new_contents: # encoding: utf-8
  
  
  class Singleton(type):
      _instances = {}
  
      def __call__(cls, *args, **kwargs):
          if cls not in cls._instances:
              cls._instances[cls] = super(
                  Singleton, cls).__call__(*args, **kwargs)
  
          return cls._instances[cls]
  
subject: Implementa metaclasse Singleton