input: Write a python function to recommend movies based on user ratings.
  assert recommend_movies({'Movie1': 9.0, 'Movie2': 8.0, 'Movie3': 8.6}, {'Movie2': 7.5}) == ['Movie1', 'Movie3']
  assert recommend_movies({'Movie1': 8.0, 'Movie2': 8.2, 'Movie3': 9.1}, {'Movie1': 8.0, 'Movie3': 9.1}) == []
  assert recommend_movies({'Movie1': 9.3, 'Movie2': 8.9, 'Movie3': 8.5}, {}) == ['Movie1', 'Movie2']
output: def recommend_movies(all_movies, user_ratings):
      recommended = [movie for movie in all_movies if movie not in user_ratings]
      recommended.sort(key=lambda x: all_movies[x], reverse=True)
      return recommended