<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def owner(self):
 """Returns whether the lock is locked by a writer or reader."""

        if self._writer is not None:
            return self.WRITER
        if self._readers:
            return self.READER
        return None