python查找缺失文件

import numpy as np
import os,sys

filepath=r'F:\gucheng\20191212d'
numfile=4163
fn1=r'F:\gucheng\20191212d\20191212x'
fn3='.dat'
nlist=range(1,numfile+1)

for i in nlist:
    
    if i<10:
        filename=fn1+'00'+str(i)+fn3
    elif (i>=10 and i<=99):
        filename=fn1+'0'+str(i)+fn3
    else:
        filename=fn1+str(i)+fn3
          
    ans = os.path.exists(filename)
    
    if not ans:
       print(filename)