<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def keys(self):
 """Return the tuple of field names of key fields."""

        keys = self._keys
        if keys is None:
            keys = tuple(self[i].name for i in self.key_indices)
        return keys