Extract a value for a given key.

def get_value(self, key):
        """Extract a value for a given key."""
        for title in _TITLES.get(key, ()) + (key,):
            try:
                value = [entry['lastMeasurement']['value'] for entry in
                         self.data['sensors'] if entry['title'] == title][0]
                return value
            except IndexError:
                pass
        return None