._is_on = switch_state.state == STATE_ON
+        self._available = switch_state.state != STATE_UNAVAILABLE
+
+    async def async_added_to_hass(self) -> None:
+        """Register callbacks."""
+        @callback
+        def async_state_changed_listener(entity_id: str, old_state: State,
+                                         new_state: State):
+            """Handle child updates."""
+            self.async_schedule_update_ha_state(True)
+
+        self._async_unsub_state_changed = async_track_state_change(
+            self.hass, self._switch_entity_id, async_state_changed_listener)
+
+    async def async_will_remove_from_hass(self):
+        """Handle removal from Home Assistant."""
+        if self._async_unsub_state_changed is not None:
+            self._async_unsub_state_changed()
+            self._async_unsub_state_changed = None
+            self._available = False