code: def write_signatures(self, signatures):
          
          self.fileobj.seek(self.signature_offset)
          sig_entries = [dict(algorithm_id=id_,
                              size=len(sig),
                              signature=sig)
                         for (id_, sig) in signatures]
  
          sigs = sigs_header.build(dict(
              filesize=self.filesize,
              count=len(signatures),
              sigs=sig_entries,
          ))
          self.fileobj.write(sigs)
          signatures_len = len(sigs)
          self.additional_offset = self.signature_offset + signatures_len
          # sanity check; this should never happen
          if not self.additional_offset == self.fileobj.tell():  # pragma: no cover
              raise IOError('ended up at unexpected offset')
docs: Write signature data to the MAR file.
  
          Args:
              signatures (list): list of signature tuples of the form
                  (algorithm_id, signature_data)