新手求助 关于self

最近在研究一个前辈写的程序，有些看不懂，查不到，故求助各位大神
def _build_chrom_strand2genes(self):
"""Build chrom_strand: list of genes"""
cs2genes = {}
for chrom in self:
for strand in ['+', '-']:
cs2genes[(chrom, strand)] = {} # only keep unique (start, end)
for bed in BedReader(open(self.f_gene)):
chrom_strand = (bed.get_chrom(), bed.get_strand())
if chrom_strand not in cs2genes:
cs2genes[chrom_strand] = {}
s_e = (bed.get_start(), bed.get_end())
if s_e not in cs2genes[chrom_strand]:
cs2genes[chrom_strand][s_e] = bed
return cs2genes