def target_sequence_length(self):
    """ Get the length of the target sequence.  length of the entire chromosome

    throws an error if there is no information available

    :return: length
    :rtype: int
    """
    if not self.is_aligned():
      raise ValueError("no length for reference when read is not not aligned")
    if self.entries.tlen: return self.entries.tlen #simplest is if tlen is set
    if self.header:
      if self.entries.rname in self.header.sequence_lengths:
        return self.header.sequence_lengths[self.entries.rname]
    elif self.reference:
      return len(self.reference[self.entries.rname])
    else:
      raise ValueError("some reference needs to be set to go from psl to bam\n")
    raise ValueError("No reference available")
Get the length of the target sequence.  length of the entire chromosome

    throws an error if there is no information available

    :return: length
    :rtype: int