code: def radicals(self, levels=None):
          
          url = WANIKANI_BASE.format(self.api_key, 'radicals')
          if levels:
              url += '/{0}'.format(levels)
          data = self.get(url)
  
          for item in data['requested_information']:
              yield Radical(item)
docs: :param levels string: An optional argument of declaring a single or
              comma-delimited list of levels is available, as seen in the example
              as 1. An example of a comma-delimited list of levels is 1,2,5,9.
  
          http://www.wanikani.com/api/v1.2#radicals-list