Guard against network errors for Dark Sky (#27141)

* Guard against network errors for Dark Sky

- Prevents network errors from throwing an exception during
  state updates for the Dark Sky weather component.

* Implement `available` for Dark Sky component

* unknown -> unavailable

homeassistant/components/darksky/weather.py

@@ -102,6 +102,11 @@ class DarkSkyWeather(WeatherEntity):
         self._ds_hourly = None
         self._ds_daily = None
 
+    @property
+    def available(self):
+        """Return if weather data is available from Dark Sky."""
+        return self._ds_data is not None
+
     @property
     def attribution(self):
         """Return the attribution."""
@@ -215,7 +220,8 @@ class DarkSkyWeather(WeatherEntity):
         self._dark_sky.update()
 
         self._ds_data = self._dark_sky.data
-        self._ds_currently = self._dark_sky.currently.d
+        currently = self._dark_sky.currently
+        self._ds_currently = currently.d if currently else {}
         self._ds_hourly = self._dark_sky.hourly
         self._ds_daily = self._dark_sky.daily
 

tests/components/darksky/test_weather.py

@@ -6,6 +6,8 @@ from unittest.mock import patch
 import forecastio
 import requests_mock
 
+from requests.exceptions import ConnectionError
+
 from homeassistant.components import weather
 from homeassistant.util.unit_system import METRIC_SYSTEM
 from homeassistant.setup import setup_component
@@ -48,3 +50,16 @@ class TestDarkSky(unittest.TestCase):
 
         state = self.hass.states.get("weather.test")
         assert state.state == "sunny"
+
+    @patch("forecastio.load_forecast", side_effect=ConnectionError())
+    def test_failed_setup(self, mock_load_forecast):
+        """Test to ensure that a network error does not break component state."""
+
+        assert setup_component(
+            self.hass,
+            weather.DOMAIN,
+            {"weather": {"name": "test", "platform": "darksky", "api_key": "foo"}},
+        )
+
+        state = self.hass.states.get("weather.test")
+        assert state.state == "unavailable"