text: """
  Support for MQTT sensors.
  
  For more details about this platform, please refer to the documentation at
  https://home-assistant.io/components/sensor.mqtt/
  """
  import logging
  import json
  from datetime import timedelta
  from typing import Optional
  
  import voluptuous as vol
  
  from homeassistant.core import callback
  from homeassistant.components.mqtt import (
      CONF_AVAILABILITY_TOPIC, CONF_STATE_TOPIC, CONF_PAYLOAD_AVAILABLE,
      CONF_PAYLOAD_NOT_AVAILABLE, CONF_QOS, MqttAvailability)
  from homeassistant.components.sensor import DEVICE_CLASSES_SCHEMA
  from homeassistant.const import (
      CONF_FORCE_UPDATE, CONF_NAME, CONF_VALUE_TEMPLATE, STATE_UNKNOWN,
      CONF_UNIT_OF_MEASUREMENT, CONF_ICON, CONF_DEVICE_CLASS)
  from homeassistant.helpers.entity import Entity
  import homeassistant.components.mqtt as mqtt
  import homeassistant.helpers.config_validation as cv
  from homeassistant.helpers.typing import HomeAssistantType, ConfigType
  from homeassistant.helpers.event import async_track_point_in_utc_time
  from homeassistant.util import dt as dt_util
  
  _LOGGER = logging.getLogger(__name__)
  
  CONF_EXPIRE_AFTER = 'expire_after'
  CONF_JSON_ATTRS = 'json_attributes'
  CONF_UNIQUE_ID = 'unique_id'
  
  DEFAULT_NAME = 'MQTT Sensor'
  DEFAULT_FORCE_UPDATE = False
  DEPENDENCIES = ['mqtt']
  
  PLATFORM_SCHEMA = mqtt.MQTT_RO_PLATFORM_SCHEMA.extend({
      vol.Optional(CONF_NAME, default=DEFAULT_NAME): cv.string,
      vol.Optional(CONF_UNIT_OF_MEASUREMENT): cv.string,
      vol.Optional(CONF_ICON): cv.icon,
      vol.Optional(CONF_DEVICE_CLASS): DEVICE_CLASSES_SCHEMA,
      vol.Optional(CONF_JSON_ATTRS, default=[]): cv.ensure_list_csv,
      vol.Optional(CONF_EXPIRE_AFTER): cv.positive_int,
      vol.Optional(CONF_FORCE_UPDATE, default=DEFAULT_FORCE_UPDATE): cv.boolean,
      # Integrations shouldn't never expose unique_id through configuration
      # this here is an exception because MQTT is a msg transport, not a protocol
      vol.Optional(CONF_UNIQUE_ID): cv.string,
  }).extend(mqtt.MQTT_AVAILABILITY_SCHEMA.schema)
  
  
  async def async_setup_platform(hass: HomeAssistantType, config: ConfigType,
                                 async_add_devices, discovery_info=None):
      """Set up MQTT Sensor."""
      if discovery_info is not None:
          config = PLATFORM_SCHEMA(discovery_info)
  
      value_template = config.get(CONF_VALUE_TEMPLATE)
      if value_template is not None:
          value_template.hass = hass
  
      async_add_devices([MqttSensor(
          config.get(CONF_NAME),
          config.get(CONF_STATE_TOPIC),
          config.get(CONF_QOS),
          config.get(CONF_UNIT_OF_MEASUREMENT),
          config.get(CONF_FORCE_UPDATE),
          config.get(CONF_EXPIRE_AFTER),
          config.get(CONF_ICON),
          config.get(CONF_DEVICE_CLASS),
          value_template,
          config.get(CONF_JSON_ATTRS),
          config.get(CONF_UNIQUE_ID),
          config.get(CONF_AVAILABILITY_TOPIC),
          config.get(CONF_PAYLOAD_AVAILABLE),
          config.get(CONF_PAYLOAD_NOT_AVAILABLE),
      )])
  
  
  class MqttSensor(MqttAvailability, Entity):
      """Representation of a sensor that can be updated using MQTT."""
  
      def __init__(self, name, state_topic, qos, unit_of_measurement,
                   force_update, expire_after, icon, device_class: Optional[str],
                   value_template, json_attributes, unique_id: Optional[str],
                   availability_topic, payload_available,
                   payload_not_available):
          """Initialize the sensor."""
          super().__init__(availability_topic, qos, payload_available,
                           payload_not_available)
          self._state = STATE_UNKNOWN
          self._name = name
          self._state_topic = state_topic
          self._qos = qos
          self._unit_of_measurement = unit_of_measurement
          self._force_update = force_update
          self._template = value_template
          self._expire_after = expire_after
          self._icon = icon
          self._device_class = device_class
          self._expiration_trigger = None
          self._json_attributes = set(json_attributes)
          self._unique_id = unique_id
          self._attributes = None
  
      async def async_added_to_hass(self):
          """Subscribe to MQTT events."""
          await super().async_added_to_hass()
  
          @callback
          def message_received(topic, payload, qos):
              """Handle new MQTT messages."""
              # auto-expire enabled?
              if self._expire_after is not None and self._expire_after > 0:
                  # Reset old trigger
                  if self._expiration_trigger:
                      self._expiration_trigger()
                      self._expiration_trigger = None
  
                  # Set new trigger
                  expiration_at = (
                      dt_util.utcnow() + timedelta(seconds=self._expire_after))
  
                  self._expiration_trigger = async_track_point_in_utc_time(
                      self.hass, self.value_is_expired, expiration_at)
  
              if self._json_attributes:
                  self._attributes = {}
                  try:
                      json_dict = json.loads(payload)
                      if isinstance(json_dict, dict):
                          attrs = {k: json_dict[k] for k in
                                   self._json_attributes & json_dict.keys()}
                          self._attributes = attrs
                      else:
                          _LOGGER.warning("JSON result was not a dictionary")
                  except ValueError:
                      _LOGGER.warning("MQTT payload could not be parsed as JSON")
                      _LOGGER.debug("Erroneous JSON: %s", payload)
  
              if self._template is not None:
                  payload = self._template.async_render_with_possible_json_value(
                      payload, self._state)
              self._state = payload
              self.async_schedule_update_ha_state()
  
          await mqtt.async_subscribe(self.hass, self._state_topic,
                                     message_received, self._qos)
  
      @callback
      def value_is_expired(self, *_):
          """Triggered when value is expired."""
          self._expiration_trigger = None
          self._state = STATE_UNKNOWN
          self.async_schedule_update_ha_state()
  
      @property
      def should_poll(self):
          """No polling needed."""
          return False
  
      @property
      def name(self):
          """Return the name of the sensor."""
          return self._name
  
      @property
      def unit_of_measurement(self):
          """Return the unit this state is expressed in."""
          return self._unit_of_measurement
  
      @property
      def force_update(self):
          """Force update."""
          return self._force_update
  
      @property
      def state(self):
          """Return the state of the entity."""
          return self._state
  
      @property
      def device_state_attributes(self):
          """Return the state attributes."""
          return self._attributes
  
      @property
      def unique_id(self):
          """Return a unique ID."""
          return self._unique_id
  
      @property
      def icon(self):
          """Return the icon."""
          return self._icon
  
      @property
      def device_class(self) -> Optional[str]:
          """Return the device class of the sensor."""
          return self._device_class
  