code: def filter(self, value, model=None, context=None):
          
          http = ['http://', 'https://']
          if all(not str(value).startswith(s) for s in http):
              value = 'http://{}'.format(value)
  
          return value
docs: Filter
          Performs value filtering and returns filtered result.
  
          :param value:           input value
          :param model:           parent model being validated
          :param context:         object, filtering context
          :return:                filtered value