'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.
:type dry_run: bool
:param dry_run: Set to True if the operation should not actually run.
:rtype: :class:`boto.ec2.spotdatafeedsubscription.SpotDatafeedSubscription`
:return: The datafeed subscription object or None'

def create_spot_datafeed_subscription(self, bucket, prefix, dry_run=False):

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