code: def find_unpaired_reads(fastq_directory, forward_id='_R1', reverse_id='_R2'):
      
      unpaired_list = list()
      fastq_files = glob.glob(os.path.join(fastq_directory, '*.f*q*'))
      for name in fastq_files:
          if forward_id not in name and reverse_id not in name:
              unpaired_list.append(name)
          elif forward_id in name and not os.path.isfile(name.replace(forward_id, reverse_id)):
              unpaired_list.append(name)
          elif reverse_id in name and not os.path.isfile(name.replace(reverse_id, forward_id)):
              unpaired_list.append(name)
      return unpaired_list
docs: Looks at a directory to try to find unpaired fastq files.
      :param fastq_directory: Complete path to directory containing fastq files.
      :param forward_id: Identifier for paired reads, forward.
      :param reverse_id: Identifier for paired reads, reverse.
      :return: List of paths to unpaired files.