replace file name for ISO8601 standard to ISO 8601 extended

iso8601ste.py

#! /bin/env python
# -*- coding:utf-8 -*-
import os

files= os.listdir(os.getcwd())
for i in range(len(files)):
  if files[i].isdigit()== True:
    print(files[i])
    tmp= files[i]
    yr= tmp[:4] + '-' + tmp[4:]
    new= yr[:7]+ '-' + yr[7:]
    os.rename(files[i], new)