code: def validate_format(self, obj, pointer=None):
          
          if 'format' in self.attrs:
              substituted = {
                  'date-time': 'rfc3339.datetime',
                  'email': 'email',
                  'hostname': 'hostname',
                  'ipv4': 'ipv4',
                  'ipv6': 'ipv6',
                  'uri': 'uri',
              }.get(self.attrs['format'], self.attrs['format'])
              logger.debug('use %s', substituted)
              try:
                  return self.formats[substituted](obj)
              except ValidationError as error:
                  logger.error(error)
                  self.fail('Forbidden value', obj, pointer)
          return obj
docs: ================= ============
          Expected draft04  Alias of
          ----------------- ------------
          date-time         rfc3339.datetime
          email             email
          hostname          hostname
          ipv4              ipv4
          ipv6              ipv6
          uri               uri
          ================= ============