'Enables the automatic copy of snapshots from one region to
another region for a specified cluster.
:type cluster_identifier: string
:param cluster_identifier: The unique identifier of the source cluster
to copy snapshots from.
Constraints: Must be the valid name of an existing cluster that does
not already have cross-region snapshot copy enabled.
:type destination_region: string
:param destination_region: The destination region that you want to copy
snapshots to.
Constraints: Must be the name of a valid region. For more information,
see `Regions and Endpoints`_ in the Amazon Web Services General
Reference.
:type retention_period: integer
:param retention_period: The number of days to retain automated
snapshots in the destination region after they are copied from the
source region.
Default: 7.
Constraints: Must be at least 1 and no more than 35.'

def enable_snapshot_copy(self, cluster_identifier, destination_region, retention_period=None):

    params = {'ClusterIdentifier': cluster_identifier, 'DestinationRegion': destination_region}
    if (retention_period is not None):
        params['RetentionPeriod'] = retention_period
    return self._make_request(action='EnableSnapshotCopy', verb='POST', path='/', params=params)
