code: def parse(self, limit=None):
          
          if limit is not None:
              LOG.info("Only parsing first %d rows", limit)
  
          phenotype_file = '/'.join(
              (self.rawdir, self.files['patient_phenotypes']['file']))
          variant_file = '/'.join((self.rawdir, self.files['patient_variants']['file']))
  
          phenotype_file_handler = open(phenotype_file, 'r')
          variant_file_handler = open(variant_file, 'r')
  
          self._parse_patient_phenotypes(phenotype_file_handler, limit)
          self._parse_patient_variants(variant_file_handler)
  
          return
docs: Override Source.parse()
          Args:
              :param limit (int, optional) limit the number of rows processed
          Returns:
              :return None