code: def distance_to_interval(self, start, end):
          
          if self.start > end:
              # interval is before this exon
              return self.start - end
          elif self.end < start:
              # exon is before the interval
              return start - self.end
          else:
              return 0
docs: Find the distance between intervals [start1, end1] and [start2, end2].
          If the intervals overlap then the distance is 0.