操作文件只留后面n行

import sys
import os
def create_file(filename,num):
    count = 0
    dest_file='{}.bak'.format(filename)
    with open(filename, mode='r', encoding='utf-8') as f_r:
        for line in f_r:
            count += 1
    count -= int(num)
    start = 0
    with open(filename, mode='r', encoding='utf-8') as f_r, open(dest_file, mode='w',
                                                                   encoding='utf-8') as f_w:
        for line in f_r:
            start += 1
            if start > count:
                f_w.write(line)
    os.remove(filename)
    os.rename(dest_file,filename)

if __name__ == '__main__':
    create_file(sys.argv[1],sys.argv[2])
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语法：
python3 xxx.py filename num
例子：
python3 file_clean.py 5000