code: def dumps(self, fd, **kwargs):
          
          if 0 <= fd <= 2:
              data = [self.stdin, self.stdout, self.stderr][fd].concretize(**kwargs)
              if type(data) is list:
                  data = b''.join(data)
              return data
          return self.get_fd(fd).concretize(**kwargs)
docs: Returns the concrete content for a file descriptor.
  
          BACKWARD COMPATIBILITY: if you ask for file descriptors 0 1 or 2, it will return the data from stdin, stdout,
          or stderr as a flat string.
  
          :param fd:  A file descriptor.
          :return:    The concrete content.
          :rtype:     str