code: def circular_contigs(self):
          '''Returns a set of the contig names that are circular'''
          if self.assembler == 'spades':
              if self.contigs_fastg is not None:
                  return self._circular_contigs_from_spades_before_3_6_1(self.contigs_fastg)
              elif None not in [self.contigs_paths, self.assembly_graph_fastg]:
                  return self._circular_contigs_from_spades_after_3_6_1(self.assembly_graph_fastg, self.contigs_paths)
              else:
                  return set()
          elif self.assembler == 'canu':
              return self._circular_contigs_from_canu_gfa(self.contigs_gfa)
          else:
              return set(f circular_contigs(self):
          '''Returns a set of the contig names that are circular'''
          if self.assembler == 'spades':
              if self.contigs_fastg is not None:
                  return self._circular_contigs_from_spades_before_3_6_1(self.contigs_fastg)
              elif None not in [self.contigs_paths, self.assembly_graph_fastg]:
                  return self._circular_contigs_from_spades_after_3_6_1(self.assembly_graph_fastg, self.contigs_paths)
              else:
                  return set()
          elif self.assembler == 'canu':
              return self._circular_contigs_from_canu_gfa(self.contigs_gfa)
          else:
              return set()
docs: Returns a set of the contig names that are circular