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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 retention_period(self):
 """Retrieve or set the retention period for items in the bucket. :rtype: int or ``NoneType`` :returns: number of seconds to retain items after upload or release from event-based lock, or ``None`` if the property is not set locally. """

        policy = self._properties.get("retentionPolicy")
        if policy is not None:
            period = policy.get("retentionPeriod")
            if period is not None:
                return int(period)