def get_opposite_emotion(key):
    """
    Utility function to get the opposite emotion of a given emotion
    :param key: emotion to be processed
    :return: opposite emotion, None if no opposite emotion is found
    """
    opposite_emotions = {"joy": "anger",
                         "sad": "joy",
                         "anticipation": "anger",
                         "trust": "fear",
                         'fear': 'trust',
                         'anger' : 'joy',
                         'afraid': 'trust',
                         'negative':'positive',
                         'positive': 'negative',
                         'model_strong': 'model_weak',
                         'model_weak': 'model_strong',
                         }

    if opposite_emotions.keys().__contains__(key):
        return opposite_emotions[key]
    else:
        return None