def METHOD_NAME(self, key: str, dest_dir: str) -> None:
    filename = S3Client.get_filename_from_key(key)
    assert filename, f"Expected non-empty filename from key {key}."
    with open(os.path.join(dest_dir, filename), 'wb') as f:
        self.s3.download_fileobj(self.bucket, key, f)