<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create_spot_datafeed_subscription(self, bucket, prefix):
 """ Create a spot instance datafeed subscription for this account. :type bucket: str or unicode :param bucket: The name of the bucket where spot instance data will be written. The account issuing this request must have FULL_CONTROL access to the bucket specified in the request. :type prefix: str or unicode :param prefix: An optional prefix that will be pre-pended to all data files written to the bucket. :rtype: :class:`boto.ec2.spotdatafeedsubscription.SpotDatafeedSubscription` :return: The datafeed subscription object or None """

        params = {'Bucket' : bucket}
        if prefix:
            params['Prefix'] = prefix
        return self.get_object('CreateSpotDatafeedSubscription',
                               params, SpotDatafeedSubscription, verb='POST')