docs: def get_variants(self, chromosome=None, start=None, end=None):
          """Return all variants in the database
          If no region is specified all variants will be returned.
  
          Args:
              chromosome(str)
              start(int)
              end(int)
          
      
          Returns:
              variants(Iterable(Variant))
          """
          query = {}
          if chromosome:
              query['chrom'] = chromosome
          if start:
              query['start'] = {'$lte': end}
              query['end'] = {'$gte': start}
          LOG.info("Find all variants {}".format(query))
          return self.db.variant.find(query).sort([('start', ASCENDING)])