Prevent nut from doing I/O in the event loop (#33420)

* Prevent nut from doing I/O in the event loop

device_state_attributes would call for an update if the
throttle happened to expire.

* _format_display_state no self use

homeassistant/components/nut/sensor.py

@@ -220,6 +220,8 @@ class NUTSensor(Entity):
         self._name = "{} {}".format(name, SENSOR_TYPES[sensor_type][0])
         self._unit = SENSOR_TYPES[sensor_type][1]
         self._state = None
+        self._display_state = None
+        self._available = False
 
     @property
     def name(self):
@@ -241,38 +243,44 @@ class NUTSensor(Entity):
         """Return the unit of measurement of this entity, if any."""
         return self._unit
 
+    @property
+    def available(self):
+        """Return if the device is polling successfully."""
+        return self._available
+
     @property
     def device_state_attributes(self):
         """Return the sensor attributes."""
-        attr = dict()
-        attr[ATTR_STATE] = self.display_state()
-        return attr
-
-    def display_state(self):
-        """Return UPS display state."""
-        if self._data.status is None:
-            return STATE_TYPES["OFF"]
-        try:
-            return " ".join(
-                STATE_TYPES[state] for state in self._data.status[KEY_STATUS].split()
-            )
-        except KeyError:
-            return STATE_UNKNOWN
+        return {ATTR_STATE: self._display_state}
 
     def update(self):
         """Get the latest status and use it to update our sensor state."""
-        if self._data.status is None:
-            self._state = None
+        status = self._data.status
+
+        if status is None:
+            self._available = False
             return
 
+        self._available = True
+        self._display_state = _format_display_state(status)
         # In case of the display status sensor, keep a human-readable form
         # as the sensor state.
         if self.type == KEY_STATUS_DISPLAY:
-            self._state = self.display_state()
-        elif self.type not in self._data.status:
+            self._state = self._display_state
+        elif self.type not in status:
             self._state = None
         else:
-            self._state = self._data.status[self.type]
+            self._state = status[self.type]
+
+
+def _format_display_state(status):
+    """Return UPS display state."""
+    if status is None:
+        return STATE_TYPES["OFF"]
+    try:
+        return " ".join(STATE_TYPES[state] for state in status[KEY_STATUS].split())
+    except KeyError:
+        return STATE_UNKNOWN
 
 
 class PyNUTData: