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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fill_datetime(self):
 """Returns when the slot was filled. Returns: A datetime.datetime. Raises: SlotNotFilledError if the value hasn't been filled yet. """

    if not self.filled:
      raise SlotNotFilledError('Slot with name "%s", key "%s" not yet filled.'
                               % (self.name, self.key))
    return self._fill_datetime