'Lists prefixes in a bucket under prefix
:param bucket_name: the name of the bucket
:type bucket_name: str
:param prefix: a key prefix
:type prefix: str
:param delimiter: the delimiter marks key hierarchy.
:type delimiter: str'

def list_prefixes(self, bucket_name, prefix='', delimiter=''):

    b = self.get_bucket(bucket_name)
    plist = b.list(prefix=prefix, delimiter=delimiter)
    prefix_names = [p.name for p in plist if isinstance(p, boto.s3.prefix.Prefix)]
    return (prefix_names if (prefix_names != []) else None)
