code: def extract_field(self, field):
          
          if not isinstance(field, basestring):
              err_msg = u"Invalid extractor! => {}\n".format(field)
              logger.log_error(err_msg)
              raise exceptions.ParamsError(err_msg)
  
          msg = "extract: {}".format(field)
  
          if text_extractor_regexp_compile.match(field):
              value = self._extract_field_with_regex(field)
          else:
              value = self._extract_field_with_delimiter(field)
  
          if is_py2 and isinstance(value, unicode):
              value = value.encode("utf-8")
  
          msg += "\t=> {}".format(value)
          logger.log_debug(msg)
  
          return value
docs: extract value from requests.Response.