code: def extract_length(self, lower_bound=None, upper_bound=None, new_path=None):
          
          # Temporary path #
          if new_path is None: fraction = self.__class__(new_temp_path())
          elif isinstance(new_path, FASTA): fraction = new_path
          else:                fraction = self.__class__(new_path)
          # Generator #
          if lower_bound is None: lower_bound = 0
          if upper_bound is None: upper_bound = sys.maxint
          def fraction_iterator():
              for read in self:
                  if lower_bound <= len(read) <= upper_bound:
                      yield read
          # Do it #
          fraction.write(fraction_iterator())
          fraction.close()
          return fraction
docs: Extract a certain length fraction and place them in a new file.