code: def _open(self, f):
          
          if None in self.files:
              fd = self.files.index(None)
              self.files[fd] = f
          else:
              fd = len(self.files)
              self.files.append(f)
          return fd
docs: Adds a file descriptor to the current file descriptor list
  
          :rtype: int
          :param f: the file descriptor to add.
          :return: the index of the file descriptor in the file descr. list