参考：https://stackoverflow.com/questions/3431825/generating-an-md5-checksum-of-a-file

计算文件的 md5

import hashlib

def md5(fname):
    hash_md5 = hashlib.md5()
    with open(fname, "rb") as f:
        for chunk in iter(lambda: f.read(4096), b""):
            hash_md5.update(chunk)
    return hash_md5.hexdigest()