code: def _marshall_fields(self):
          
          values = {}
          for key, value in self.fields.items():
              if (isinstance(value, int) or
                      (isinstance(value, str) and value.isdigit() and
                       '.' not in value)):
                  values[key] = '{}i'.format(value)
              elif isinstance(value, bool):
                  values[key] = self._escape(value)
              elif isinstance(value, float):
                  values[key] = '{}'.format(value)
              elif isinstance(value, str):
                  values[key] = '"{}"'.format(self._escape(value))
          return ','.join(['{}={}'.format(self._escape(k), v)
                           for k, v in values.items()])
docs: Convert the field dict into the string segment of field key/value
          pairs.
  
          :rtype: str