c = boto.connect_s3()
bucket = c.get_bucket('mybucketname')
bucket_files = bucket.list('subdir/file_2014_')
l = [(k.last_modified, k) for k in bucket_files]
key_to_download = sorted(l, cmp=lambda x, y: cmp(x[0], y[0]))[-1][1]
key_to_download.get_contents_to_filename('target_filename')
Boto script to download latest file from s3 bucket