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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 unlock(self):
 """Closes the session to the database."""

    if not hasattr(self, 'session'):
      raise RuntimeError('Error detected! The session that you want to close does not exist any more!')
    logger.debug("Closed database session of '%s'" % self._database)
    self.session.close()
    del self.session