如何用python写一个循环，或者其他方式，完成批量选取内容并保存。

下面是将文件 split__1.fasta中选取一段序列，如下421至480. 存于selected_split_1.fasta文件中__。
如何写一个循环，可以从split__1.fasta 至 split__68.fasta，中分别选取421至480. 并存于selected_split_1.fasta至selected_split_68.fasta文件中文件中

from Bio import SeqIO

fin = open('split_1.fasta', 'r')
fout = open('selected_split_1.fasta', 'w')

with open("selected_split_1.fasta","w") as f:
        for seq_record in SeqIO.parse("split_1.fasta", "fasta"):
                f.write(">")
                f.write(str(seq_record.id) + "\n")
                f.write(str(seq_record.seq[421:480]) + "\n")  #start 421 to end 480 base positions

fin.close()
fout.close()

from Bio import SeqIO

for xx in range(1, 68):
xn = "split__" + str(xx) + ".fasta"
yn = "selected_split_" + str(xx) + ".fasta"

fin = open(xn, 'r')
fout = open(yn, 'w')

with open(yn,"w") as f:
for seq_record in SeqIO.parse(xn, "fasta"):
f.write(">")
f.write(str(seq_record.id) + "\n")
f.write(str(seq_record.seq[421:480]) + "\n") #start 421 to end 480 base positions

fin.close()
fout.close()

for i in range(1,69):
inf="split_%s.fasta" % i
outf="selected_"+inf
with open(inf,'r') as infp:
data=infp.readlines()
with open(outf,'w') as outfp:
outfp.writelines(data[421:481])

# -*- coding:utf-8 -*-
import os
from Bio import SeqIO

# root_dir为要读取文件的根目录
root_dir = r"C:\Users\2350586L\PycharmProjects\split\splitE"
# 读取批量文件后要写入的文件
with open("FANCE1020_1080.fasta", "w") as f:

    # 依次读取根目录下的每一个文件
    for file in os.listdir(root_dir):
        file_name = root_dir + "\\" + file
        filein = open(file_name, "r")
        # 按行读取每个文件中的内容
        for seq_record in SeqIO.parse(file_name, "fasta"):
                f.write(">")
                f.write(str(seq_record.id) + "\n")
                f.write(str(seq_record.seq[1020:1080]) + "\n")  #start 481 to end 540 base positions

        filein.close()
print("FINISHED")