code: def get_checksum(self, encoder=base64.b64encode, hasher=hashlib.sha256):
          
          assert self._closed, "Archive not closed"
          with open(self._temp_archive_file.name, 'rb') as fh:
              return encoder(checksum(fh, hasher())).decode('ascii')
docs: Return the b64 encoded sha256 checksum of the archive.