docs: def read_history_file(self, filename=None): 
          u'''Load a readline history file.'''
          if filename is None:
              filename = self.history_filename
          try:
              for line in open(filename, u'r'):
                  self.add_history(lineobj.ReadLineTextBuffer(ensure_unicode(line.rstrip())))
          except IOError:
              self.history = []
              self.history_cursor = 0