code: def pformat(self, traceback=False):
          
          buf = six.StringIO()
          if not self._exc_type_names:
              buf.write('Failure: %s' % (self._exception_str))
          else:
              buf.write('Failure: %s: %s' % (self._exc_type_names[0],
                                             self._exception_str))
          if traceback:
              if self._traceback_str is not None:
                  traceback_str = self._traceback_str.rstrip()
              else:
                  traceback_str = None
              if traceback_str:
                  buf.write(os.linesep)
                  buf.write(traceback_str)
              else:
                  buf.write(os.linesep)
                  buf.write('Traceback not available.')
          return buf.getvalue()
docs: Pretty formats the failure object into a string.