code: def push_stream(cache, user_id, stream):
      
      stack = cache.get(user_id)
      if stack is None:
          stack = []
      if stream:
          stack.append(stream)
          return cache.set(user_id, stack)
      return None
docs: Push a stream onto the stream stack in cache.
  
      :param cache: werkzeug BasicCache-like object
      :param user_id: id of user, used as key in cache
      :param stream: stream object to push onto stack
  
      :return: True on successful update,
               False if failed to update,
               None if invalid input was given