code: def assembly_length(self):
          
          for sample in self.metadata:
              # Only determine the assembly length if is has not been previously calculated
              if not GenObject.isattr(sample, 'assembly_length'):
                  # Create the assembly_length attribute, and set it to 0
                  sample.assembly_length = 0
                  for record in SeqIO.parse(sample.bestassemblyfile, 'fasta'):
                      # Update the assembly_length attribute with the length of the current contig
                      sample.assembly_length += len(record.seq)
                  # Write the updated object to file
                  self.write_json(sample)
docs: Use SeqIO.parse to extract the total number of bases in each assembly file