看评论区，才弄懂的

import base64def safe_base64_decode(s):    x = 0    # s若不是4的倍数，则肯定被去掉[4-len(s)%4]个'='    if len(s)%4:        x = 4-len(s)%4    # 将去掉的'='的补上    s = s+'='*x    return base64.b64decode(s)assert b'abcd' == safe_base64_decode('YWJjZA=='), safe_base64_decode('YWJjZA==')assert b'abcd' == safe_base64_decode('YWJjZA'), safe_base64_decode('YWJjZA')print('ok')