code: def coverage(self):
          
          if self._targetLength == 0:
              return 0.0
  
          coverage = 0
          for (intervalType, (start, end)) in self.walk():
              if intervalType == self.FULL:
                  # Adjust start and end to ignore areas where the read falls
                  # outside the target.
                  coverage += (min(end, self._targetLength) - max(0, start))
          return float(coverage) / self._targetLength
docs: Get the fraction of a subject is matched by its set of reads.
  
          @return: The C{float} fraction of a subject matched by its reads.