code: def query_locus(self, filter_column, filter_value, feature):
          
          loci = self.query_loci(
              filter_column=filter_column,
              filter_value=filter_value,
              feature=feature)
  
          if len(loci) == 0:
              raise ValueError("Couldn't find locus for %s with %s = %s" % (
                  feature, filter_column, filter_value))
          elif len(loci) > 1:
              raise ValueError("Too many loci for %s with %s = %s: %s" % (
                  feature, filter_column, filter_value, loci))
          return loci[0]
docs: Query for unique locus, raises error if missing or more than
          one locus in the database.
  
          Parameters
          ----------
          filter_column : str
              Name of column to filter results by.
  
          filter_value : str
              Only return loci which have this value in the their filter_column.
  
          feature : str
              Feature names such as 'transcript', 'gene', and 'exon'
  
          Returns single Locus object.