<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def valid_modes(self):
 """Valid modes of an open file"""

        default_mode = (self.mode,) if self.mode is not None else None
        return getattr(self, '_valid_mode', default_mode)