new_contents: class Base(object):
  
      _instance = {}
  
      @staticmethod
      def instance(key):
          if not Base._instance.get(key):
              Base._instance[key] = 11
  
          return Base._instance[key]
  
  
  
  class A(Base):
      pass
  
  
  class B(Base):
      pass
  
  
  if __name__ == '__main__':
      a = A()
      b = B()
      b.instance('hello')
      a.instance('h')
      print(b._instance) 
      print(a._instance)
      print a._instance == b._instance 
subject: add class-attr