def calc_paired_insert_stats(in_bam, nsample=1000000):
    """Retrieve statistics for paired end read insert distances.
    """
    dists = []
    n = 0
    with pysam.Samfile(in_bam, "rb") as in_pysam:
        for read in in_pysam:
            if read.is_proper_pair and read.is_read1:
                n += 1
                dists.append(abs(read.isize))
                if n >= nsample:
                    break
    return insert_size_stats(dists)
Retrieve statistics for paired end read insert distances.